BMS Pooched?

[Hammer247]

I can't seem to get the BMS to do anything. Not sure what happened to it. The bike it came on stopped working the day after I got it. The wiring was a mess and I didn't have time for it so I tore it down till I was ready to build another bike (now-ish). I tried charging it a couple months ago but had no luck. I did notice a flashing blue light on BMS.
Yesterday, I tried anything I could find online to wake it up to charge the battery, but nothing worked. I decided to open the plates and check out the PCB.,etc.
Can anyone tell me if it's still any good? Or if there's a way to test it? Or who/where to take it for help? Any help is appreciated.
Emily

 

[amberwolf]

Most battery problems are not caused by the BMS. The BMS usually prevents charging or discharging for a reason, so that you don't damage the cells and cause a fire.

The fault is usually the cells, pack construction, or physical damage to the pack causing damage to wiring, interconnects, or cells (or all of these).

What are the cell voltages, for every group of parallel cells? List them starting with the most negative ones first (starting at the main battery negative where the thick black wires from the BMS connect to the cells, going up to where the thick red wire connects to the cells.

 

[aae34]

I think there's atleast two relatively obviously damaged parts on the pcb:


Unless there's something special about the bms, i would replace it with something else.

 

[Zambam]

According to Will Prowse, it's the BMS that usually fails. Granted he's coming from solar backup storage where LFP cells are used, not sure if this applies to ebikes, which use a different chemistry mostly.

 

[amberwolf]

AFAICT the balance resistor (lower right corner) is probably not damaged, just installed upside down.

The FET just looks like it's back is more polished or wiped clean so it's shinier and reflecting back the darker surfaces facing it from the camera POV.

Regardless of whether the BMS is functional or not, the cells still need to be checked to see if the battery needs to be repaired or replaced.

 

[aae34]

AFAICT the balance resistor (lower right corner) is probably not damaged, just installed upside down.

Look at the solder. Would the resistor being upside down affect the solder&vias ?


edit: and also the fet does stand out in two separate pictures. no camera pov will explain that.

 

[Zambam]

Need better pictures that's in focus and not blurry if you want visual diagnosis of heat damage. Resistors are not polarity sensitive and can be installed upside down. If I have to guess, the blackened resistor and FET are damaged. Do you have a DVM? Check the resistance of the blackened resistor and compare to other resistors. If very different, then it's likely damaged. Between any 2 terminals of the FET, if you get zero ohms, you have a shorted FET (usually happens between the Drain and Source pins)

 

[amberwolf]

Look at the solder. Would the resistor being upside down affect the solder&vias ?


edit: and also the fet does stand out in two separate pictures. no camera pov will explain that.

No, you're right that looks like overheating damage.

(when I zoom in on the images I can't see anything like that clarity here--it blurs and pixellates, so I didn't see that)

So you're probably right about the FET too.


But I would still recommend testing the cell voltages (at minimum), since a fried balance resistor shouldn't happen unless it saw a really high current thru it, which implies a really high drain from that bank of cells, or a short circuit in the balance wires across multiple groups of cells, which means shorting out the cells themselves.