The Offshore Voyaging Reference Site

Lithium Batteries Buyer’s Guide—Balancing and Monitoring

In the last chapter I explored why we offshore voyaging sailors need a battery management system (BMS) that can intelligently control charging sources rather than panicking and shutting down the whole works—totally unacceptable on an offshore voyaging boat.

If you still don’t agree with that after reading the last chapter and the above linked one, that’s fine with me, but let me be clear that as far as I’m concerned, this is a fundamental requirement for a seamanlike lithium-based electrical system, and everything that follows is based on that.

Given that, I will answer any comments that argue with this premise with “fine, let’s agree to disagree”, rather than get into it yet again—that’s as nice a way as I can come up with of saying “you’re welcome to your opinions but please respect mine”.

“Drop Ins”

That said, if you have batteries with a BMS that does not communicate, or that’s the route you want to go—”drop in” batteries with an internal BMS that does not have process control—I will cover how to do that in a seamanlike way in a later chapter, so let’s not go there now, other than to say that such a system will never be as good as one done right…in my opinion.

More Required Capabilities

Before we dig into the simple wire or CAN Bus decision, which I will now cover in a later chapter, let’s explore two other buying criteria that either type should satisfy:

Cell Balancing

Let’s start with cell balancing, because understanding that informs our decision on monitoring—this is sure to get everyone warmed up.

First off, we need a BMS that balances properly and automatically, and a system set up to let it.

Who says? I do…oh, not good enough? You’re right. How about this? Al Thomason at WakeSpeed says.

I spent over an hour on the phone with Al before writing this article and most of that was on balancing and making that happen.

Al has worked with most of the major lithium-battery and BMS manufacturers over some five years to develop profiles that will charge their batteries as efficiently as possible, while not shortening their lives by over-charging.

No one, but no one, and certainly not that handsome silver-tongued presenter on YouTube who purports to make all this simple, or that guy on the forum who has all the answers with absolute certainty, has even a fraction of Al’s understanding.

Here are the key things Al told me, and other credible sources confirm:

Balancing is Real

All lithium batteries, no matter the quality, tend to drift out of balance over time.

For the purposes of this article I will assume that the battery was fully charged and properly top balanced when new, and focus on ongoing balancing while in service.

If you are interested in the initial process, the Victron Lithium Smart Battery Manual has a good explanation (section 4.3) of what needs to be done before new batteries are put into service.

13.5 Volts is Not Enough

One of the biggest myths around lithium batteries is that if we never charge them above around 13.5 volts1 (or close) we don’t need a BMS that controls charging sources since we will never be close to the BMS cut-off voltage of ~14.4 volts.

Yes, that’s true…for a while, but the problem is that at that voltage the battery won’t balance and so will drift over time.

How long before problems show up? That depends on usage and how well balanced the cells were in the first place, how the BMS works, the owner’s astral sign, and on it goes. Could be years, could be months. More likely the latter than the former.

So, yes, just charging at low voltage will work, but it’s far from best practice, and on an offshore voyaging boat only best practice will do, particularly since it’s not that hard or expensive to do it right.

Keep in mind, it only takes one weaker or stronger cell to start causing blackouts if not properly managed.

No, Al and I didn’t make this up. This is just one example of the instructions from a battery manufacturer:

We recommend a minimum absorption time of 2 hours per month for lightly cycled systems, such as backup or UPS applications and 4 to 8 hours per month for more heavily cycled (off-grid or ESS) systems. This allows the balancer enough time to properly balance the cells.

Victron Smart Battery Manual

Usage Dependant

Wait, it gets worse. Lithium cell balance drift is usage dependant.

