Cross-Bank Battery Charging—DC/DC Chargers

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The DC/DC charger I installed on our J/109. Note that there is no point in getting the isolated one for most applications. We only ended up with it because that's what Ocean Planet Energy had in stock.

In the last chapter in this Online Book we looked at splitters and relays to combine battery banks for charging and concluded that while usable, both had a fundamental problem: engine battery overcharge.

Now let's move on to a solution that fixes that, albeit at a price in both money and complexity.

There are two variants to think about:

  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—Loads and Options
  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. Eight Steps to Get Ready For Lithium Batteries
  28. Why Lithium Battery Load Dumps Matter
  29. 8 Tips To Prevent Lithium Battery Load Dumps
  30. Building a Seamanlike Lithium Battery System
  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. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  39. Battery Monitors, Part 1—Which Type Is Right For You?
  40. Battery Monitors, Part 2—Recommended Unit
  41. Battery Monitors, Part 3—Calibration and Use
  42. Battery Containment—Part 1
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Foster Lee

Have you looked the Battery Bank Manager Plus for Emily and Clarks Adventure? This seems like an interesting device for mixing lead and lithium batteries. Would be interested to here thoughts on this device.

Foster Lee

Not harsh at all to me (I have dog in the fight either way) Thanks for taking the time to answer and generally I agree with what you are saying. Say their device a while back and never heard it mentioned anywhere. Wanted to hear some experts advice/opinion. thx

Alex Borodin

Hi John,

Victron’s solar charge controllers can and do use current data from BMV monitors when networked over bluetooth (no monitoring device necessary). The tail current needs to be adjusted in battery settings in expert mode. Otherwise they default to 1A.

I wish Victron made an alternator regulator. They have all the pieces to make a competitor product for Wakespeed WS500 and probably with even better user experience.

Bruce Kavanagh

For an all lead acid system, with a modern sealed starter battery how big an issue is the overcharging of the start battery that you’ll get with a charging relay system? Is this actually likely to shorten the start battery life enough to justify the additional cost and complexity of a battery to battery charger? I guess that’s probably one of the many unanswerable “it depends” type questions.

Arne Mogstad

Hi. John answered this pretty good in the relevant chapter, but as you say, it depends. On my boat, I set everything for the house bank, and just let the starter battery suffer. The voltages are not very different for the two batteries, and I don’t motor much, so I can’t imagine that it will have a huge impact, at least not until I am upgrading the whole system and implement a DC-DC charger.

Ben Garvey

As has been said – probably not a problem. I have run our boat this way (with a simple Cytrix charge relay to tie in the engine start batt when voltage is over 13V) for 10 years with zero issues, on the same batteries (all FLA), same alternator and no changes to the system over that period. Is there some reduction in reserve capacity of the starter battery? probably – it’s 10 years old! but it has not seemed to matter to me – and I (regrettably) have done a fair amount of motoring on our last 3 summers excursions to Newfoundland, Bermuda and Sable Island – probably in the region of 550-600 hours (I could find out exactly – but i’m not on the boat right now…).

James Greenwald

Hi John,

I have a separate alternators for house bank and engine start batteries 24V. And for that matter generator alternator. For gen start battery 12v. The big main alternator with WS500 and all the bells and whistles. Do to recommend regulators for the start batteries and gen. I thought the internal regulator would take care of overcharge but not sure. Start batteries 2 Northstar AGM31’s and 1 12V for gen. By the By; 10 FireFly carbon Foam and so far no quality issues as of yet.

James Greenwald

They dedicated to the start batteries only.

James Greenwald

When I purchased the boat it appeared to be a factory setup (Swan) which at the time I thought was brilliant. Also I used the existing Victron Skylla 24V 80amp charger for the engine start batteries only; Big Quattro inverter/charger for mains, probably complicated overkill. I would love to have you out for an audit on all the refit work done, mostly inspired by what you have written about.

Mal L

John have you looked into Planar Transformers? they get over the limited DC/DC Convertor output limitations as you say Victron currently max out at 360W but these Planar Transformers are 3000W! I know of only two companies selling them ‘American Power Systems’ & ‘Safiery’. They are not Chargers (as far as i’m aware) only Converters but thought i’d mention them. I wouldn’t be surprised to see one with a ‘Victron Blue’ Cover on it one day;)

Matt Marsh

A planar transformer is just a transformer whose windings are made of thin copper sheets, or etched on a PCB, instead of being wire-wound. They’re a common component of DC-DC converters. (All DC-DC converters work by inverting DC to AC at high frequency, usually tens of kilohertz, changing its voltage with a transformer, and then rectifying that output back to DC.)

There’s no fundamental limit to how big you can make a DC-DC converter. The ones sold for boats are only a couple hundred watts because that’s all we ever need in our applications. But I can go buy a 4 kW one off the shelf today from Bel Power or Vicor, and semi-custom DC-DC converters rated at over 1 megawatt have existed since the early 1990s.

I don’t know which specific products you are referring to, but APS & Safiery both sell a variety of power electronics parts that are all based on standard switching-mode and DC-DC conversion topologies.


