8 Tips To Prevent Lithium Battery Load Dumps

Preventing Instant Darkness

In the last article about lithium batteries, I defined load dumps and shared why they matter.

And, further, I wrote, and believe strongly, that we cruisers should not listen to salespeople who downplay this fundamental characteristic of lithium batteries—they will be home safe in their beds when a load dump puts our boat and our crew in jeopardy.

It's Basic Seamanship

As boat owners we should not tolerate a system that will load dump, any more than we would ignore a broken strand in a shroud or a sloppy rudder bearing.

So let's look at some ways to banish load dumps:

#1 Never "Drop In"

If you have not already bought "drop in" lithium batteries, don't.

The problem with "drop in" batteries is that, since the BMS is inside the battery, it has no way to communicate its intentions before load dumping.

This is a fundamental system architecture issue that's not ever going to get fixed properly.

For example, even if "drop in" vendors add something like a buzzer, or a warning over Bluetooth to our phone, prior to shut down, that will not help much. (See Tip 2.)

And for those who say, "but wait, plenty of people have "drop in" batteries on their cruising boat", I say "yes, and plenty of people invested with Bernie Madoff".

Already Have "Drop In" Batteries

That said, I don't think any the less of you if you have already bought "drop in" batteries. I have made far worse mistakes than that.

After all, the very words "drop in" infer something that's very seductive—all of the benefits of lithium with no added expense or modifications to our boat.

Just as the words "guaranteed high returns" are seductive—high investment returns with no risk.

Sadly, neither are true.

Still, all is not lost. If you already have "drop in" lithium batteries, Tips 4, 6 and 7 will help at least mitigate the load dump problem.

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Do You Need A Generator?
  22. Efficient Generator-Based Electrical Systems For Yachts
  23. Battery Bank Size and Generator Run Time, A Case Study
  24. Battery Options, Part 1—Lithium
  25. Battery Options, Part 2—Lead Acid
  26. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Load Dumps
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Ion Batteries Explained
  31. 11 Steps To Better Lead Acid Battery Life
  32. How Hard Can We Charge Our Lead-Acid Batteries?
  33. How Lead Acid Batteries Get Wrecked and What To Do About It
  34. Equalizing Batteries, The Reality
  35. Renewable Power
  36. Wind Generators
  37. Solar Power
  38. Hydro Power
  39. Watt & Sea Hydro Generator Review
  40. Battery Monitors, Part 1—Which Type Is Right For You?
  41. Battery Monitors, Part 2—Recommended Unit
  42. Battery Monitors, Part 3—Calibration and Use
  43. Battery Containment—Part 1
  44. Q&A—Are Battery Desulphators a Good Idea?
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Daniel Barthel

Would two side by side banks connected by a 1,2 both battery switch minimize the potential for a load dump?


Partly, but it’s a lot more complex than might be expected at first glance.

If you have lithium batteries at differing states of charge, and you put them in parallel, one will rapidly discharge itself to charge the other until they’re roughly in equilibrium. And lithium cells do not like to be very far out of state-of-charge balance with each other, at a considerable hit to longevity if balance is not maintained. So a simple 1/2/both switch is not a very effective solution; it might be more likely to damage the batteries than to get you out of a pickle.

The very best system architectures I’ve seen have not just two, but three buses with individual relays.

  • Bus 1 is all the charging sources, including the alternator, whose regulator is wired to accept a “stop charging” command from the BMS at a slightly lower threshold than the “panic, open the relays!” level. Bus 1’s relay will open for a high voltage, over-charge, over-current, or over-temperature condition, and sounds an alarm.
  • Bus 2 is the regular DC loads. Its relay will open at some nominal low, but safe, voltage / state of charge, and also for conservative over-current or high-temperature conditions, and sounds a different alarm.
  • Bus 3 is the most safety-critical DC loads. Its relay will open at the low voltage, high current, and/or high temperature levels just before the battery damage risk threshold.

In some cases, Bus 3 is implemented on a completely separate bank with its own charging bus, and no connections (except for a common ground) to the main bus.

And, even if using lithium for most service loads, I would always keep a completely separate lead-acid engine cranking bank.

Andy Schell

Nigel’s Integrel system sorta does this with or without lithium. The big battery bank is like an electrical fuel tank – it gets charged by the big charging sources, then in turn charges the buffer 12V bank that the house loads are connected to.

This thinking seems like the best of both worlds – super high efficiency charging on a big lithium bank, much longer times between charges, added real capacity for the same rated capacity, but a separate lead acid bank on the load side which gets charged constantly by a DC-DC converter.

On FALKEN we’re doing this for the critical systems – nav lights & nav instruments. The rest of the non-critical loads will connect directly to the lithium bank.

Thomas Ingram

Along these lines, for coastal sailing, I’m planning to install USB sockets and a cigarette lighter adapter to the AGM bow thruster bank as a way to charge cell phones or an ipad (with Navionics or other charts loaded), a handheld VHF, lights, etc. These sockets will be nice to have, anyway, and will be a backup in the event the house lithium bank goes down. A couple of LED cabin lights would be nice, too. The bow thruster batteries would be charged via a DC/DC charger from the lithium bank. It also avoids trying to create new/unusual/improvised interconnections between battery banks, which risk creating new problems if not perfectly done. There’s only so much time, money and expertise out there. I’m having a lithium system with external bms and wakespeed regulator installed right now. It was a challenge to find experts with the time to design and do the work, even in Annapolis.

Ray Hendricks

It seems like the folks at Battery Balance https://batterybalance.com/x2-bms-12-24/
have a system that integrates lithium with lead acid very well.

Kit Laughlin

John, all these points above are excellent. From my perspective, the Achilles heel of any single bank lithium system is that its BMS can pull the whole bank out of play, as you describe in the Load Dump problem. When I started thinking about this, the first solution I cam up with was simply to build two completely separate house banks that were able to be selected, but without the option for load sharing. This way, if the active bank went down (for whatever reason, and assume BMS on bank 1 is alarmed) a simple mechanical switch over to bank 2 would get over the ‘lost everything’ scenario you outlined, until you can fix the problem, hopefully the next day when the weather is clear, and coffee has been consumed!

Ernest E Vogelsinger

Coming from software development I tend to break more complex problems down to separate manageable requirements. Same as Matt outlined above I would see three major blocks:

charge dump – this should never ever affect the load bus, so any BMS that is to be chosen must clearly support separate overcharge and overload switchingcritical loads, which are mostly electronicsauxiliary loads where a loss might be annoying but not mission critical, such as most cabin lighting, or fridgeSince a battery bank has only one output for load it would be wise to split this into two separate load busses, one for the critical loads, and one for the rest. The “critical” bus should have an additional backup battery that kicks in in case the regular source is lost.

Both load buses must be electrically separated to avoid the aux bus being “back powered” by the critical bus, this might be achieved for example using a standard MOSFet battery separator. The backup battery for the critical load bus would be connected using an automatic switchover module.

The backup battery should ideally be of lead acid type and kept in charge by means of a DC/DC charger, from the main battery.

And, of course, there should always be a separate (lead acid) bank to start the engine.

MOSFet batty isolator: https://www.victronenergy.com/battery-isolators-and-combiners/argo-fet-battery-isolators
DC/DC charger: https://www.victronenergy.com/dc-dc-converters

Note: although the battery separator might support current up to 200A per output the load on the critical bus should not exceed 25A continuous as this is the limit of the switchover module. This should be able to power all necessary electronics, even including radar, however.

