running 36v packs with bms in series - blew a diode

So I was riding home last night after work, I was going through a pedestrian underpass (not underpower) pretty much at walking speed. I hear what sounded like my rear wheel puncturing or riding over a bursting plastic bottle. Anyway after losing a bit of power I realised id don't something stupid with the packs.

Normally I get to work and charge both packs, that day i'd forgotten to charge one so they were unevenly charged. One was full one was about half.

There is no regen enabled on the bikes controller though it is a direct drive hub motor so there my be some regenerative forces happening.

I took the Macalister pack apart when I got home and found the s3m m1c diode I bit less together than it should be smelling of the magic smoke. I placed an order for some more and will replace it and see whether this pack will charge again.

my own naïve explanation is that the regen was being taken disproportionately into the two packs and trying to charge the emptier one faster so blew that diode. I don't have a better explanation than this but I open to learning what the real issue is here.

AFAIK the most common thing that happens with packs in series is if the BMS on one shuts off for any reason, the FETs in it now take the full voltage of both packs across them. If the FETs aren't rated for that, POOF.


I don't know about the diodes; if they are rated for the full voltage of both packs they should be ok, but if they are not, then that's also the most likely explanation, and I'd replace them with ones that can handle that.

So the S3M diodes arrived in timely fashion from ebay. Hand soldered the part as i dont have a hot air rework station yet. Anyway I kinda had a big spark when my iron accidentally slipped and made contact with the p- contact and probably arced the pack voltage. Nice green spark. Being a touch more careful i managed to get the new diode in place and have put it on charge. appear to be taking the charge so maybe it was the only thing to go on this bms board. Very chuffed as it has saved a pack for me.