Jorvick trike electrical problems (inrush?)

[Clivington]

The BMS cutting off power when the pack was at 36volts is a pretty good giveaway that one cell group is much lower than the other cell groups. The pack seems to be unbalanced. Sturmey's suggestion of opening the pack and checking the voltage of all of the cell groups is a good one.

My guess is that you have one parallel group of cells that has lost capacity ( or possibly very much out of balance). Its coming up ok on charge but it discharges early and this low voltage of one cell group will cause the BMS to do its job and disconnect the load.

I don't get it. If you leave the battery connected to the controller, there is no inrush when you switch on because the capacitors are on the unwatched dive of the controller.

Thanks very much everyone. I'm sure you're right - that the cells are faulty. The considerable inrush current must load the weak cells such that one or more parallel groups exhibits a low voltage which is enough to trip the BMS, so the display can't power the trike up. Had it been my battery, I would have opened it long ago and probed around. But it isn't, and I don't want to return a tampered-with battery to my friend because I wouldn't be able to guarantee its safety. What I've done is I've fitted the inrush limiter and it's working OK - enough to enable the trike to operate with assist, so I can return it at least. I'll discuss the options with the owner.
Thanks again!

 

[Ghost1951]

@ Clivington

You seem to be very fixed on this idea of 'inrush current' .

I don't think inrush current is an issue at all. The whole system is designed to demand lots of Amps from the battery when you apply power to the motor. That is what the controller does.

I think the battery is just out of balance and a low group of cells is causing the BMS to shut off power when the normal voltage sag happens as you demand power. That particular cell group falls (as they all do) by a volt or two, and that triggers the BMS to shut the system down. It happened to me the other day when I was range testing my battery and after 43 miles, one particular group went too low while the others were all OK. I thought I had about five miles left judging by the overall battery voltage, but the screen went dark and I had to pedal home unaided. At home I checked the voltages and found the low group and sorted it out by charging it. All OK now.

However, I can see why since it is not your battery, you don't want to mess about with the battery insides.

 

[saneagle]

Thanks very much everyone. I'm sure you're right - that the cells are faulty. The considerable inrush current must load the weak cells such that one or more parallel groups exhibits a low voltage which is enough to trip the BMS, so the display can't power the trike up. Had it been my battery, I would have opened it long ago and probed around. But it isn't, and I don't want to return a tampered-with battery to my friend because I wouldn't be able to guarantee its safety. What I've done is I've fitted the inrush limiter and it's working OK - enough to enable the trike to operate with assist, so I can return it at least. I'll discuss the options with the owner.
Thanks again!

You can prevent over-current from shutting down the battery by adding some solder to the shunt in the BMS, which will cause the BMS to allow higher current. If inrush is indeed the problem (I don't think it is), that will get you a bit more life from the battery. If you add solder to 25% of its length, you'll increase its tolerance to current by 25%, which should be enough, given that it doesn't happen every time.

I had to do that one time because I had a battery that shut down every time I went up a hill, even though the controller only drew 15A. It completely solved it.