Parallel Battery Wiring Best Practice?

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nebulight asked Feb 15, '23

Parallel Battery Wiring Best Practice?

I've got a DIY Lifepo4 battery bank that I'm going to be installing in a somewhat unique fashion. I have 16 304AH 3.2v cells that I'm going to put into a 12v bank with 4 cells in parallel then series those groups for a 12v bank. However the unique part is I'm going to install the 4 cell in parallel into a battery box so if they need to be removed from a camper van it's manageable weight (ignore the series busbar connections as I have matching flexible busbars on order). The BMS is going to be a REC Active BMS so will be external to the battery boxes.

My question is the first photo. Per wiring unlimited, it says to wire parallel strings in one of these suggestions:

Since the "load" is going to be the battery box terminal, would there be any benefit to running a wire from each cell or would that not matter since I already have a bus bar? Just go with option 1 (Diagonally)?

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klim8skeptic ♦ commented · Feb 16, 2023 at 09:24 AM

@nebulight interesting.

Given how much spare room there is in the box, the "posts" method looks promising. 4 equal length cables crimped into a lug, bolted to the battery cover terminal posts. Any over length cable can reside in the empty section of the case, directly below the cover's terminals.

Thought about how the balancing / monitoring leads will exit the case / interconnect with the next "cell" ?

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nebulight klim8skeptic ♦ commented · Feb 16, 2023 at 02:38 PM

There actually isn't as much as it appears. Plus I'd need thick wire to handle the the current I'd like to run through these batteries so probably 4-6awg wire. Fitting 8 equal length cables is going to get messy and may not fit.

As for the cell monitoring, since they are all in parallel I was just going to monitor from the terminal on the outside of the case.

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Answer 1 · 2023-02-16T07:57:55Z

Alexandra answered · Feb 16, 2023 at 07:57 AM

@nebulight

In the most correct set up the path from each battery to the load ideally be the same. But sometimes that is not practical.

Take offs from the ends usually ends up being the easiest / most convenient set up, but since you already are planning on posts on your box is makes sense to run same length individual cables to the posts for equal current share. How would you work the cable routing though?

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nebulight commented · Feb 16, 2023 at 02:41 PM

That's the challenge. I think it would be too difficult to fit all 8 cables inside that box when they are sized appropriately for the current to match a MP3k. I think I'll be sticking with the busbar diagonal option.

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Answer 2 · 2023-02-16T11:32:27Z

the-doon answered · Feb 16, 2023 at 11:32 AM

I have mine setup using the diagonal method as you have mentioned. I only have 163Ah cells and wanted to put them in a 4S4P configuration but was told to split them into 2 individual batteries of 4S2P by my supplier as the fault current was too high for 1 BMS to handle. Not sure what the REC is rated at but might be something to consider in your design.

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nebulight commented · Feb 16, 2023 at 02:44 PM

I too have thought about interrupt current, specifically inside the box. I plan on 3d printing an isolator board along with terminal covers inside the box should there be a lose connection and the cable falls off there should be no chance of them shorting. Though I don't plan on sealing these battery boxes and will be doing inspections frequently. 1240ah 3.2v cell is a lot of energy.

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