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Forum Index : Electronics : 8 Cell Balancing BMS

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Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1158

Posted: 01:43am 10 Mar 2021

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Currently splitting up an old lifepo4 battery bank into several 24v packs, the old 40AH cells are pretty much on the way out, 4 have started to swell markedly if left after charging; the rest 30 odd are probably at < 50% capacity.

Cannot really use them without a BMS as their capacities vary all over the place, the current BMS I published a while ago is a bit complex and requires several pcbs; so have designed this version which is a bit easier to make.

Spec: 8 cells, can vary between 2.5 - 12 volts each, balancing current 0.5 - several amps, using a switched floating isolated measuring circuit. Extender output allows expansion to more groups of 8 cells.

Just as I completed this, latest silicon chip has a nifty design using switching transformers for lossless balancing of 4 cells, not sure how much that it can be extended, wait for part 2 next month.

And those 4 bad cells, cannot find anywhere in Wellington to take for disposal or recycling, not sure what to do with them, may drill a 10mm hole in each and drop in a bucket of water for a week to make inert.

PCB top: 260 x 115mm

I dont have a schematic as yet as its very similar to previous designs.

Cheers
Mike

Solar Mike
Guru

Joined: 08/02/2015
Location: New Zealand
Posts: 1158

Posted: 10:54am 11 Mar 2021

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Update: Have cut and paste to clone an extender pcb, this allows multiples of 8 cells to be measured. Made some minor alterations to original at same time, still don't have a schematic for this, not even a pencil drawing, but have sent both sets of gerbers off today regardless; cannot see any errors... famous last words.

Extender 115 x 178:

Once I get it going will publish gerbers and software.

Cheers
Mike

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