Symptoms of Damaged BMS?
So in my haste and excitement to charge my battery pack and get it rolling, I decided to wire up some connectors rated for 12v on a 48v battery pack and got a nasty lesson in dielectric strength. The charge connector completely shorted out and spot welded itself shut. After that I soldered in some XT90 connections and now it charges but whenever I'm riding the ebike will just shut off. While under minimal load (using Bafang C965 display), and no throttle, only Pedal Assist. I'm curious if I blew the BMS.
Unplug the battery and plug it back in and the display turns on. The battery pack doesn't even get mildly warm.
It's hard to say. This can definitely be a symptom of a damaged BMS, but other issues can also cause this. Generally, shorting the charging wires usually only damages the charging circuit on the BMS but when surges of current flow through sensitive electronics, unpredictable things can happen.
Last time I shorted the ends of battery cables, the symptoms of a damaged BMS included several chunks of missing material from the FETs, and a distinct smell of burning plastic. It wasn't real hard to diagnose, as I had just essentially welded the ends of two wires together. Ka POW!
Since then I have been VERY CAREFUL when working on battery wires. In many packs, there is a fuse. REMOVE THE FUSE. In many packs, there is a center-of-pack disconnect. OPEN THE DISCONNECT (my old pack had a bullet connector as center-of-pack disconnect.) Once the center of pack disconnect is open, COVER IT WITH ELECTRICAL TAPE. Worse comes to worse, have a friend hold the bare terminal while you WRAP IT WITH ELECTRICAL TAPE. all these measures are designed around the principle that you must only have one bare terminal wire at a time while working on battery connections.
So after a few months of testing and riding, I'm getting fewer and fewer issues with random shutoffs. It's not 100% reliable though, and I'm losing my mind tracking everything down. First thing I did was fatten up each cable I used to 12 gauge, and I might fatten them again to 10 gauge to see if that further increases reliability. However, now a different issue is popping up, which I still don't know if it's related to the BMS or if it's in the battery pack itself, but I have a fairly high voltage sag as I'm riding. I'll watch the voltage display on the Bafang drop significantly if I apply throttle for more than about 3 full seconds, and it will continue to drop until the display flashes low voltage. Am I missing anything here?
Can you remind me what your battery setup is (13s4p, for example, or something else?) and which bafang mid drive you're using? It sounds like maybe this could be a resistance issue causing voltage sag, either from too few cells in parallel, or not enough series connections, or something else entirely.
13s5p, 48v, and using a bbs02. I was also thinking resistance issues but I'm not sure how to calculate voltage drop on high voltage packs like this. For a 3v pack usually you could measure internal resistance using a resistor and calculate the value, but from my calculations even using something like a 20ohm resistor would need to be rated for up to 200 watts and I don't know where to scavenge those from.
5p is pretty good for a bbs02. That should be about 22-25A max, right? so about 5A max per cell, which isn't too high.
For testing the battery, I use halogen bulbs as a dummy load, the same way you use a resistor. For a 48V battery, I'll put 4 50W or 100W halogen 12V bulbs in series. You can also parallel more bulbs for a bigger load.
One other thing to consider is the terminals on the battery. Did you use the brass wire connectors on the -1 and +13 terminals? And did you use two on each terminal? Using only one means more of a bottleneck, since current from the few cells on the end have to travel further and burn off more energy as heat in the bus bars until they reach the single brass connector.