Lithium battery build questions - adding ah capacity

[Saks]

Hello.

I am going to assemble a lifepo4 pack for my 36v golf cart.

I appreciate there is heaps of knowledge here and wanted to tap into it for the build, which will have solar charging added once all up and running.

I can get my hands on a set of 3.2v lifepo4 cells from an electric truck.

I will add a balancing bms - jk brand to control it.

They test at 90ah which should be plenty (its only used for golf!), however the set comes with 14 3.2v cells. I was going to connect 12 in series to give me 36v leaving 2 unused. This seems like a waste. Am I able to connect the extra 2 in parallel and will that add any capacity?

If I do connect the extra 2 in, do I need anything special in terms of the bms ( the one I'm looking at says 20s 20p) and will there be any charging issues, or will the bms take care or this?

The price for the pack is so good I have to use it, even if I only use 12 cells.

I have added a rough schematic of how I could do this, but do you add the parallel batteries to cell 12 (left diagram) or do you add them to the total pack as per the right side diagram attached)

Thanks!

 

[sunshine_eggo]

Hello.

I am going to assemble a lifepo4 pack for my 36v golf cart.

I appreciate there is heaps of knowledge here and wanted to tap into it for the build, which will have solar charging added once all up and running.

I can get my hands on a set of 3.2v lifepo4 cells from an electric truck.

I will add a balancing bms - jk brand to control it.

Make sure the BMS you select can handle the surge current of the motor. Motors tend to draw about 5X their run current, and this will bring most BMS to their knees. You may need to significantly oversize your BMS.

They test at 90ah which should be plenty (its only used for golf!), however the set comes with 14 3.2v cells. I was going to connect 12 in series to give me 36v leaving 2 unused. This seems like a waste. Am I able to connect the extra 2 in parallel and will that add any capacity?

No. You can't. The capacity of a battery is determined by the LOWEST capacity cell. All you're doing is making two "cells" higher capacity than the other 10. You will still be limited by the lower capacity cells.

Keep them as spares in case one fails.