MOSFet separator: ~ $150,-
DC/DC charger: ~ $200,-
Switchover: ~ $200,-
plus backup battery

Ryan Ellison


Respectfully. You are doing a lot of harm to the lithium battery industry with this series. In full disclosure I am the founder of a ‘drop in’ battery company and I have lived and worked with installing and testing these types of batteries in boats. So i believe I have some education and experience behind this post. I also have a lot more data on failure rates than anyone else will have.

While I believe in the theme of discussing dump loads, what they are and why they are important to plan for is important to flat out say don’t install a ‘drop in’ is simply wrong and based on misguided information.

In your post you discuss the outcome of a dump load or a failure of a battery but that’s only the severity piece of the risk equation your missing the probability piece. Just saying ‘this could happen and it would be bad’ is fear mongering and as far as I can tell not based on any practical data which you have presented. Of the 350k batteries we have sold in the last 7 years, not only on boats but in other extreme conditions we have no reports of dump loads which lead to severe failures or blackouts.

Now thats not to say other manufactures produce the same quality but your grouping everything in to one. Nor is it to say it will never happen at some point in the future (it probably will but thats probability for you). Its not a everyday event.

I will say that the pleasure craft business is very bad at collecting data. Our company has data on reports, nothing more. I do agree that we need to have some type of probability rate for BMS shut down would be very helpful in this discussion.

People have also spent a ton of time running through the streets talking about the dangers of lithium dump loads being one of them. Fact is non of these issues are new and there are ways in todays electrical systems that can create a dump load.

To be clear, i am not saying a person should just ‘drop in’ (in fact I hate that word) a battery to their system. There needs to be careful planning on the charging, monitoring and over all system setup but there are a lot of way a drop in system can be safe without spending 30k on a dual buss, external monitors battery setup (you forgot to mention the price difference of all this).

A lot of smart people have come together on ABYC TE13. No where in these standard (or soon to be standard) does it say you cant have a internal BMS or you cant use a drop in. It does imply that they system should be thought out and designed with charging, discharging and dump loads in mind.

A lot of people read your blog because its thoughtful and well written (myself included), thus they follow your advice but flat out saying an entire group of batteries can not be installed in a safe manor is doing a disservice to your readers and the yachting industry.

Andy Schell

So maybe we should define ‘high consequence’? As I’ve said, I don’t actually think a blackout has to be that high consequence if you always have it in the back of your head when making plans. You mentioned a few cases in part 1, but they’re still not ‘immediate threat to boat or crew’ instances. The crash jibe yes, but there are so many other ways that can happen too. And if you’re keeping a good watch and notice the blackout, it should be instinctive to take over the helm and steer out if it. If you’re not ready for that eventuality then you’re not ready to go offshore. The ability for an internal BMS to sound an alarm preceding a blackout, and thereby alerting crew to go into ‘blackout’ mode so to speak, is something Ryan had mentioned about the new ABYC regs.

Furthermore, you’ve got the risk of the blackout itself, the probability of that happening at ANY given time as Ryan said (with no real data which we can’t do much about), then the timing of a blackout happening at a CRITICAL moment. You get different outcomes with a blackout depending on when it happens, which makes the risk even harder to quantify (and the probability even lower).

I tend to agree with you that low probability/high consequence events must be avoided. Nassim Taleb’s book The Black Swan is about exactly this. But his idea of high consequence events were way worse than an electrical blackout. If we were talking about load dumps causing fires or explosions, that’s a high consequence event. To me, a blackout isn’t in the same category.

I’m actually not defending either side here, and I have both – Dakota on my little boat SPICA, and Lithionics, designed with Bruce & Nigel, going on FALKEN.

I think the question is defining what a high consequence event means to each person, and what their personal risk profile is for that. There’s a way you can frame this that removes your opinion and let’s people decide for themselves based on information. And anyway your opinion is clear based on what you put on your own boat – that’s the best opinion too because you’ve got skin in the game.

So why did we put Dakota on SPICA, fully understanding the risk? Because the boat is dead simple, I navigate on an iPad, we don’t even have an autopilot and the performance gain is totally worth the risk, under my risk profile.

Andy Schell

I don’t mean it as confirmation bias, what I meant is that the best way to get advice from the ‘experts’ is to simply do what they’re doing. Taleb famously hates journalists and academics because they have no skin in the game. It wasn’t a criticism of you, on the contrary – your actions speak loudly to your audience, in a good way, and you’ve got the experience x studied knowledge that usually leads to good outcomes.

Ryan Ellison


What do you believe acceptable failure rate is for a a lithium system with regards to a dump load? E.g. regardless of the system construction how many hours of use before a dump load could occur?


William Deertz

I’ve been following this series with interest. I installed “drop in” LFP batteries about 18 months ago and have been loving the performance. I have the alternator charging sorted with a wakespeed regulator and .5C AC charging through Victron chargers. My start battery is AGM. All my critical loads are to my NAV/COM panels (radar, chart plotter, running lights, etc) and based on this article I am contemplating connecting these either to their own dedicated lead acid bank or my start battery. While underway the NAV/COM loads are about 25A so the 80A dedicated alternator will keep the start battery fully charged. At anchor the loads are 2-3A so the batteries should be sufficient for the 22-23 hours between generator run of 1-2 hours to charge the house LFP bank. The start battery has its own dedicated 60A AC charger. This would leave the house LFP bank only powering non-critical loads such as refrigeration, cabin lights, fans, etc. And I have Maretron Alerts setup with very conservative parameters for high/low voltage, temperature and loads.

Would connecting my NAV/COM loads to the start battery be a good risk mitigation move? My only concern is that if at anchor long term the 1-2 hours might not give the start battery a sufficient absorption charge, although I tend to move at least every 2 weeks so a 3-4 hour run will get the battery fully charged. Appreciate any feedback.

William Deertz

John, yes thanks, good point about the voltage sag during starting. I should have mentioned in my original post that my house bank is 3×4 batteries wired in parallel. Would that make any difference in the risk of a bms shutdown?

Petter Mather Simonsen

Alternatively, one could install a DC/DC 12-12v isolated converter to the starter battery and feed the electronics from them. I guess that should give stable voltage to the electronics.

Dave Warnock

We are installing LiFePo4 batteries with built in BMS. This is very helpful.
However, we do not have a diesel engine, we have no alternator. It does appear that we therefore have eliminated the most significant dangers.
Our charging is via Solar (victron MPPT with breakers both sides of the MPPT and a fuse), mains (Victron Galvanic Isolator then Victron Multiplus II). It seems that none of these would be damaged by a over charging dump.
Our big loads (mains appliances and the electric motor) will all be used only when a person is there – who could react to alarms from the Victron battery monitor). Gradually we will add more integration using Signal-K which will allow us many more alarms and both automated shutdowns plus remote access to shut things down etc.
We are being careful to read and follow all the guidance about fuses, wire sizing etc.

Dave Warnock

I am definitely looking at adding external BMS.

Dave Warnock

It would be if I could find an external BMS for whole 12v batteries rather than individual cells.

Lindsey Gill

John, I am one of those people who already have batteries with built in BMS. Last Summer our house batteries (Rolls AGM) died and I needed to replace and decided to go Lithium. Having considered the options, I decided to go for the Victron Lithium Super Pack 25.6V 50A batteries. I have 9 of them in parallel. I did consider external monitoring but personally I didn’t see it got me much more than I get with the built in BMS and it seemed to me to be adding more complexity than necessary.

