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WIRING 2 BMS IN PARALLEL.
#1
Hello, please is it possible to wire two separate 15amps bms in one pack to get double the power (30A) ?
Thank you.
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#2
Have also been wondering this. I have ordered 2 and will do some testing when they arrive, can’t see any reason why not.
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#3
(03-05-2019, 05:33 AM)Idiwzr Wrote: Hello, please is it possible to wire two separate 15amps bms in one pack to get double the power (30A) ?
Thank you.

The problem will be how to distribute the currents equally to both.

its a problem of resistance on the paths.

Personally i consider that approach into the category "different, but not good".
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#4
You will get around 30A for short bursts. But as the BMSes warm up, the combined limit will keep falling.

This has to do with the quirk of semiconductors to have less resistance as they warm up. Thus unless you get two perfectly matched BMS, one of them will have a little less resistance and let through slightly more current, warming it up slightly more than the other BMS, thus reduce its resistance even more... feedback loop with one BMS carrying more than half the current and hitting the 15A limit.

So if you only need 20A~25A max, you'll probably be fine with 2 parallel BMSes.
If you need 30A, you really should look for another solution. Perhaps add a 3rd BMS.
Best solution: get a BMS with 40+A to have some headroom.
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#5
Yes, you can run two in parallel but buying a single larger BMS would be best. If running two, make sure to balance leads from both directly to the battery so that there are no shared balancing wires.
#18650
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#6
Thank you very much for the contribution. I really helps
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#7
I agree with the other comments… it would work, but not ideal.  I would just pay attention to the wiring and do your best to have equal resistance paths to help reduce the chance of cascading failures during high loads.
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#8
(03-05-2019, 11:50 AM)ajw22 Wrote: This has to do with the quirk of semiconductors to have less resistance as they warm up.  .... warming it up slightly more than the other BMS, thus reduce its resistance even more... feedback loop with one BMS carrying more than half the current and hitting the 15A limit.

My understanding is that yes this applies to regular transistors but the opposite applies to MOSFETs (as used in BMS units). MOSFETs are actually quite good at current sharing (they parallel lots of them in inverters & they share current nicely). If a MOSFET gets hotter, its on resistance rises = less current flow, so next device carries more current, heats, equalizes, etc

My concern with parallel BMS's with load switching would be one switches on/off before/after the other leaving one to carry the load.

For cell monitoring/balancing it shouldn't matter much.
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#9
Redpacket have the most crucial comment and info off all.

The problem is when One switch before the other..ie this makes the parallel dangerous and should not be used.

Its not like a 30a bms cost 10* what an 15 does...
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