Bike turns off under low loading

Luckyeatwell

1 µW
Joined
May 11, 2022
Messages
1
Hi ebike gurus,

I'm stuck in my troubleshooting processes to understand why my battery keeps turning off.
I've been building my first ebike. It's been great and very fulfilling. However my battery is 2nd hand and I'm thinking this is the source of failure, but I don't understand why. So I'm hoping my description below will steer one of you to explaining what's going wrong, and how I might fix it.


The Specs:
- Bike frame, wheels and drive are all custom built or sourced as component kits.
- BBS02b system, purchased new, as a kit on ebay.
- Battery is a 48V 17ah, Dorado type, purchased 2nd hand (see further description below)
- Charger is new, SANS Model SSLC109V55 (54.6V, 2A output)

The Symptoms:
I power on the bike via Bafang SW102 controller.
I can ride the bike a short while, or just spin the rear wheel while it's on a stand. But as soon as I start taking on load (gentle uphill, or acceleration in Level 2 PAS) the display goes black and I need to disconnect & re-connect the battery before I can start the SW102 display again. I actually put a large isolation switch between the battery and the BBS02 motor so i just switch it off, then on and then I can re-start the SW102.
This seems to get progressively worse as the battery charge goes from about 54V at full charge down to 52V (still pretty full) where it shuts off the instant I start to pedal in PAS1.
The same results occur when using the Bafang thumb throttle control.


Background:
I thought I'd found a bargain when I purchased a 2nd hand Dorado type battery, described by a bricks & mortar ebike retailer as "a few weeks old, dropped by customer. Needs new BMS or re-build".
I replaced the BMS with an ebay product that seemed to suit the specs https://www.ebay.com.au/itm/1747317...SzAkIT2QQO&var=&widget_ver=artemis&media=COPY
It turns out the BMS may have been fine and what was still causing a fault at the BMS (preventing discharge from the battery) was the On-Off button & LED cluster in the Dorado battery enclosure. I disconnected this and suddenly the battery delivered 50V+ with my new BMS installed.
However I'm now not so sure I've won the war with this battery.

Discussion:
There were sparks at times when soldering all the BMS sense-leads into the battery cells. But I checked all the cells and found them to be balanced. The battery has been doing just a couple of top-up charges and finishing at above 54v just fine.
Despite problems with circuitry, the battery has always held a charge, at or above 50V even when I wasn't able to use it during the build over about 4 months.

Any suggestions and thoughts on the matter would be appreciated.

Thanks
 
Most likely cause is that bms cuts the power due to one or more cells being weaker and therefore diving in voltage during load or the cells are badly out of balance.

This should be obvious if you measure the cell voltages with a multimeter.
 
My bet- in no particular order:
BMS set up incorrectly with too high a voltage
Major system overload (a short for example) causing major voltage sag to LVC
Low charge on battery
Crappy connection causing voltage sag.
 
Back
Top