I already have Victon Multiplus Charger Inverter (120A charge) and Victron MPPT controller (aprox 30A max solar charge) and reprogramming them was straightforward for the Lithium batteries. Charging from the two alternators was more complex. I have ended up with one alternator connected to the bow thruster batteries and also linked to two Victron DC to DC chargers (17A max each) onto the lithium batteries and the second larger alternator connected to the starter batteries and also to 4 Victron DC to DC chargers which feed the lithium batteries.

One important thing I considered is the fact that the Victron Super Pack batteries don’t just disconnect the load and charge if there is a fault condition. For example, in an over temperature situation charging is stopped per the specifications at 45C (slightly higher in the test below) whilst discharging is still permitted until a higher temperature (50C) is reached.  So, it is not as simple as a relay cutting the battery of from discharging in the event of any specification being breached. This I think is shown well in this YouTube video from Victron. (617) Extreme Lithium Victron SuperPack Test – Pros Cons of Lithium Batteries – YouTube

I did consider the risk of the batteries shutting down whilst in use but your article has certainly made me think more about it. In the first article you list the various fault conditions that may cause shutdown. I have given some thought to each of them:

Over temperature – I guess this can come from 2 places. 1) Internal battery fault generating heat – in which case I want the battery shutting down as soon as possible to prevent risk of fire, which is a much greater danger than the battery shutting off the load. I don’t see any downside to internal BMS for this situation, and unless I am very unluck some of my other 8 batteries may not be overheating in this way 2) The environment where the batteries are kept is hot. Again, I want the batteries shutting down. Generally, I am aware of the temperature of the engine compartment where the batteries are kept (not ideal I know but that is how the boat is set up), so I think I would be on the lookout for the risk of a shutdown due to external over temperature conditions. I monitor the engine and engine room temperature on a raspberry pi and have an alarm if they are going too high.

Under temperature – Our boat is built for high latitudes and if we are aboard and sailing then the temperature will be kept well above the -20 at which shutdown becomes a risk. If the heating systems fail then again, I would be on alert for the risk of a shutdown.

Charge over-current – My 9 batteries have a maximum charge current of 50A each giving 450A maximum charge current. My Victron Multiplus has a maximum charge current of 120A, my alternators can theoretically generate (but never come close) to 100A and my solar can provide a little under 30A, so even with all three charge sources running an over charge current is not possible. Furthermore, the main fuse for the batteries will blow at 300A.

Charge over-voltage – The absorption voltage for the batteries is 28.4-28.8 volts. I have the settings in the various Victron units at 28.4 volts and I have reduced the absorption times down to short intervals. Victron recommend you don’t charge at Absorption for more than 4 hours. I have reduced the times down to under an hour for the various controllers. My theory here is that it is better not to try to overcharge the Lithium batteries to improve longevity. Once Absorption is finished the system floats at 27V if charging.

Too low state of charge – I monitor the batteries using a Victron BMV 712 Smart and when at sea I keep an eye on the state of charge (and voltages but I know this might not help until too late). I normally try to keep the state of charge above 50% and I follow Victron guidance and fully charge the batteries every couple of weeks to re synchronise the monitor. So, if I see the state of charge getting low I would switch on the generator or (engine if the generator fails) to bring the batteries back up to a better state of charge. So, I don’t see how there is a big risk of failure from this situation for me.

Too high state of charge – I think I have two layers of protection here. Firstly, my Victron charging units should not permit this as they are correctly configured and secondly the internal BMS should prevent over charging. If the BMS cuts off the batteries due to overcharging then I guess there is a risk of overvoltage on the main bus if one of the Victron units fails but I would hope to detect this when I am sailing, as I said above I regularly check the MC712 for state of charge and voltage information.
One or more cells in the battery too far out of voltage balance with the others – So in my case I have 9 batteries so if this situation occurs in 1 of them that one may shut down but the others should continue to function.

In the event of a failure, I can connect the starter batteries to the main bus in under five minutes. Perhaps I should wire this in with a switch so I could do it in 30 seconds.

Clearly, I am biased as I have invested heavily in my Lithium setup however I just don’t get your Bernie Madhoff comment. I could just as easily say “And for those who say, “but wait, plenty of people have “drop in” batteries on their cruising boat”, I say “yes, and plenty of people invested with Berkshire Hatahaway  (Or pick your own fund that generally outperforms the market a bit)“. It means nothing either way, it is the facts about the performance of the Lithium batteries in the real world that matter. My experience so far has been good. Zero load dumps in 9 months and I think Ryan provides some good feedback about the lack of reporting of load dump issues to Dakota Lithium.

Looking at some of your specific suggestions for batteries with built in BMSs

#4 Two busses – I don’t see the need for this. If there is an over charge issue it can only come from the generator, the alternators or the solar (unlikely given its output). If it is the generator I can dial it down to zero on the Victron remote controller (in the pilot house), and if it the alternator I can switch the DC to DC controllers off or worst case disconnect the field wire to the alternators. Since my two alternators run independently I would hope to identify where the problem was and only disconnect one of them.

#6 Smart regulator – I have gone for smart DC to DC chargers with 17A limits each rather than trying to control the alternator. My solution is probably not as cost effective as a smart alternator but I think it works.

#7 Amp Counter backup – I don’t understand the need for an alarm at 85% of charge. I would not see that as an alarm situation. Surely it is more complex than that. If I wanted an alarm it would be at 100% of charge and voltages at above the specified absorption voltage of 28.8v. I want to know the batteries are full and the chargers are still trying to add more to them rather than moving to float.

As regards the risk of failure I would ask the question, how many boats catch fire each year because of faulty wiring. If all those boats had good fuses would that not limit the number of fires. However, if the fuse blows whilst entering Halifax harbour in fog are we not in a similar situation, the power supply has just been cut off.

So let me close with my disclaimer. I am not an electrical engineer, but I do have a basic understanding of electricity. I may have misunderstood how things are working in my set up; hence my long post as I would be interested in constructive feedback around my misunderstandings so I can fix them and possibly improve my situation having chosen batteries with built in BMS (I too don’t like the term drop in).

Thanks for the interesting article.


If you’re using Victron’s thoroughly pre-engineered and standards-compliant system, then you are in a very different state from someone who dropped a generic “12V 100Ah RV Boat Solar Lithium LiFepo4 Integral BMS Group31 Battery” off Alibaba, or a domestically-relabelled variant of the same, into the spot where a Group 31 lead-acid used to be.

Jeffrey Werner

I don’t read this as requiring the BMS to have anything other than warning capability. If I can build a system where the charging sources are immune to charge load disappearance based on the lithium bank’s inherent behavior, then that is up to me and the capabilities of my MPPTs, Charger/Inverters and Alternators. Either the wording of the standard needs to be far more prescriptive of the Li BMS responsibility or the interpretation can be “my system doesn’t stop the charging source in a manner that causes damage” since I can offload and absorb the dump in any number of alternative schemes to BMS-controlled charge sources. I expect that the ABYC will stop short of being that prescriptive as these are only safety guidelines.

I understand your position, you have been very clear on this, but the ABYC standards can not, and should not, live up to that hyper-conservative offshore risk adverse stance. These are safety standards and thus need to be a compromise, just like the rest of ABYC, a baseline.

Ernest E Vogelsinger

Well, actually this requirement would also be fulfilled if the BMS provided some dummy load to the charging source when disconnecting the load bus.
Would nevertheless be the wrong way to implement this, IMHO.