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More Articles From Online Book: Electrical Systems For Cruising Boats:

  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. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. Lithium Buyer’s Guide—Budget: Economy Options
  36. 10 Reasons Why Hybrid Lithium Lead-Acid Systems are a Bad Idea
  37. 11 Steps To Better Lead Acid Battery Life
  38. How Hard Can We Charge Our Lead-Acid Batteries?
  39. How Lead Acid Batteries Get Wrecked and What To Do About It
  40. Equalizing Batteries, The Reality
  41. Renewable Power
  42. Wind Generators
  43. Solar Power
  44. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  45. Battery Monitors, Part 1—Which Type Is Right For You?
  46. Battery Monitors, Part 2—Recommended Unit
  47. Battery Monitors, Part 3—Calibration and Use
  48. Battery Containment—Part 1
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Arne Mogstad

Hi. Great articles! I have not gone the lithium route yet, but I will once my lead acid bank dies, so this is a timely article since I am trying to educate myself as much as I can. Personally I will probably go the full DIY bank route, but I don’t think that matters much for the purpose of this article. Anyway, I just wanted to say that more citations and sources within your article would be nice, since I’m sure you manage to find some things in your research that I don’t. Even if not directly in the article like an academic citation, just a “further reading” list at the end with various sources you’ve found would be appreciated. I realize a lot of this comes from Al and therefore it is hard to add citations to it (which does not mean that it’s any less relevant!!).

Lastly for anyone interested, I would just like to mention that Marine How To have a few good (albeit a bit old and very technical) articles where he also have some tips for further reading (and also HEAVILY advocate for further research).

Kindly, Arne

Arne Mogstad

I totally see your arguments John! And after having done some fairly heavy research myself (on both this and other totally unrelated topics), I absolutely get what you mean! And this is one of the reasons I like your site, when I started out as a TOTAL beginner on a sailboat 2,5 years ago (never even set foot on one before), I still managed to singlehandedly sail my new-to-me yacht from UK to northern Norway with no issues, mostly due to this resource. So I (and I believe many others) trust that you are very able to make sense of what actually matters. Just to let you know that at least some of us appreciate all the links to sources/resources you find worthwhile! 🙂

Kindly, Arne

Stein Varjord

Hi John,
Rod has recently started “recommending” drop-in batteries with no process control, but they’re not real recommendations. He’s just accepting the market power and that this is what part of it is asking for, no matter what he says. So he gives them a decent example of such a product, which also gives him a small revenue, which he really needs due to his health issues.

Still, he makes no secret about his opinions other places on his site, which have been the same for over a decade: Lithium batteries MUST have a properly communicating BMS and an external system that can react to the communication. Anything else he considers swindle. That includes at least one of the batteries “recommended” on his site.

Henrik Lundin

Great post.
I really think lithium is fun right now, but as with many new fads/techniques people sometimes tend to get their temper worked up.

Some reflections in no particular order:

I personally think that for cruising, large lithium banks should have an active balancer. Passive balancers does simply not seem effective enough.
Most “drop-in” batteries have passive balancing.

Individual cell monitoring:
For most DIY systems this is easy to accomplish. It should also be easy to accomplish for “drop-in” batteries, but most does not seem to have this feature right now. I simply think most companies are not adding this “feature” because it hasn’t been requested by the customers, it is mostly a software question.

I have watched/read some info from
He has an offgrid solar system with diy batteries.

He has done some good videos regarding charging at a slightly slower voltage (13.8 volt for a “12v” system). I think when my system is finished I will try out his settings and see how they work for me as a cruiser.

Stein Varjord

Hi John,
Confirming your statement: Even 13,5 Volt “float” charging has resulted in the death of multiple lithium banks. Probably most premature lithium deaths are caused by extended low Amp and often low Voltage charging. Anything above 13,3 Volt is risky if prolonged. Lithium cells should ideally ONLY be bulk charged. They can tolerate some saturation charge as long as the Voltage is a slow as possible. They must NEVER be float/maintenance charged at a Voltage above 13,3 Volt.

Setting a lower limit for the bulk stage is not in any way a sufficient method for cell safety. It’s only a method to reduce cell ageing by staying a bit further away from stressing them. It’s only useful if that strategy fits the rest of the system, which it won’t if there are automatic “balancing” circuits anywhere.

I have to nag about this: What the automatic “balancing” circuits do is NOT real balancing. It’s only equalising the Voltage. Real balancing of lithium cells is a specific process that changes the chemistry of the cells. It takes time and precision at higher Voltages. It leaves the cells in far greater “harmony” than the automatic systems can ever get.

Each cell still absolutely needs constant attention for possible drift, but actual rebalancing is usually not needed often on matched banks in use. I know of very large 48V propulsion banks that have stayed within 0,03 Volts between cells for 3 years now, still counting. No “balancing” circuits. These are run daily 100% to below 50%, often further. Harder work than any cruising boat house bank in history. It works, but it has to be good components in a good system.

