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Help designing a 600ah LifePO4 battery bank for sailboat

Becaris

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Mar 4, 2021
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I am thinking of building a LifePO4 prismatic form factor 600ah 48v battery bank for a sailing catamaran. It has two electric pod motors, 7.5 kw each at 48v. At maximum, running the pod motors full out, each pod pulls about 156 amps, or total for both would be 312 amps. My question is, what size and configuration of LifePO4 batteries would be best?

I have read that smaller size batteries take the pounding better on a platform that is often in motion, in my case, a sail boat. However, I have also read that putting too many banks in parallel is not a good idea either (2 or 3 maximum). If I went with 100ah cells, I would have to put six strings made of sixteen cell banks (100ah @48v) each to get to 600ah total. This keeps the cell size smaller, but makes six banks in parallel. Or, I could go with 310ah cells, making two strings of 16 again, but each would give me 310ah @48v. That would only require two banks in parallel but I would be using larger individual cells. I could split the difference and go with 200a cells for three banks in parallel.

What would be best? Or is there another configuration I am missing? I am somewhat new to this so be kind. :)
 
This is probably beyond my knowledge to answer fully, but one thing to consider is that bigger battery cells allow higher discharge rates, often "1C", meaning a 280Ah cell can discharge at up to 280A. It depends on the actual battery cell what it is rated for. It might be nice on a boat to be able to use one of the two packs; for example in case one or more of them dies (for example from a fried BMS or blown fuse).

If you have multiple 100Ah (1C, 100A) packs in parallel to reach your 312A draw, I'm not sure how you match the current to ensure you don't draw more than 100A from any one of the packs, without hitting an over-current limit from any one of the BMSs. Therefore, my initial inclination would be to use the biggest cells possible with the fewest number of packs in parallel, so I guess that means 280Ah or 310Ah; I'm not sure if they come any bigger.

Alternatively you could build a higher-voltage pack and step the voltage down to 48V; that's probably a better solution than many 48V packs in parallel but does require a high-current and high-voltage DC/DC conversion. I'm assuming your "pods" run on DC power. I think this is what electric cars do: they have something closer to a 600V pack and step the voltage down / step the current up to drive motors which run at lower voltages.

Most people don't run into this problem because they're adding packs in parallel to increase capacity, not because they need high instantaneous current draw. Maybe someone else can comment on how you do the current balancing / current limiting between parallel packs without over-drawing current from any one of them. There might be some sort of balancer circuit to do that.
 
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