Jan Gils

I’m just giving my 2 cents after having had a load dump and it destroyed 2 plotters, fridge, freezer, whinch control boxes, etc… Reason for the load dump was an incorrect temperature sensor in the Lithium battery bank and no automatic shutoff of the alternator with an external wakespeed controller. There was a Sterling protector installed, but it could limit the voltage spike that destroyed all over-voltage protectors in most electronics.

This happened 3 months ago and to make it even worse, the whinches started running automatically because of the voltage spike, tore the boom down that broke at the gooseneck. So I do know the seriousness about a load dump.

The issue would have been prevented if the Wakespeed killed the field of the alternator before the BMS opened the relay of the battery bank. Reason it didn’t: I didn’t have time to implement it and I never got to it. Besides there was a sterling battery protector to absorb to overvoltage and protect the system. But I can garantuee you it didn’t.

At this moment the BMS sends out a prewarning over a hardwire relay which kills the field to the alternator. I discussed this with Wakespeed, and you can also use an input to kill the field, but then it goes through software. Nothing beats a hardware relay just to be sure.

Lithium banks should be designed with layered safety which also is important for keeping your bank in top shape. A factor I didn’t realize fully, untill we started our circumnavigation last year.

Layer 1: CANbus communication
I call this normal operation, the BMS (Batrium in my case) controls the whole system (everything) and sends out parameters for chargers and inverters. This also means that when you’re top balancing your bank automatically, you need to limit the charger to the amount of amps you can burn off. The Batrium BMS does this excellent and even let’s you define ramping curves to avoid overshooting / undershooting. The BMS communicates the voltage required and the chargers stick to it, ignoring their own predefined values. This also includes alternators : get an external controller that has CANbus control). The CANbus also controls the inverter, and limits discharge currents when the cell voltages go to low.

Layer 2: Inverter / charger settings
On all chargers you set the required voltages (bulk, absorption, you name it how you want). These voltages are either the same as you define in the BMS or a little bit higher, but well withing safety reasons (look up long-life LifePO4 settings). In case the CANbus fails, every chargers goes back to its own settings. In this case you have a “dump” system (like drop-ins) but it’s safe. This includes also your alternator (get a Wakespeed WS500) and you can fully control these parameters.

Layer 3: hardwired critical stops
A decent BMS allows you to control some relays, which you can wire to the chargers, inverters, alternator regulators. When the BMS says stop, you can fire off pre-warning-shutoffs to all chargers (and especially the alternator).

The BMS itself is also layered with alarms / setpoints on several levels for high / low voltage, high / low temperature, high / low amps, all on cell level and on shunt level.

To give an idea on equipment:

  • BMS: Batrium with CANbus
  • Chargers/inverters/mppt: Victron all with CANbus
  • Alternator: Wakespeed WS500 with CANbus

The whole system works together smoothly and the biggest advantage is that I can forget about it. I want to cruise and not worry about batteries.

I do agree that drop-ins are popular on Youtube, but I would not consider them in my system for the above reasons.

Ryan Ellison

Jan, what type of battery where you running? This is not clear from your post.

Petri Flander

Hello Jan Gils. It would be very good if the circumstances of your case could be known, in order to start looking the solutions. Could you please elaborate a bit on what type of alternator(s) you got running when you got that load dump?
Based on the damage, it sounds like you had something like 200 amp Balmar/Prestolite big case running on full bore.

What’s inside the Sterling APD device is most likely a Littlefuse automotive Varistor that is manufactured to 12V or 24V system. Technically, there is no reason why it failed, other than that it’s Joule rating was heavily exceeded. Or, the ~5 Amp series fuse got blown. Did anyone inspect the APD after the dump for signs of melting, or measured it with multimeter?

Namely, it could be that the varistor component that Sterling chose is dimensioned for regular 12V/70A automotive system, and it is way out of it’s league with big frame alternators.

btw. lightning surge suppressors on powerline high voltage switching fields are varistors also, and they work just fine. Just that their dimensions are more like on a main sewer pipe.

Rob Gill

I commented previously about our LiFePO4 battery bank experiencing a low SOC load dump and how I believed this wasn’t a problem. Here’s my layman’s description why:

When our 30% alarm happened we were at anchor and the crew ashore. The BMS shut down the lowest SOC battery using its contactor. Had we not returned in time, it would presumably have shut down the other two batteries when they got to 20% also.

In the MasterVolt system there is a master / slave(s) setup on the management bus, and as I understand it the slave(s) drop out first. The master being the last to be isolated.

Upon return, I saw the low SOC alarm, started the engine to charge the bank, but didn’t realise the contactor had been triggered. We only figured this out when the battery would only charge to 66% level. On re-setting the contactor the bank returned to normal.

Had we been aboard / at sea, and ignored three separate low SOC alarms (visual and audial) at 30% SOC, then continued to draw current until all three Lithium batteries had been isolated at 20% SOC by their contactors (at least three hours), we could simply start the engine and then switch the house load to the start battery (having manually turned off any non-essential load at the 12V switchboard), until our lithium house bank was sufficiently recharged bring back on-load. Note: pre-check the contactors had triggered, and each lithium battery was electrically isolated before bridging the starter battery.

Having a complete MV system, I don’t ever expect to experience a high SOC alarm condition as this is all managed by the MV BMS talking to the MV smart regulator and MV solar controllers. If it did happen, I would expect the BMS or alternator regulator had failed. We carry a pre-programmed spare for each.

But if (when) things really go wrong, we can operate the boat normally on just one of our lithium batteries (180 Ah), and thanks to a previous AAC article on lithium some years ago, we left in place our original AGM battery cable runs to the engine bay, so in the event of a total battery system failure (say the control system), we can isolate the lithium bank totally, purchase two truck batteries ashore as a conventional lead acid house bank, connect them both up using bridging cables and continue our cruise until we return to NZ. We have a process diagram in our vessel manual on the right way to connect them up using bridging cables.

The best way for me to describe the actual system is to provide the 12V electrical generation and distribution schematic (see attached). We have a separate 240V diagram.

When I asked permission to share the attached schematic and explained why, our marine electrical supplier and installer Enertec NZ advised they are now installing 80% lithium systems to 20% lead acid.

Enertec now supply their own brand of lithium batteries (still sell and support MV gear) and they shared a view that most internally managed lithium ion batteries wouldn’t meet the Australia NZ Electrical standard for boats under; “Additional requirements for lithium ion batteries”. And such non-compliance would be problematic in the event of an insurance claim.

Having the single +bus and -ve bus system is very simple, relatively easy for me to understand and manage, and has proven reliable in operation for us. Retaining the separate lead acid start battery and old house battery cables gives us a back-up.

So I have to say, I can’t see the need to have separate buses for the different load profiles being discussed here, unless we were struggling with supply and / or capacity issues. Maybe I’ve missed something (entirely possible). Our power budget is modest in comparison with our generation and battery bank capacity.

But I am absolutely open to understanding what the unforeseen consequences (on my part) could be and what might be done to improve things without starting again.

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If I’m not mistaken, your setup allows the individual BMSes to, via the bus, command the alternator regulator, combi charger, and solar MPPTs to stop charging when the batteries are full.

That is, effectively, a multiple-bus system. It has the ability to automatically and smoothly stop the charging sources when it hits a full charge / high temp / high voltage / high current condition, without cutting the batteries off from the loads and without a sudden disconnection of a fully-loaded alternator that would cause a damaging voltage spike.