The vast majority of electric tourist boats in Amsterdam are still better off with, and indeed have, flooded lead acid 2V traction cells. Lithium is only chosen for boats struggling with space, weight or insufficient charging time between running times.

Robert Cart

Well done, John. You covered the balancing issue well and made it accessible for all. It’s but one aspect to consider of many and often misunderstood or ignored.

I do have a comment to help others not stray outside the channel. At this stage, only the battery cells themselves can provide the information about their particular behavior and status and thus only a device that can read and understand cell data can provide good advice for cell management. Devices that attempt to infer the information from the pack rather than the individual cells simply don’t have adequate intel. For example, I have two Victron BMVs, one for the lead start battery and one for the house lithium. The BMV does great on the AGM battery, but I have no other reference to compare. It just seems right. However, with my LFP bank, the BMV is useless for state of charge. It isn’t even close. My Orion BMS provides excellent monitoring and it is easy to see that it is correct with data logging and charts. I’m sure Victron is doing the best possible but right now it says my LFP bank state of charge is 92% when it is actually 44%. That’s really awful. I only use it for quick voltage or current status checks.

With lead acid, you only need peukert and coulomb counting to get close. Not so easy with LFP with only those and bank capacity added on. This also means a smart regulator cannot know exactly what the battery needs. It’s actually much more of a problem than you may think. Beware.

There are two schools of thought here. One has the BMS as the authority and it must instruct all charging sources including alternator, solar, shore power, wind, generator, etc. Those devices need to be told what to do by the BMS and if they don’t behave, they get cut. Same for loads with the final straw being contactors that should never have to open but will do so to protect the batteries. The final straw for bad behavior of all the charge and load centers.

The other approach is that the BMS can only be trusted for that last protection and maybe some opaque balancing behind the curtain. In that approach, each big charge and load source – and of course a very nervous operator – must attempt to understand what the battery needs sans advice from the BMS. That’s the drop in world where you need smarter charge regulators and battery monitors than we have today. I would love it if we could buy some Battleborns, a Wakespeed, and a Victron BMV with other blue boxes for solar and be off to the races. It might even work fine for light duty local and coastwise cruising. That tech is just nowhere near ready for heavy duty offshore cruising. I hope it gets there because the alternatives of high cost integrated systems or high geek DIY ones are not attractive to most.

Henrik Johnsen

Hi. Great article. As usual, very informative.
I have one comment regarding the drawing of the connections between WS500 and the Victron smart Lithium Battery Bank.
The Victron Smart Lithium Batteries has got two dedicated BMS signal cables, connecting the batteries to the BMS. If a system has multiple batteries they are to be daisy chained to the BMS. Those cables doesn’t  show on the drawing and as far as I have understood the Victron system, that has to be a foul since the BMS cables are vital to have the BMS controlling the charging of the batteries from the alternator and WS500. 

Eric Klem

Hi John,

Just catching up on your latest lithium articles which I applaud you for trying to enforce good system design with, it isn’t as simple as it may seem. I have largely ignored lithium batteries in boats as I fall relatively cleanly in the group of people who are served perfectly adequately by lead acid and do not see the effort and expense as being worth it. On the other hand, I do follow it for cars as it matters directly to my car. And this is where I see an interesting similarity in that the information around cell balancing for cars is all over the place and kind of poor just like you have found with boats. This seems rather alarming as cars are much larger investments and done by much larger companies with huge engineering, training, etc. teams and not single person stuff like Al. On our car, I could find no real documentation on what to do and only some internet forum posts which are generally much more concerned with only charging to 80% or so than to balancing occasionally. There doesn’t even seem to be a mention of how the cells are balanced and only by using diagnostic software can you find that they are top balanced (pretty predictable). Some of the other cars out there have documentation that mentions balancing and some even suggest doing it as often as monthly.  

It is going to be really interesting to see whether they either automate these functions well enough or do a good enough job of educating consumers. Given how poor people are at maintaining things, I fear that the educating method will fail. It certainly seems possible to convert much of this from battery speak (“if you have x chemistry, charge to 80%”, “top balance”, etc.) to normal speak (“press for maximum range/capacity when needed”, “do an extended charging session soon”). Cars provide a lot of data points and while what they are doing is not perfect, the more modern packs do seem to hold up pretty well.

