Bafang display didn't switch on, but I did fix it...

[Manc33]

I've got a Bafang rear hub DIY kit. I did hundreds of miles on it last year and the year before, with no issues.

I didn't use the bike for 7 months. I switched it on every few weeks in that time and it always said 53% battery left. I was quite surprised that it didn't lose even 1% battery in 7 months. I went on it for a ride the other day (after 7 months of not being used) and had no issues.

Yesterday though, I went to switch it on and the display wouldn't switch on. I looked online at what might be wrong and a page I found said on two of these same units (850C) he opened the case and found a battery soldered to the circuit board, so he shorted the contacts for a few seconds and it fixed it. I did the same thing and it fixed it.

"My thought being if the Microprocessor was not booting for some reason maybe this battery was holding it in a bad state and shorting the only visible power supply might reset it?" from here:
https://empoweredpeople.co.uk/2022/11/0 ... -the-dead/

I did notice after 7 months of not using it (on the ride I did the other day) that the display's clock had reset to 12:00 and it said it's 2021.

The thing is why did it let me use it after 7 months but then a few days after that, it wouldn't switch on?

It's like the ride I did somehow made it get in a mess with itself, after so long not being used?

What I want to know is, why would shorting the little battery inside the unit fix it?
Is that the thing that keeps the time and date set?
How could it keep working unless it's rechargeable?

EDIT: Oh it is rechargeable:
From the manual: "Note : There is a battery inside the screen to ensure the clock keeps time even when display is powered off. This battery is charged when the screen is turned on. If you don’t use your bike for a very long period of time, it can go flat." - https://custombuilt-ebikes.co.uk/wp-con ... Manual.pdf

I wonder how long that little battery takes to charge, but going off what the manual says, the display has to be switched on for it to charge it. I think it auto powers off after 15m, but maybe that's all it needs to charge it up.

It's not like you can just replace that battery when it runs out (if it ever does) because it's soldered to the circuit board and the unit isn't meant to be taken apart (it's glued).

I just wonder what triggered it and when it might happen again, if ever. It's not difficult to fix it but I don't want it happening when I'm 25 miles away somewhere - but then again if you've got a 3mm allen key, a Philips glasses screwdriver and a shiny piece of metal to short the battery with (even foil would work) you could fix it while out.

[rjb]

Sounds like an issue with the battery management system. I had an issue with my Tongsheng mid drive system when my battery was fully charged but only able to deliver a pulsed trickle current.
Found online advice was to reset the battery management system. It involved charging the battery briefly (couple of second) at the output terminals (ie the power feed to the motor). Alternative method involved opening up the battery case and unplugging the cables. My battery had no plugs internally with the battery cells spot welded to the circuit board.
You must be careful and observe the battery polarity. IE charge + to + ve terminals and -ve to -ve terminals. Wear rubber gloves if you aren't familiar with high power battery's.
Since doing this 1000 miles ago it's been faultless.

[Manc33]

Didn't use the bike for about 6 or 7 weeks and had to do this fix again tonight.

The part I can't understand is, I switched it on and off a few times earlier today and it was fine.

I think I might just swap to another display, if any don't suffer this same issue.

[LasseF]

Just wanted to say thank you! and add that this did the trick on my Bafang 860c display too. Soldered on a microswitch protruding from the back of the case since this problem most likely is recurring.
Regards from Norway,
Lasse

[Manc33]

It's happened again!

After months of not riding the bike, the display won't switch on and I'll have to do this battery shorting trick again. Over those months of not using the bike, I have switched on the display at least once per week (leaving it on for 15 mins each time) to try to make sure that button cell stays charged, but it's made no difference.

It seems to take 6 months to 1 year to happen, the dates each time:

May 2023 (after 7 months not using the bike)
July 2024 (don't know how long)
July 2025 (don't know how long)

Soldered on a microswitch protruding from the back of the case since this problem most likely is recurring.

https://i.imgur.com/j85qCEX.jpeg

I wish I knew how to do it!

EDIT: Right it's an actual wired switch like this one, I think I am starting to get it now.