Battery Bank Separation and Cross-Charging Best Practices

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Best Practices

In the last chapter in this Online Book, we looked at battery switches and how to use them to build a cruising boat DC electrical system that properly separates the engine start and house banks.

And earlier we specified a third critical-load backup lead-acid bank to make lithium-based systems offshore capable and seamanlike.

So now we have all these batteries separated, how do we make sure they all get charged, particularly when the engine is running?

I have an article all written, that we will publish in a few days, analyzing six options for charging more than one bank of batteries at once.

But first let's cover good practice for battery bank separation and multi-bank charging:

  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. 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. Lithium Ion Batteries Explained
  32. 11 Steps To Better Lead Acid Battery Life
  33. How Hard Can We Charge Our Lead-Acid Batteries?
  34. How Lead Acid Batteries Get Wrecked and What To Do About It
  35. Equalizing Batteries, The Reality
  36. Renewable Power
  37. Wind Generators
  38. Solar Power
  39. Hydro Power
  40. Watt & Sea Hydro Generator Review
  41. Battery Monitors, Part 1—Which Type Is Right For You?
  42. Battery Monitors, Part 2—Recommended Unit
  43. Battery Monitors, Part 3—Calibration and Use
  44. Battery Containment—Part 1
  45. Q&A—Are Battery Desulphators a Good Idea?
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Andrew Todd


I guess a lot of people would like to augment their capacity by adding lithium to an existing LA based system, but most advice assumes a complete changeover to all or mainly lithium. Would this work… (simplified re protection, monitoring, alarms etc.). The purpose is to add extra capacity in lithium to an existing LA system so that any risk of change is avoided as much as possible.

PV panels feed a 2 way and off switch. One way goes to MPPT chargers configured for LA and onto the existing common charge/load bus with the LA house bank on it. The other way goes to another MPPT charger configured for lithium going to prismatic cells with an external BMS. The lithium battery go through a DC to DC charger the LA with an isolator switch in that connection.

This assumes you have enough PV to recharge the LA everyday with some time to spare. One could add more ​LA, but that would probably mean more PV. With the spare PV time you can charge the lithium and keep it in reserve for a string of dull days. You could also have a basic inverter on the LI bank to use for non-critical 240VAC appliances when you didn’t need to keep the reserve.

One hole in this configuration might be the DC to DC converter. I’m told they run hot, are not very efficient and aren’t really suitable for ‘always-on’ use. (Hence the isolator mentioned above) I’m sure there are other challenges?​​​

Matt Marsh

I don’t see how one could gain any significant benefits with that setup.
The main reasons for choosing LiFePO4 over lead-acid are:

  • Lighter weight for the same capacity.
  • Larger capacity for the same physical volume.
  • High charge acceptance rate (thus, the engine is well loaded when being run for short periods for charging).
  • Improved safety (it is extremely difficult to create a battery fire, electrolyte spill, etc. with a modern, well-engineered LiFePO4 system protected by a good BMS).

If you have a large lithium house bank AND a large lead-acid house bank, both cycled regularly, then you have the worst of both worlds — all the complexity, cost, installation labour, protection circuitry, etc. of lithium, plus all the weight, monitoring labour, and maintenance of lead-acid.

Generally, in a LiFePO4 system, I like to see lead-acid cells used only for two specialized places:

  • As a dedicated engine starting battery.
  • As an uninterruptible power supply for a small subset of critical circuits that would pose a major safety risk if they were to suddenly lose power while underway. (John wrote about this here). This is charged from the main bank by a DC-DC charger, and is maintained at full charge; it is not cycled in normal use.

DC-DC converters and DC-DC chargers are established, proven technology. The good ones are efficient and highly reliable in 24/7 multi-decade service. When one does fail, it takes only a few minutes to swap for a spare. They are not to be feared.

Andrew Todd

Matt, John, Your answers assume a blank canvas. I, and many others, have a well proven, already paid for and low risk LA based solution. I just want some extra capacity for a string of cloudy days without having to festoon the boat with PV. What’s the best way to do that?

I often here people talking about the risk associated with the technology, but this is nothing compared to the risk of change and the situational risk. ‘Situation’ in my case is …about to make an ocean passage to remote locations. So completely changing from a LA based system to all lithium is a non starter. Also, the failure rate of lithium vs LA is immaterial against the binary nature of failure of lithium and the lack of user intervention. Which one might not think a problem if your coastal cruising.

Matt Marsh

“Extra capacity for a string of cloudy days” in a system that you’re otherwise quite happy with sounds like a generation vs. demand matching issue, not a storage issue. Reducing consumption, followed by a really good mounting & belt scheme for a really big alternator and a really good regulator, would be my go-to recommendation there.

Peter Thornton

Hi John, I agree with you on your comment, do it once, do it right. I think there is a strong argument for building a lithium in 48v. All the heavy lifting, charging, storage, inverting is done at 48v. Step down DC-DC for existing 12/24 loads are easily and cheaply achieved with only a slight loss. I understand the perils of YouTube but here is some kit from Balmar that looks worthy of further investigation.