Pros and Cons for splitting a large LFP bank into two.

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eliott asked Jun 12, '19

Pros and Cons for splitting a large LFP bank into two.

Hi!

I am designing an electric boat with 40kW of LFP batteries. Altogether it will have 8 25v/200Ah LFPs.
The question is if it would make sense to split the bank to two 2s2p packs or have one large 2s4p pack. The discharge current would be well within limits even for the two bank config

PROs:
- most of the time only one bank would be used, the other can sit idle, so it won't wear
- if the owner does not need it can leave it around 60% for longer periods
- Since on the shore only 16A per socket is available, I would need two chargers anyway than why not charge separate banks
- Redundancy

CONs:
- TWO CCGXs, BMSs etc

I was also thinking about turning on individual strings with a master switch. In this case there would only be one bms needed. The only problem is that it would drive the BMV nuts :)

By the way, how safe it is to parallel connect two different SOC battery systems? I am afraid that it can blow the fuse on the serial strings.

Any ideas or points to pros / cons?

Lithium Battery

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Answer 1 · 2019-06-13T07:24:38Z

John Rushworth answered · Jun 13, 2019 at 07:24 AM

I often wondered about two separate banks. Personally I think it better to go for one as that way if you don’t expect to discharge much you’ll get longer cycle life than discharging a smaller bank separately to a deeper discharge. Also, even though I use 25.6V LiFePO4 in my direct drive electric yacht, had I been able to afford more Lithiums I’d have gone for a 48V system, so lower current draw.

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Answer 2 · 2019-06-13T19:21:00Z

kai answered · Jun 13, 2019 at 07:21 PM

The "unused" bank will still degrade with time - just not as fast as when its actively being cycled.

I guess I'm not seeing the advantages of having 1/2 of capacity onboard sitting idle, unless the redundancy requirement is absolutely not negotiable.

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