You still have the low-state-of-charge / undervolt cut-off risk, but in that design, it doesn’t look like it would be any worse than the equivalent risk with a lead-acid system, and two of the batteries can (briefly) keep things running long enough for you to get things under control if the third cuts out.

Rob Gill

Hi Matt, appreciate your thoughts and reassurance.

The charging sources are all visible from our MasterView display, which we have in our nav station, We setup soft-switches that allow us to not only monitor, but manually turn off/on each charging component, individually.

In terms of risk management, our main concern is more electronics failure than electrical. So we work on monitoring and controlling the storage environment carefully.

MasterVolt advise keeping batteries and controllers separate, but we found having the one bigger and well ventilated space easier to fit out and manage in our yacht.

In case members are interested; Mass-Combi top-right in picture, solar controllers top-left, the three contactors to left, alternator controls on bulkhead to right (out of picture sorry), and the three LiFePO4 batteries strapped in with truck tie downs. Large vents out of picture front and back.

We monitor the temperature and humidity inside this space, as well as being able to interrogate each battery for temperature, voltage, etc etc, on the MasterView display.

We regularly change out two “DampRid” containers and try to think of the space more as an IT server room than a battery locker. But we currently have no active cooling / fans as we find the fan in the MassCombi circulates the air well enough for us, and it comes on automatically as the ambient temperature rises.

We also have small portable 12V clamp-on cabin fans we could install temporarily in the tropics – but they have not been required so far even with 30+ degrees sea temp, in Fiji. Ngā mihi nui, Rob.

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Rob Gill

Hi John, valuable pause for consideration on my part, notably:

  • One of our four charging devices (MassCombi) is only ever charging in dock, attached to the mains – we don’t have a generator.
  • The two solar charging devices could only create an over voltage / charge issue during daylight, when an event would be more easily handled and the BMS would first try to shut the solar power down using the MV bus.
  • The most likely troublesome charging source would be the alternator. And as I understand it, if the Alpha-Pro regulator malfunctioned, the batteries would not load dump in an over-voltage / over-charge event, but first the BMS would take the alternator offline (it would free-wheel I understand). If the BMS and the Alpha-pro fail together and then the LiFePO4 batteries load dump, then we need to parallel over to the start battery.
  • Then how critical is losing the navigation instruments or auto-pilot for a few minutes? The first scenario is not critical for us, as a professional navigator I am always allowing for such an event. But maybe as we get older?
  • The second scenario we experience about five times on our way to Fiji (intermittent fault with our Lecomble and Schmitt electric motor driving the hydraulic pump), including several all-standing gybes. We got really good at recovering.
  • If we did implement a second “essential service” bus with dedicated lead acid battery for the critical navigation gear and VHF (which Enertec say we could do relatively simply), would we then create new points of failure and extra complexity to manage?

Thanks for raising this issue – we will certainly explore this with Enertec before our next offshore trip.

Rob Gill

Thanks John – the more I think about it, the more I like having our “get-home” back-up installed and pre-tested.

David Parsons

First post here after reading many relevant excelent and helpful knowledge and learning articles on offshore sailing / cruising. Thankyou!

I passed on the two recent lithium battery articles to my son who has just bought a 45ft ali humm catamaran project yacht. A full electrical redo is to happen. He is a remote area (land) solar project engineer familar with large battery and solar installation, so he nows much about that level of BMS moniroting.

He asked me in response:
I think the problem with lithium cutting out at too high state of charge can be fairly easily managed by ensuring the voltage setpoints of the different charging devices are lower than the cutoff voltage of the BMS – if you do this I can’t see how the batteries would get charged to the point where they would cut off?”
He was happy for me to post this question (as my battery/BMS / litium knowledge is not good enough to answer him).


David Parsons

Thanks John. And apologies for the terrible spelling in my post – my reason, but no excuse, is my rush at the time to send it off before I got to my train-stop.. 🙂

Dave Warnock

In an earlier response to John I glibly said I would be looking to add an external BMS to our system. Trouble is I have been looking and I am struggling to find an external BMS that works for banks of large drop-in Lithium batteries.
Our main bank is 4 x 12V 300AH drop in batteries.
Does anyone know of an external BMS that will work with these?

Rob Gill

Hi Dave,

Having been through the lithium decision myself six years ago, I believe (I’m no expert) that many (most?) external BMS implementations treat the batteries as one bank, and are set up to trigger one contactor. In this case a fault in just one cell (which is not uncommon with lithium) will take down your whole bank. As I understand it, this is how even the Victron BMS worked at the time (still works?).

Ideally for resiliency you will find a BMS that will allow each battery to have it’s own contactor, and so they drop out individually under fault condition.

Otherwise, you may be better staying with your internal BMS setup, or wouldn’t you be at risk of losing your electric drive?

Good luck.

Frode Rognstad

A fault in just one cell (which is not uncommon with lithium)

What is the basis for this statement? Other than hearsay, where are the statistics showing single-cell faults with the types of lithium batteries we will use on boats (LiFePO4)?

There appears to be an unfortunate amount of misinformation, and speculation masquerading as actual events, making its way into these discussions, as well as unfounded fears.

LiFePO4 batteries are not delicate little creatures, in need of constant babying. If you operate them within their “near-flat-voltage” area (avoiding high or low voltage knees), they are rock solid.

I also see no reason to recommend adding lead acid batteries of any kind, for any purposes, when the main bank is LiFePO4. My nominal 12V bank, at 900Ah, cranks my 185HP diesels without any noticeable voltage drop (at the bank).

I have a self-designed, self-built EMS (Energy Management System), which monitors and controls all charge sources and all loads, in addition to the batteries. The charge control is not smart, just simple on-off, based on voltages, not Ah counting, which is all that’s needed.

My BMS is also self-designed and built. Its only job is to monitor the batteries, and report into the rest of the system. However, when it detects that the EMS has gone offline, or the battery bank triggers any of the severe fault conditions, it has the ability to trigger a battery disconnect.

My system will possibly not handle all unlikely conditions, but neither will I, my wife, nor our vessel. It will, however, handle all normal and edge conditions that could occur while the vessel is operated by a reasonably prudent operator.

This is offered not as an advice for others, to copy my setup, nor as negative criticism of other suggested approaches, merely to serve as one possible approach.

Rob Gill

Hi Frode,

Having no electrical training or specific expertise to offer, I rely on advice from those I trust. And our marine electrician Enertec tell me each year they receive calls for help from cruisers arriving in NZ, or stuck in the Pacific Islands with lithium battery banks where a cell has failed, the whole system has dropped out and can’t be brought back on line. I think that qualifies as “not uncommon”?

And being recent technology, many/most lithium battery banks are still early in their life cycle. Disclosure; this is speculation on my part of course, but for instance according to our BMS, we have only used 315 cycles (based on 80% discharge) from the MasterVolt specification of 3500 cycles for the bank, in six years. We have averaged about 110 days aboard each year since the batteries were new.

So I wonder what might happen to LiFePO4 cell / battery reliability as they get closer to the end of their life cycles, especially after long term use in a salt-air boating environment?

I’m not sure anyone knows yet, but I do feel happier having comprehensive management at cell, battery and system level from a specialist and reputable marine electronics supplier.

Rob Gill

Hi John, now that is encouraging. When we looked six years ago the choice was pretty much single vendor system, or home spun. Not being electrically trained it was an easy call for me.

Lindsey Gill

Hi John,

Is it the case that in the situation where you have a bad cell in one battery the external BMS will shut the whole battery bank down even if there are other batteries in the bank that are operating normally. In this situation does not a system with multiple batteries each with its own internal BMS come out on top; as the battery with the bad cell will shut down but all the other batteries will remain active. If as is the proposition here that this is a common occurrence then is that not a reason to take a less negative view of batteries with an internal BMS, providing off course the rest of the system is set up reduce the risk of a load dump from the whole system?

As with all things there are positives and negatives (excuse the pun) with differing approaches and what may be a problem in one circumstance may be a blessing in another.

Al Thomason

DISCLAIMER: Al here from Wakespeed.

Not that I want to get into the habit of posting here, but this is such a keen topic I must share:

We have two Li battery manufactures which at present support ‘BMS Aggregation’ via CAN. What this means is the WS500 is able listen to two (or more, max 10x) BMS’s which are attached to the same DC bus and have a number of cells under each of them.

Lithionics and MG Energy System

What happens with aggregation is the WS500 will dynamically adjust the charging goals based on not only what each BMS is asking for, but also how many there are.

Example: in the case of the passage mentioned above, this vessel had two battery banks installed; One in the bow, one in the lazarette. About 2/3rds of the way into a passage to Hawaii (Fully under alternator power BTW –  never used the gen!) the bank in the bow had a cell go south. That BMS reacted and took the bow battery bank off line. At this point the WS500 adjusted the charge goals to that of the remaining lazarette BMS/battery. 

All automatically and transparent to the operator and crew. In fact it was a couple of days later when the operator noted the issue while using the battery app to check their status – otherwise the system carried on uninterrupted. (Ya, monitoring issue here. Carbon Based Units <smile>)

I think this is an architecture to consider for true High Reliability installs: Gives redundancy, takes that decision point away from the operator who might be able to make a change under relaxed conditions, but would be SOL in the case of making a bar crossing when things went south…

And to be clear: Internal or External BMS, it is all the same. Does not matter.

This capability also makes it KISS when installing one of our supported battery partners: Simple connect up the battery(s), install the config, and go. And later if you add capacity, largely just add them in parallel, extend the CAN and all is adjusted automatically. (Leaving out that all important detail around new vs. old and balancing of course)

(BTW, the most common reason I see for installing multi BMS’s is current limits per BMS. We have done some OPE installs where the peak load was over 1,200A – and that required 4x high capability external BMS’s IIRC)

Al Thomason

Good point. So, there are CAN cable BMS’s that are in an external box, as well as some batteries with an internal BMS that is also CAN cable (See Lithionics GTX12V315A for an example).

The key here is a CAN interface, and one that the engineering work has been done to assure we good level of communications between the BMS and the WS500.

Not to be confused with internal BMS’s which do not have any external communication capability, neither CAN nor charge enable wire. Aka, most ‘drop ins’

But rest assured, ‘internal’ BMS’s like the mention Lithionics is fully capable of delivering the aggregation value I noted above. And when deployed in a Belt-n-suspenders way (See Bruce at OPE, they are experts at this), gives a lot of capability and reliability.

Jim Picerno

Interesting discussion. I upgraded my house bank to Lithium in early 2020 working with Ocean Planet. I think most if not all of your tips wound up on my design checklist. My system has an external BMS which supports a separate load and charge bus. This was considered best practice for LFP battery banks at the time particularly with a boat intended for off shore use. And obviously there are folks who still consider this a better architecture. I assume a battery with an internal BMS has only a single bus supporting both load and charge current. It would be interesting to hear the experts pontificate on the single bus design vs. a separate bus for load and charge.

Dan Perrott

I have just made the decision to skip lithium this time round and drop in a fresh set of lead acids, but read this with interest as I did consider a move to lithium, and maybe next time.
I have read of and know people who have left a lead acid battery in the system permanently in parallel with the Lithium bank. On the basis that whatever the bms does there is always somewhere for the power to go or be drawn from. Never a danger of a load dump. And very little system changes required.
I am struggling to see a large down side with this system but would like to know what others with more knowledge think of this solution?

Dan Perrott

Thanks John. As I suspected there is no good easy shortcut to doing this right.

Rod Collins


BMSLoad dumps can be caused by numerous reasons not just high cell or pack voltage. I hear it repeated over and over “I can just program my charger for a lower voltage to be safe” while this can lead totlonger cell life o one needs to be cognizant that each time you push the voltage to an absorption level that level is high enough to engage cell balancing. For example Kilovault batteries initiate cell balancing at 3.5 V per cell or 14.0 V pack. Battle born initiate cell balancing at 14.2 V. If you charge to a voltage that is lower than these will not balance the cells and will eventually wind up with an unbalanced pack.

Mitigating the damaging effects of the BMS load dump can be done in a number of ways. I recently published a new article on Marinehowto.com dealing with drop-in batteries.

All of this and more are covered.


Philip Wilkie

About two years back I made a comment about the Lithium Titanate Oxide 24v 5.5kWhr (LTO) bank I built. Short answer – zero problems. Most of the issues being described here arise because LiFePO cells are relatively less robust than we would like them to be.

My LTO cells have proven to not care about over or undercharging. They don’t care about temperature and can sustain ridiculously high charge or discharge rates without blinking. As a result you really do not need much in the way of a BMS – just a simple monitor of total bank voltage will suffice.

The have a very predictable SoC to voltage relationship so it is perfectly OK to just set a high and low threshold bank voltage – 2.8v high and 1.8v low per cell is what I have chosen. Balancing is handled in a separate dedicated unit and has proven completely reliable – all the cells have always been within one or two mV of each other. If anything goes wrong with the balancing the cells will likely recovery just fine. In that event I might add a more sophisticated cell-level BMS but for the moment it would just add complexity I don’t think I need.

As described in John’s article I used a Charge and Discharge Bus each with a separate latching vacuum contactor that can handle switching 400A easily. The total bank has a 400A Class-T fuse and each bus a magnetic thermal breaker to protect the cables. All the rest of the electrical install is Victron.

The only real problem that prevents LTO’s from being a good marine solution is that you have to build all of this yourself – which unless you have a solid electrical background is probably going to be a show-stopper. No warranty for a start.

The alternator load dumping problem is something I have yet to properly address – for the moment I still have a very crappy alternator that awaits a big upgrade. A WakeSpeed unit will be very nice – but totally wasted for the moment. My engineering rationale at this stage – and I accept it is suboptimal – is just to avoid Charge Bus dumps in the first place. And the LTO system I have is so simple and robust this is easy to achieve.

Philip Wilkie

The main performance constraint for LTO is a nominal 2.3v per cell compared to 3.2v for LiFePO – for the same weight. This translates into a specific energy density that is 33% worse for LTO. But for a yacht I do not think weight as a critical parameter. For instance my 5.5kWhr (11S6P) bank weighs 90kg total – compared to around 450kg for lead bank with similar overall performance. A LiFePO bank on might be closer to 60kg – but honestly that 30kg difference does not matter on a monohull that weighs13 tonne.

I am not in a good position to know why the marine battery industry seems to have universally headed off down the LiFePO path. The most accessible LTO cell manufacturer I know of is YinLong. but their price and availability varies a lot and this may well be the biggest barrier for people putting together commercial battery banks. And probably the tradeoff I have made around not using a full cell-level BMS is not attractive for them either.

Otherwise LTO chemistry seems to combine most of the performance of lithium with a robustness similar to lead. I am not claiming what I have built is the ideal way forward for everyone – but I am very happy I made the effort to do it and learned a lot along the way.