Hi John
I’m using the Victron DC/DC charger. Stock Yanmar alternator (80amp) Since the Victron charger max’s out at 30amp I should be well within your recommended 75% max of alternator’s output. I’ve been keeping the maximum charge time at 3 hours. Not sure if this is correct. I would like to be able to charge longer but I don’t want to toast my stock alternator. Overall, love the system and the adjustment available. Just need to do some finally tweaking.

Raymond Webb

Hi John
Thanks for the great articles on battery systems.
I suggest to finish it off it requires one or two proper electrical drawings detailing all you have recommended? Cheers Ray

Brian Stannard

Hi John

The Victron Orion can have higher output than 30 amps. It is rated at 360 watts (30 amps) at 40 C but 430 watts at 25 C. I know of at least one person who has installed a fan below it for cooling and achieves 35 amps continuous.

Arne Mogstad

I have tried looking at the Victron demo app and the webpage, but these DC-DC chargers don’t seem to have a “storage” setting, only bulk, absorption and float. Is it correct that they will leave the starter battery on float voltage “permanently” then until you pull the fuse to give it a rest? That seems to me will fry the AGM starter battery eventually, unless you put the float voltage a fair bit lower?


Paul Borondy

I have been using an Enerdrive DC2DC+ for a few years now, Input is alternator and separate mppt solar, automatically switches between the 2. Output is up to 50 Amps. Charge parameters are adjustable.

Elliott Bennett

I am looking into using a 24v alternator to charge a large 24v house bank from the engine. I had looked into using a B2B 12 to 24v charger but the currents are quite low for a large bank. I see the new (available 1Q 2023) Sterling B2B chargers have been completely redesigned and one model can support 12 to 24v at 120 amp input amps:
This is much higher than a Victron 30 amp Orion (Orion more of a choke point unless you parallel a few of them). Do you see any downside to using such a higher capacity B2B device instead of my replacing the 12v stock alternator with a 24v one, or adding a second alternator that is 24v?

Richard Phillips

I don’t think your criticism of the Sterling unit is fair – because you are applying different standards when commenting on this compared to other bits of kit. And I think one your main criticisms is wrong as a point of fact.

The criticisms about the alternator requirements make no sense to me because *whatever* approach you take to achieve high charge rates will involve replacing the alternator. Anything over maybe 120ah and you will need to replace the pulley as well.

So this is not a disadvantage of the sterling kit – it is just an underlying fact that achieving a high charge rate will need a high output alternator. It is a ‘whole category’ issue that applies to every alternative approach.

Indeed your own recomended approach is “good big continuous-duty alternator, external regulator, and DC/DC charger.”

And I think using terms like ‘magic’ to refer to one bit of kit – but not your favoured approach – runs counter to your generally high standards of objectivity!

The sterling unit is no more ‘magic’ than all those blue boxes you recomend and *certainly* no more so than the highly sophisticated Wakespeed.

Finally, you say “ If the thing dies there is no way to use the engine to charge any of the batteries on the boat without replacing it.”

This is simply NOT TRUE – because it requires no modification of the alternator, if the sterling unit fails you can simply failsafe back to the (be it inneficient) factory default.

I am not arguing that your own recomendation is wrong, but that the dismissal of an alternative is using arguments that do not really stand up.

Sorry that seems harsh – maybe you have answers on these points!

Richard Phillips

I guess the obvious question is what happens if your wakespeed fails?

My approach is two Sterling B2B chargers. If one fails I can switch to the other. If both fail at least the alternator is providing charge and I am not entirely stuck.

My most likely point of failure is if the alternator itself crashes out – and I very much have in mind a wise man who recomended carrying a spare alternator!

Richard Phillips

In addition to my comment about the Sterling Alternator to Battery Charger – I think there is another solution to the problems you describe that is – for *some* people at least – that better than either of the options you describe here!

That is, the Sterling Battery to Battery Charger. This offers a simpler than your recomended approach – and I think is more fail safe. It is not the same as their alternator to battery charger but has some similarities.

You have a normal configuration of vanilla high output alternator with factory controller – wired to charge the engine starter battery. So *no* changes to alternator wiring at all. If anything later in this system fails, engine starter charges as normal and the alternator is not left in suicidal open circuit.

Then, you simply wire a B2B between the starter battery and LifePo4 or LA cabin battery and select the chemistry on the B2B settings.

Thats it. You are done. You now have an optimised multi stage charge of your cabin – with your starter batter also getting charged.

So you have gone from two bits of magic (expensive alternator controler and DC – DC charger) to one, that does both of these tasks.

As I say, if the B2B fails, the normal alternator controller takes over so you have starter charge and thus options.

The only problem I have identified is that it is possible for the B2B to drive the alternator too hard and overheat it, but that is easy to configure out of the system.

I have a 120ah alternator and have a 60ah B2B so it drives the alternator at 50% reliably and without overheating.

And since I got this so cheap (far less than half of the cost of a wakespeed plus victron dc->dc) – I got two of them. Rather than having one in the locker as fallback, I have wired in both of them but one has an isolation swich which takes it out of service.