In our case, I have a quarterly calendar reminder to top balance as we usually charge to 75%. I certainly hope that the boating world figures this out more quickly than the automotive one is but the odds seem stacked against it.


Eric Klem

Hi John,

Good to hear that Victron are thinking about it as more user centered by asking the user to do something but not alarming them than battery centered. And agreed on how long it has taken people to understand how to charge lead acid, heck most of the stock charge programs are still terrible. I learned how to take care of those from building electric cars that used golf car batteries but still killed plenty of house batteries in boats until solar came along.

To be fair, longevity studies on recent BEV’s have actually shown great battery longevity to the point where I have no worries about needing to replace a battery pack due to mileage and while gains are possible, they may not be deemed to be needed. The news does love to dwell on stories of ride share drivers killing packs in 100k miles but when you dig a bit deeper, the pack was badly abused and an internal combustion engine would not have lasted with comparable abuse either. However, some of the older BEV’s had quite poor longevity. The most well known case I believe was due to using passive thermal management primarily which proved definitively that active thermal management is key to longevity in that use case. For example, our car when unplugged will use the battery to heat the battery at <0F and if you are plugged in, it starts doing it at 30F with a similar routine for hot temperatures, it actively manages while driving, and most of the cars now preheat the battery before fast charging. I don’t know if balancing has had anything to do with the better longevity of recent batteries but it is certainly possible.

Watching the automotive world is going to be really interesting as there are many more data points and they are better at collecting data than the marine or RV world. It is obviously a different use case and sometimes different chemistry but still instructive. The 2 that I am really interested in watching are BEV’s with no home charging as they will tend to do much deeper cycling (which has been shown to hurt longevity although not catastrophically like lead acid) and they won’t keep their batteries in as narrow a temperature range, and plug in hybrids which are designed to really cycle the battery hard unlike full BEV’s that tend to use <20% of capacity on a day to day basis.

I hope this wasn’t too much off the subject of boats. We will likely replace our 9 year old house bank in another year or 2 and that will be lead but I could see the replacement after that going lithium and I will need to educate myself then.


Alan Sexton

Hi John,
Interesting comparing your views on “must haves” for monitoring Li Batteries vs the claims for the “Juice” Li batteries assembled (they buy in the cells, package them and fit their proprietary BMS) and marketed by Enertec Marine in NZ
In particular they say the BMS is monitoring voltage (3), there is active cell balancing (8) and there is data logging (11).
Question for me is how much of this info is directly accessible to the user, ie does not need a technician with a laptop to interrogate the BMS. I will be catching up with these guys in a couple of weeks so will be asking these questions.

Alan Sexton

The Juice BMS is stated to have specific outputs for Charger and Load Control per (6) in
This is a requirement from “AS/NZS 3004.2 (Part 2)2014, Electrical Installation Standard for Marine Installations”, the controlling/obligatory standard in our part of the world.
If I can source it I will post a link to their manual.

Rob Gill

Hi Alan,

Enertec installed our Mastervolt electrical system;
back when they were the MV agents in NZ. MV subsequently bought BEP electronics in NZ so establishing MV NZ direct. This prompted Enertec’s move to create their own Juice range of products.

Anecdotally, a friend was fitting out a new power-cat for fishing and diving – he wanted to fit Lithium batteries and asked me who we had used and I referred him to Enertec. When I saw him a few weeks later he told me Enertec had persuaded him against installing lithium, as he didn’t have a dedicated battery locker that could keep them dry and cool, meaning Enertec didn’t secure any business.

We average over 100 nights aboard our yacht each year and as of last month, since commissioning in 2016 we have used 165 cycles without any system glitches. The MV integrated electronics and BMS handle the issues John describes above. So Enertec will fully appreciate these issues and as I understand it, worked with Auckland University to design and test their own BMS functionality.

If you ask good questions, I am 100% confident you will get straight and helpful answers.

Jon Moss

Hi interesting what you say about SG200 , I have one and it’s caused me all sorts of hassle. In view of the inevitable switch to marine Lithium over next 5-10 y I presume this relatively new monitor is increasing of historical interest and a “do not buy “