Retrofit Bafang motor help - cargo bike not working

Viva

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Hi, long post alert...hope someone can give me some advice.

Does anyone have experience with retrofitted Bafang motors? We have a Bicicapace longtail cargo bike which we bought second hand. The previous owner had fitted the Bafang motor themselves about 3 years ago - we bough it 1.5 years ago, and it's been working fine. A few months ago, the battery was full and then appeared empty on the display, and the display and motor kept cutting out when using the pedal assist. We left it for a week or so whilst trying to arrange to pop it into a workshop but before I'd had a chance to take it in, decided to check and suddenly everything was working again. Last week, it began doing the same thing:
  • Battery appears fully charged (when fully charged), then suddenly it's showing as empty on the display, then display cuts out.
  • Motor works for a short time, then cuts out along with the display when using pedal assist.
  • Throttle seems to cut out the display and motor straight away.
  • Lights are coming on, all fine there.
  • Display will turn back on again after cutting out, but will cut out again once trying to use the Pedal Assist
  • It all works fine when testing it on it's stand - display stays on, motor seems to be working, throttle doesn't cut it out when using pedal assist, but the back wheel isn't touching the ground.
  • We just spent a lot of money replacing the battery cells after being told it was a battery issue, but this hasn't changed anything. It's not the battery.
From a brief internet search, it looks like it may be the controller. I've read Bafang motors can start to let water in - it was wet the first time it cut out and wet this time (although it's been wet many times and fine). I'm just confused if it is the controller, then surely the motor wouldn't work at all? HELP! We use the bike daily for commuting with the kids and I feel like I've already wasted a big amount on money on the darn battery
 
Check all wiring connections pull them apart and check for moisture. It will look green or white residue around the metal pins and sockets of your cable ends
 
Definitely sounds like wiring issues. Could be something frayed or damaged somewhere that isn't visible, in addition to the potential for connection issues described above.

Disconnect the brake cutouts on the brake levers. Does the problem go away? Bad cutouts are a not-uncommon cause of inexplicable motor shutdowns. However since your problems are evidenced on the display this is very unlikely. Still, its free to do it and see what happens. Leave them disconnected to minimize variables at the next steps.

Put the bike in a stand so the rear wheel is off the ground. Now wiggle the battery wires around at the root where they come out of the motor. Is there intermittent performance as you move them? I'm hoping no because if your power wires going into the motor are bad its unlikely you will be able to salvage the motor. Also at this point its hard to be totally sure you are really isolating the problem to those wires.

After that, I would replace the throttle. A Bafang-compatible throttle is cheap (about US$10) and you've reported immediate failures with its use. If a replaced throttle doesn't fix it, then go one step down the chain and replace the main wiring harness. Thats the one-to-many big wire that is coming out of the motor, and which connects to the throttle, the display etc. Should be about US$35.

The next methodical step - and we're doing this in the order of increasing cost - is to replace the display. Maybe buy a cheap one for about US$65 to minimize cost.

And lastly... replace the motor controller. Thats about double the cost of a display and is sort of a pain to do. Hopefully you will get this nailed down with one of the cheaper and easier earlier steps.
 
Start at the beginning. "The battery was FULL" According to WHAT DATA??? Four bars on display, three dots on battery, numeric readout, Digital Voltmeter perhaps? NEXT, "and then appeared empty on the display". WHAT DID YOU DO? Press a button, pedal, mount the bike, did a squirrel fart on it, WHAT. Display showed full, maybe, and then empty, without you even breathing on it? No load whatsoever, is the important point.

Also, when you say you replaced the cells, so it is not the battery, a battery contains cells, and also many wired connections, a BMS, maybe a switch, a fuse, several other pieces. It might not be the cells, but it COULD be the battery.

Check for loose wires, DVM on the battery repeatedly, ideally with a load, disconnect everything possible. Especially ebrakes.

I would disconnect throttle rather than replace, if still issue with pedelec only, throttle ain't it. They are cheap and handy to have a spare.

The wiggle test and careful inspection should find problems with the wiring harness, display really not indicated as source of problem. Controller possible but not common to be so load related. Bad connection or nicked wire, I come back to the battery as most likely culprit.

I would rig a way to put a load on that battery with a wattmeter and see if it drops power quickly.

What happened to it immediately before the issue began. Did your idiot cousin use it? Ask him what he did to it.
 
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