(Incidentally the Gigavac contactors I used are brilliant. Totally reliable and ideal for this duty. I’m about to shell out for a couple more to manage the anchor winch reversing as well. The contacts are sealed inside a small vacuum chamber and this means apart from zero marine corrosion, there cannot be a switching arc and no possibility of welding. Capable of reliably switching very high currents.)

Fred Calnan

This is a very interesting thread and one person above mentioned what I believe is a simple elegant solution to deal with the scenarios John has outlined in the Halifax example. It also deals with the alternator disconnect.
The solution is to keep your Pb in parallel with the Li. As the writer said this provides somewhere for the alternator to put power and maintains power to critical devices in the event of a Li shut down.
The Li will cycle up and down every night and the Pb will mostly remain in float. Clearly the charge profile will not be perfect for the Pb bank but these batteries are very resilient and in the above situation being used purely for back up, which is mostly what lead was supposed to do.
The video here explains the concept and the author has further developed the idea by creating a product that connects and disconnects the two systems (Pb-Li) based on multiple criteria. Simplicity often makes for better reliability than complicated designs.
I am interested in peoples thoughts. I hope this adds to the discussion. https://www.youtube.com/watch?v=tAuPfgZgXec

Dave Warnock

Regarding the two busses (my summary to check my understanding: separate charging from load so that they can be controlled separately by an external bms. Then only shutdown charging bus for an overcharge problem, only shutdown load for a low charge problem).

We are using a Victron MultiPlus II. They are clear that dc solar should feed between the multiplus II and the batteries. This way if the battery is full the Victron can use the solar towards the Inverter loads to reduce shore power use.

It seems to me that with our multiplus II (protected by a victron isolator) and victron mppt controlller as the only battery charging routes then these do a lot of the protection needed (certainly from overcharging and from our Inverter loads emptying the battery bank).

We are now going to add an agm “buffer” battery between the 48v lithium and the all 12volt systems. It will be charged by a dc to dc converter. If the main bank load dumps our 12 volt should all continue. That means all navigation etc will continue to function.

Our electric motor could still cause a load dump. Obviously we can easily add alarms to battery state. To be honest everything I see of electric motor users is that they are acutely aware of the battery state. As we are not going to be motoring anything like as long as a diesel allows it is not the same risk as if motoring for 12 hours.

We are going to have separate switches for the motor, the victron (and within that for the solar) and the 12 volt so can isolate any independently. Also for the two sets of batteries that get paralleled to make the full bank. We are using victron battery balancers within the bank.

So the next key issue to address is the battery temperatures. At the moment I think I will use our Raspberry Pi to collect temperatures from the outside of every battery. We can start with basic alarms and full logging. Later more intelligence can be added.

I think we have at least mitigated most of the risks.

Dave Warnock

Yup, we are happy with that.

Robert Cornah

Thank you for this very interesting article. Load dumps are a significant issue as I recently experienced: Standing watch on autopilot, squally southerly about 25=30 knots downwind, poled out genoa, autopilot coping (just) Suddenly everything goes black, I jump up and grab the wheel, look at the compass – no light, fumble around trying to get my bearings, and luckily manage to catch the all standing gybe as the main starts thinking about it.

Someone had knocked the battery switch off!!

Interesting experience which left me with the following thoughts – Its hard to get your bearings quickly when you have been sitting quietly in a deck saloon and have to dash outside and grab the helm
-On this boat the wind indicator is not visible from the helm and there is a lot of stuff (bimini, panels, davits that make wind awareness a bit tricky at the best of times. Not ideal.
-Had the motor been running the alternator would have been destroyed and a voltage spike may have done further damage.
-check your battery switches and consider how you wire your alternators and starters.

Robert Cornah

I have built my own LiFePo system using 8 x 180A calb cells with and orion junior 2 BMS. I have no background training nd am an absolute amateur. It has been a great learning experience and I m happy with the result. However – it was more complex took longer and cost more than I expected, and I have had a couple of interesting issues along the way. I think the concept is quite simple, but good robust implementation turned out to be a lot more challenging.

I used Eric Bretscher’s excellent articles as a base. ( https://nordkyndesign.com/category/marine-engineering/electrical/lithium-battery-systems/ )

It is a dual bus system using the start battery as a buffer via a splitting diode to cope with alternator spikes. The BMS has high and low voltage cutoffs which completely isolate the battery, s well as high and low voltage warnings that I have configured to sound a buzzer and light, and to cut off either charging devices or high loads (inverters, refrigeration, heating, stereo, windlass) The voltages at which all these relays trigger at can be individually set.

The start battery has its own alternator as well, via internal alternator regulator (OEM) and no other duties.

High voltage cut off of the alternator is managed by
1) setting the Balmar regulator to max voltage below the warning threshold
2) by a relay cutting the ignition feed to the regulator if the warning voltage in ny individual cell is exceeded, and
3) the buffer battery if the aforementioned fail and the BMS cuts out unexpectedly,

Low voltage is similar, a relay cuts sounds the buzzer and cuts off the non critical loads. The voltage that this occurs at cn be adjusted, and thus I can trigger this at 15% of capacity which should leave plenty of time for me to react before the battery disconnects. I though about putting in backup battery, but decided that was overkill.

The biggest issue I have experienced is thermal cutouts due to faulty thermisters. This was due to poor installation. There is no way of predicting this and although it shuts down in a safe way from a voltage spike point of view, it still shuts down!

The 2 solutions – either 2 parallel banks each with their own BMS and relays connected to the common charge and load bus bars or a buffer battery.

The BMS itself can output to a display – via canbus, serial port or analog output. I have an independent external battery monitor but I dont think this is a great solution on its own. It is difficult to accurately calibrate and shows battery not cell voltage. If you only used this it could easily drift and an unexpected shut down could still occur.

Most of the problems of cell drift and balancing go away if you program conservative values for charge/ discharge and operate in the flat part of the battery voltage curve. I think pushing the batteries to their limits makes reliable management much harder, shortens battery life and is a poor trade off that a lot of the ‘drop ins” make.

Thoughts and suggestions much appreciated.

Robert Cornah

Hi John.
No – there’s no useful balancing at my conservative values. I have 2 profiles – a ‘operational’ profile and a ‘balancing” profile. When I have time and am plugged in, I just upload the balancing profile and balance if necessary. Generally, if the cells are not being pushed hard, they don’t go out of balance much. It is at the ends of the profile that balance becomes an issue. The balancing profile also ensures I don’t get a memory effect.

I don’t feel the slight loss of capacity due to my conservative profile is an issue. One of the great advantages of Li over Lead is how easy it is to add ridiculous amounts of capacity. Space is no longer a limiting factor as it is with lead on smaller boats. Putting in the smallest capacity and then pushing them hard is where a lot of problems come from.

Tom and Deb Jarecki

I don’t think anyone else has mentioned this BMS that is designed, manufactured and sold by an offshore cruising sailor: TAO Performance https://www.taoperf.com/. It provides all the functionality that is needed to meet and exceed the ABYC TE-13 standard (https://abycinc.org/store/viewproduct.aspx?id=16759395), particularly for configurable advance warnings of low voltage and high voltage cut-offs.

With this BMS we have three levels of low voltage protection (and corresponding ones for high voltage):

  1. CANbus command to limit discharge currents based on an SOC value or voltage, with the voltage set above the low voltage warning level. Separate limits apply to discharging or charging at low and high temperatures and cell differential voltages and/or temperatures.
  2. BMS relay-initiated low voltage warning, which in our case disables the inverter from supplying load current based on voltage.
  3. BMS relay-initiated low voltage cut off, which first sets off a visual and audible alarm and then 5 minutes later signals a contactor to cut the connection between the load bus and the battery. We set the delay to 5 minutes to provide time to manually parallel the start batteries to the load bus.

