Blown MOSFETs Due To Chain Jam, Possible BMS Arrives for Li-Ion Pack

I have been having balanced karma of late--every time something really good happens, something equally bad compensates.

Yesterday on the way home from work the bike's rear chain (regular bike drivetrain part) somehow derailed and got tangled up, which actually pulled the entire right end of the rear axle and the derailer bolted to it off the dropouts and bent the rim up against the chainstay on the other side (the left end stayed bolted in), and then everything jammed and both bottom-leg MOSFETs blew.

The power meter registered over 3500 watts peak, 153 amps peak, which is a more than any of it is rated for--the MOSFETs I installed in my 2QD are only rated at 80A continuous, and can handle ~150A for *maybe* a second absolute max, with sufficient cooling (which they don't have in my setup, by any means). So it's no surprise they blew up.

All that only took about half a second as I was accelerating away from an intersection, crossing two lanes to get to a left turn I have to make to get home without going down a very busy stretch of road.

Then I had to drag the bike across the rest of the lane (while cars that weren't yet coming when this all started kept trying to go around me instead of letting me get out of their way!), up onto the concrete median, and spend an HOUR with traffic roaring by me, while untangling the chain enough to get the wheel off, unbend the rim and true it enough to get it able to roll. Then I had to take out around 6" of chain due to bent and broken links, and set it up to run only in lower gear in front since it was now too short for the big ring. Plus I had to take the motor chain off, since my pedals are linked to it and it is nigh impossible to pedal the bike without the motor running otherwise.

After that it was a heck of a ride home, at about 9MPH, which is just above the unstable speed of the bike, and I was totally worn out when I got home (and then had to go to a different job as handyman for someone else).

I didn't have the camera with me to take pics of the chain/etc, but wow, it was awful looking. It looked like someone had hit my rear wheel from behind and then stepped on it sideways, but it was just from being yanked so hard by the chain as it tangled.

The frame and dropouts all appear intact, but the wheel was seriously messed up. I will need to take it completely apart, restraighten the rim, and then re-lace it to get it back to anything close to really true.

The derailer cage was slightly bent but not badly, but the bolt that secures it to the little shaped nut that recesses into the dropout behind the axle had sheared thru as it was all pulled out. I am just lucky that I carry spare nuts/bolts in the toolkit, one of which happens to be the same thread pitch and diameter as that bolt--it is a bolt from a rackmount kit--it's so long that it prevents use of the highest (smallest) sprocket on the rear cassette, but as I had to pedal with no assist there was no way I was going to be shifting up that high anyway.

I guess the good news is that I *could* fix the problem on the road well enough to get home.


Here's some pics of the controller with toasty MOSFETs, before repair. I still haven't had time to go back and fix it yet; just replacing the MOSFETs and gate resistors (whcih also burned open as they are supposed to) didn't fix it. I'm too tired to try more now, after another day of work. Tomorrow and the next day I have to go help do a roof and some other things, so I won't be able to do it then, either. Fri and Sat I'm working again, so I don't know when I will find the time to finish troubleshooting it. Maybe Sunday. Should be easy, when I have time and am not too worn out to concentrate. (writing this post has taken several tries).

A close up of the damaged area. The discoloration around the screw on one MOSFET is actually the melted remains of the plastic insulator ring...it's still not shorted to the heatsink, but that insulator is totally destroyed from the heat.




This blown LiFePO4 BMS (sent by an Endless Sphere forum member) arrived today:

All that is known about it's problem is that a puff of magic smoke escaped from it somewhere, but not specifically where. With luck it's the MOSFETs, as those I have around here already.

Once I find and fix the blown part(s), then if I can figure out how to change it's HVC and LVC voltages to match those of my Li-Ion cells, I can use it as a BMS for my custom-built pack.

First, I just need to invent a time-machine that gives me extra time to do everything and still get enough rest....