If my B2B fails, I can flip a switch and the fallback comes into play. I also have the option (tested, but not used in anger) to turn on both B2B at the same time, pushing the alternator to work at full capacity – though shortening its life.

I like this system on KISS principles – and I like it that it provides a robust seperation between the alternator and the LifePo4 bank, reducing chances of voltage spikes wrecking the BMS or battery.

Oh and in the event the BMS shuts down, the B2B shuts down as well – but (presumably by magic!) manages to settle the alternator voltage without frying anything. The alternator settles at the normal float voltage for the starter battery.

But there might be something I have missed and really am open to feedback!

>> it’s the only option for systems with, for example, 24-volt house and 12-volt start, as is becoming more common.<<

There is a sterling B2B that has that one covered as well

Richard Phillips

>>if the alternator charging solution can supply at least 150 amps at ~14 volts to the house battery and for many situations 200 amps at 14 volts<<

The approach I use would scale to this, but obviously I can’t get 150 amps out of my 120 amp alternator!

My approach can get 120amps out of a 120 amp alternator at the flick of a switch – but I choose to run it cool most of the time.

But the same approach could get ~ 200amps out of a 200 amp alternator.

That is just a question of scale – not the underlying technology choice.

Dan Tisoskey

I am using lithium house and flooded cell for start. Charging: 120 amp alternator with brush box and Balmar regulator charging the start battery and Victron Dc to DC 30 amp charging the house bank. Works great. I know I am not taking advantage of the alternator but my intentions are to add more DC to DC charging to make 90 amps output. Your thoughts?

JP Procyon

I have a 200Ah lithium house battery and 80Ah lead acid starter battery. The starter battery hangs directly on my stock Yanmar alternator while the house battery gets charged via a Redarc BCDC1225D charger from the alternator or solar. House and starter circuits are isolated by the Redarc. Since my boat lives on a mooring and solar / alternator are my only charging sources, I needed to figure out a way to maintain my starter battery. I installed a Victron DC/DC charger that uses the house bank to charge the starter battery. I use a relay that switches off the charger whenever the starter battery isolator is closed, i.e. when the engine may be running and Redarc would be charging the lithium battery as this would “short-circuit” the Redarc and the Victron and drain the house bank rather than charge it. On the Victron I have set a minimum input voltage level to prevent excessive discharging of the lithium battery (its internal BMS serves as ultimate protection). I also have a Victron battery monitor which could control the DC/DC charger based on state of charger rather than voltage of the house battery but feel like this is not adding much value. This is system is simple, quite cheap and once installed pretty much fool proof.

JP Procyon

Hi John, that’s correct. I use my boat for day sails and the odd holiday cruises. The reason why I decided to leave my starter battery connected to the alternator is to protect the alternator where the lithium battery charger switches off because the lithium house bank is fully charged. The dc/dc charger from the house bank to the starter battery is merely used to maintain the starter battery when the boat is not used for a few weeks.

Stephen Brady

How about an Echo Charger, which is connected to the House Bank along with all charging sources, and takes from the house bank to charge the starting battery?

Gregg Rivara

Any opinions on the Renology DC-DC 50A charger? Dual inputs for alt and solar. Flooded start to two 200A Li house batteries.

Gregg Rivara

Thanks Cap!

Mal L

John you touched on the fact you installed an ‘Isolated’ DC/DC as thats what Ocean Planet Energy had in stock but a ‘Non Isolated’ is all we need, i assume both your Negs tie in together anyway? but is there a place for the ‘Isolated’ DC/DC type aboard?
I was thinking of Nav Gear like VHF & HF to give cleaner ripple free power supply (as we all have Inverters these days), or will fitting any DC/DC Isolated Ground or Not give cleaner power regardless as its interupting the House Supply?
Or my new boat has a separate Bow Thruster Battery Bank not in the original wiring drawings presently charged via a Battery Isolator/Splitter, i plan to change that and charge via a DC/DC but could i/ should i charge that as if it was a separate Power Source via an ‘Isolated’ DC/DC and keep it away from the House Negative? Or should we be tying everything onboard to a common Negative! which then begs the original Question, where to use an ‘Isolated’ DC/DC? Something iv’e always wondered about;0

Justin Francis

The article states a DC-DC charger must be directly wired to batteries:

Must be wired directly to the batteries, so is always energized unless the fuse is pulled.

Is that a vender recommendation? Is there a reason a regular battery switch could not be used to interrupt the input or output wire instead? That would be way easier than pulling a fuse, if a little overkill for a low-amp charger

Justin Francis

Or wiring it to a breaker on the DC panel which avoids the use of a separate fuse (on one end at least)? I’m planning an install between two battery banks and I want to have a way to turn off the charger, say, for storage to avoid one bank draining the other while self-discharging

Justin Francis

Thanks, John. Yeah, I’ve poured over the Victron docs, and they definitely seem to want you to use the little remote connections for a switch and wire them direct to each battery. I assume they have their reasons.

Setting a lockout voltage could also make me feel less anxious.