One quibble I have with your recommendations is the absurdly high value for the low battery alarm at around 30% SOC. That’s well above the low voltage knee for LFP batteries and loses about 20% usable capacity. Why do you recommend such a high value? There’s no issue cycling down to 10% SOC (or 20% if you want to be conservative) and putting the actual cut off at a cell voltage of 2.9V or 2.85V per cell – still above the low voltage knee for quality LFP cells such as Winston of CALB.

Regarding alternators and their regulators, as far as I know only the Wakespeed WS500 communicates via CANbus. For all the others a relay that cuts the ignition wire rather than the field wire is a much safer way to disable charging and is the way recommended by Balmar. Again, having a BMS that sends a high voltage warning signal separately from the high voltage cut off signal is required. Adding a delay between signals provides time for a shutdown even in the case of rapidly increasing voltages.

Tom and Deb Jarecki

I’m not sure what you mean by “given that most lithium batteries will load dump at about 20% state of charge”. Do you mean they (or a (dumb) BMS) cut off the discharge bus at that point? Routinely cutting off discharge at 30% or even 20% SOC is quite high and gives away a fair bit of usable battery capacity. What’s the point of LFP batteries if not the high capacity usage (easily 10-90% SOC and very conservatively 20-80%)?

A minimum 10% SOC safety margin to start charging above load dump level (aka cut off discharge) is OK. You definitely want a good BMS that can turn on charge sources based on SOC or voltage (or rather, turning off charge disablement). And you do want this automated so a warning is nice to have but not necessary.

In our case, we have the BMS cut the inverter off at 2.95V per cell and cut the discharge bus at 2.85V per cell. These voltages are both below 10% SOC for our 700Ahr @12V battery built with 4S Winston Thundersky cells.

In the year since we’ve lived aboard and installed the LFP battery we’ve never reached either of those limits as we use SOC-based charge profiles to start/stop our charge sources. On shore power we set the lower limit at 20%; when off shore power for sunny weather it’s 30% and for cloudy days or on passage it’s 60%. We stop charging usually at 80%, or 90% when on passage.

Regarding syncing a battery monitor with battery SOC, that’s what the BMS is for – it precisely counts the amps in and out (or should), monitors individual cell voltages, and knows very accurately the actual SOC. A battery monitor should use the BMS’s calculation rather than it’s own. We find our BMS and Victron Energy battery monitor are generally within 2% SOC.

David Barker

Hi John and Tom and Deb
This bms looks worth a look.
I found Phillip Tao’s suggestion for handling a load dump in his notes.

“Option to use the lead-acid battery as a backup:

In case the voltage of a lithium cell drops below the minimum allowed, the BMS disconnects the loads and you are in the dark. TAO BMS has a way to avoid that:


use a relay to connect the backup battery to the load bus (red dotted line on the diagram) and command that relay with the BMS output 4.
configure output 4 so that the ‘backup battery connection” relay is opened in normal operation.
configure the “load disconnect” trigger to also activate output 4 (meaning that the “backup battery connection” relay will be closed just before the “load disconnect” relay is opened and the lithium battery is disconnected from the load bus).

if the BMS detects a low voltage fault requiring a load disconnect:

it gives advanced visual and audible warning
if you have not corrected the situation after a time set by you (default is 5 minutes), it will:
connect the starter battery to the load bus (output 4)
disconnect the lithium battery from the load bus (output 6)
And you will not be in the dark!”

dorn hetzel

Some (perhaps many) electric cars have a high voltage DC lithium battery for propulsion, but also a typical 12V lead-acid “car battery” for all the accessory systems. The 12V lead-acid battery is charged by a DC-DC converter from the main lithium pack. Perhaps a similar system with a 48 VDC lithium “main” battery bank for larger loads and for supplying AC via an inverter where required, which also charges a 12V lead-acid battery via a DC-DC converter, and all critical loads are connected to the lead acid battery, where all the usual low-voltage warnings and usual can apply. The lead-acid battery doesn’t need to be terrible large, since under normal circumstances it is kept topped-off by the DC-DC converter, and alternate emergency charging sources for it might also make sense. The only loads on the lead-acid battery would be critical systems. Would this make sense?

Grant Du Plooy

Would you recommend one of these to help prevent battery dumps, or is that different? (sorry for the novice question)

Devon Rutz-Coveney

Thanks for posting this info. Real good info.
Our 1st hand experiences:
I looked at researched the LFP replacement for many years starting about 2011. It was Rod Collins’ site that really laid out the entire process of our upgrade which we have been using since March 2016.
As Rod says, it is ‘a no brainer’ : get rid of the ‘dead lead’…. all the extra weight and capacity one needs to be able to get just that 25% of usable capacity.
The LFP has been a game changer.\
BUT…. it is a total system upgrade.

OK… our’s is a DIY system like Rod’s. Orion Jr External BMS….. yes we had unexpected load dumps…. immediately after commissioning. It took me awhile too figure out why. It turns out these were from the way I wired the system: the skinny, small, unshielded wires the BMS uses on it’s battery harness were prone to ‘inducted’ currents causing the system to momentarily shut itself down. Only momentarily. The fix: do NOT have the harness wires loop back upon themselves.

While I was figuring out what had happened I read a lot and came across a really super informative website for LFP energy on boats: written by Eric Bretsher of ‘Nordkyn Design’ …. The part that really grabbed my attention deals with the load dump issue very well (and charge dumps/disconnects as well). Here is the link:


On this web page, scroll down to: “Alternative 1 – Lead-Lithium Hybrid Bank”
As Eric writes in this section, “The simplest way of resolving all the challenges mentioned at the beginning of this article is running the lithium bank in parallel with some standard lead-acid capacity.”

We have been using this configuration for 6+ years and it works great. The battery type we have installed as the ‘hybrid’ lead acid is a AGM 100ah lead crystal.

Rod Collins recommended on his site that those wanting more technical info about LFP look at Eric’s site. Like RC, Eric depends on donations (PayPal) for all the great work he does to help the boating community.

Devon Rutz-Coveney

PS to the above post:
The Hybrid battery (in parallel) is fused and switched: the switch is the important part: we have placed a Off+1+2+ both switch in front of the load & charge busses: Position 1 goes to load, Position 2 to the charge buss.
Most of the time we have this switch in the both position. If the Orion Jr. BMS kicks off for some reason, their is an audible alarm. Meanwhile the small hybrid battery takes over for loads & charges.
I can supply photos of you want.

Erik Rudels

I fitted a winston lifepo4 system in my Ebbtide 33. Consists of 4×3.3V 400Ah cells and a REC active BMS with Victron interface. This is a fully programmable BMS and connects to a pc. The system has two contactors, load and charge. Fitted with precharge unit for the charge bus – has capacitors etc in the charger inverter.
The system is reliable. Avoiding deep discharges at high loads helps upceeping cellbalance well. With solar the system topbalances nicely and properly. When using the charger it also tapers off the charge current when nearly full and cuts of charging at 100%.
The Victron interfaced BMS controls all victron chargers connected to the Victron cerbo.
Cells are easily monitored, stays balanced. Toppbalancing every now and then.
And most important, dont float the lithiums, let them cycle!
If the boat is moored a long time, I switch over to the lead acid 220Ah backup, they like to float. And the lithium may rest at 40-70%.