bafang bb02b - battery drops to 0% after throttle use in 7, 8, 9 assist, is this a safety feature?


New member
Jun 8, 2022
Hi all, I’m a new bafang bb02b 750w owner.

While riding, the battery dropped from about 25% to 0% under load from using the throttle.

This issue happened both when I first used a friend’s bike, which tempted me to get my own. I used the throttle, probably with PAS on 9, in a high gear uphill, all not recommended which I now know.

It happened again on my own setup yesterday, slightly different scenario, hit the throttle on a slight incline with PAS on 7 in about a middle gear.

Battery instantly dropped to 0% from around 23%.

After 5 minutes of pedalling at PAS setting 1, it recovered to 4%, slowly back up to 15%, but then at the next hill it dropped down to 0%

The fact that I was just about able to make it up the hill, make me wonder if is was still giving some very minimal amount of assist, to compensate from the additional weight/resistance of the bafang kit.

I have two questions really.

1.) when the battery drops like this, is this a sign of a fault or temporary overload?

2.) at 0% in these conditions is the motor still giving some sort of assist?

Thanks in advance

Deleted member 4845

What you are seeing is called voltage sag. Very common.

Voltage sag increases as the battery nears end of capacity. Voltage sag increases as batteries age. Sag to this degree indicates a low AH-capacity battery, especially if new or nearly-new.

It is a sign of overload, could also be considered a "fault". Not a designed-in feature.

Both the controller and the in-battery BMS have an LVC, or low voltage cutoff. As long as you have not triggered either of those, then motor is still getting some power. Once an LVC is triggered, all battery output is ended. You have to turn off and on again, ideally after charging.
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