Troubleshooting 48V Bafang Battery

[osoadv]

I was directed here from a friend who said this forum has been very helpful to him.

I have a 2018 Giant Sedona with a Bafang e-bike kit. It sat for about a year in storage and now will not power up, even after an overnight charge. It would power up with the charger connected, but the motor would not operate and it would power off immediately when the charger was disconnected.

I removed the battery and disassembled it for troubleshooting. Below are the voltage readings I get at the critical points.

Does this appear to be a BMS issue or something else?

Thank you for your guidance.

Voltage readings:
54 volts across battery terminals.
50 volts at charger connection
50 volts at main discharge port
0 volts BMS charge terminal to negative battery terminal
4 volts BMS discharge terminal to negative batter terminal
4 volts BMS discharge terminal to BMS charge terminal

 

[goatman]

id be looking at the battery on/off switch, even jump the wires to turn the bms on but maybe take a picture of whats happening with those little wires. whats that thing with the black ground wires. is that the off/on or a temperature

 

[osoadv]

id be looking at the battery on/off switch, even jump the wires to turn the bms on but maybe take a picture of whats happening with those little wires. whats that thing with the black ground wires. is that the off/on or a temperature

Thank you. There is no on/off on the battery itself, only on the handle controls.

I believe the wires you mentioned are what go to the battery level meter on the battery. There is only that and the charging port.

 

[goatman]

I don't know why you have 3.96v at b- and p- theyre both negative. sounds like bad bms.

id solder these two wires together to bypass the bms, unsolder them from the bms and solder together

 

[goatman]

its actually -3.96v and your black probe is on the bms p- so ya it should be 0volts bms is toast

 

[amberwolf]

Typically the voltage reading across the discharge FETs (the 4V-ish) indicates the FETs are turned off, which means the BMS is trying to protect the cells against a problem it has detected. Whether the problem is real or not...that's a separate thing. The 0V reading across the charge FETs may mean it hasn't turned off the charge port, though, which is odd.


First, if you connect the battery to the bike, what does the main discharge port + and - wire pair read in voltage?

If it drops to basically nothing, the BMS has shut off to protect the cells against some problem, which is likely to be overdischarge of a cell group.

If it stays at the 50v, then the problem is more likely to be in the controller system or wiring.

What are the voltages across the cells themselves? Meaning, if you use the voltmeter with black lead on the main cell thick negative wire, and the red lead on the first cell balance wire at the negative end of the pack, what is that voltage? Then move the black meter lead to where the red lead is, and the red lead to the next wire, what is taht voltage? And so on all the way up to the last pair of balance wires.

If any of htem are different from the others, that group has a problem, either in the cells (defective cell(s) or in the BMS (balance shunt stuck on draining cell continously, or BMS is powered from just those cells and drained them while sitting)).

Typically the inability to charge even overnight means there is a likelihood of at least one cell group below the "safe" recharge level (typically between 2v and 2.8v, depending on BMS).

 

[osoadv]

Thank you for the guidance. I was able to swap out the BMS and everything works. Unfortunately it now will not fit back into the case due to the BMS being almost twice as big.

 

[goatman]

Thank you for the guidance. I was able to swap out the BMS and everything works. Unfortunately it now will not fit back into the case due to the BMS being almost twice as big.

that's good news, now you get to learn how to build a battery case