Building a Seamanlike Lithium Battery System

So far we have published five articles on lithium batteries (see Further Reading).

Now let's pull all that together into a lithium battery system design that's truly ready to go offshore voyaging.


First off, a quick overview of the required capabilities:

  • Has no potential for a sudden blackout of electrical power (load dump).
  • Requires little human supervision for reliable operation.
  • Places no added stress on the skipper and crew, particularly when the chips are already down.
  • Be fault tolerant: will keep supplying power even after a hardware or software failure.

If those requirements don't make sense to you, please read, or reread this article.

Now that we have the requirements defined, it becomes clear that, due to intrinsic fragility and level of complication, no lithium-based system available today can satisfy all of the above requirements without a backup system.

Gotta Be Lead

Yes, you got it, to make a lithium-based system truly ocean ready, it must have a lead acid backup that:

  1. Is simple and easy to understand.
  2. Will take over critical loads instantly with not even a millisecond blackout. (Read or reread this article if that did not make sense.)
  3. Does not rely on complex software for operation.
  4. Allows the skipper and crew to keep operating the boat safely for several hours after a main system failure until there is a good time without a lot of other stuff going on to investigate the problem.
  5. Will supply critical loads for several hours, while the main system is being fixed.
  6. Will get us home if the main system can't be fixed at sea or in a remote place.

Backup System Options

OK, now we know we need a lead acid backup bank, how do we wire that up?

There are three options:

Relay Switch Cutover

A lead acid bank that has the load and charge banks switched over to it by relays commanded by the the main lithium bank BMS.

This one's easy to drop from our consideration list since it's dependent on a major and highly-complex component of the main system (the BMS) for successful operation. Makes no sense at all.

After all, one of the main goals here is to have backup against a BMS failure and now we are relying on that device to activate the backup? Need I write more? No? Good.

Lead Acid Bank in Parallel

A lead acid bank permanently wired in parallel with the lithium bank with the load and charging buses connected to both, with the latter separated with some kind of splitter—several options, but that's another article.

The big advantage here is that we don't have to worry about alternator voltage spikes in the event of a load dump since the lead acid battery will absorb them. That said, there are other ways to ameliorate the voltage spike problem.

Anyway, paralleling batteries with different charge profiles and chemistries is a fundamentally bad practice:

Yes, people still recommend it, but given that paralleling lead acid batteries of different constructions and charge profiles, say liquid filled and gel, has long been frowned on it makes no sense to parallel lead acid and lithium. Rejected.

Lead Acid Bank in Series

Once we drop the two above from consideration, the correct solution becomes obvious:

A lead acid battery supplying the critical loads at all times, and charged from the lithium bank, satisfies all of our criteria above and in a simple and elegant way.

Here's a system block diagram of the positive side only and omitting fusing, service disconnects, and BMS, etc. Click and save to download a PDF copy.

Of course there are details to be worked out:

  • What loads should be supplied by the critical bank?
  • How should the lithium bank charge the lead acid bank?
  • Can the backup bank double as the engine start bank?
  • How do we charge the lead acid bank if the lithium bank has failed and we can't fix it at sea?
  • What lead acid battery type should we use and what brand?
  • What capacity should the backup bank have?

Let's answer those questions:

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