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:
- Is simple and easy to understand.
- Will take over critical loads instantly with not even a millisecond blackout. (Read or reread this article if that did not make sense.)
- Does not rely on complex software for operation.
- 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.
- Will supply critical loads for several hours, while the main system is being fixed.
- 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.
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: