36v 15.5 ah Lithium polymer battery just cut out

[Topcat33]

Bought second hand eMate City bike recently and had no problems with battery.
20 mile ride yesterday on full charge and hardly lost any, charged up last night no problem
Today after about 14 miles whilst on power level five the battery just went dead.
Changed the fuse, tried battery off our other bike which worked fine, tried dead battery on other bike, still dead.
Got home and put it on charge but nothing happened.
Any advice would be great, I am fairly handy with electronics and am willing to take apart
Photos show type of battery and model
Thank you

 

[Deleted member 4366]

Check the fuse and measure the voltage on the outer terminals at the bottom and the outer holes on the charge socket.

 

[Topcat33]

charge socket is reading 38v
Bottom terminals reading 41v
Fuse is good

 

[Danidl]

charge socket is reading 38v
Bottom terminals reading 41v
Fuse is good

Suggest you attach a car headlamp bulb .. say 55 w accross the power terminals of the battery, it should be very very bright, you only need to have it there for a second or two. Much longer and the bulb will burn out. This will show if there is defective cells in the battery. .. if one of the tiers of the battery has gone nearly open circuit, it will still display full voltage on a multimeter, but not if it's being asked to drive a heavy load. This is a good use for those combined main and dipped beam bulbs where one of the filiments has blown.
A domestic incandescent 100w bulb will also serve , but not as well as the load will be less.
A car bulb would draw 4a at 12v, and theoretically 12a at 36. But it's not exactly linear... But is a good test.

 

[Deleted member 4366]

charge socket is reading 38v
Bottom terminals reading 41v
Fuse is good

I've done a lot of repairs to those batteries, and I would say that it's not possible to have 38v on the charge socket and 41v on the main terminals. It's possible yo be the other way round.

At this stage, my guess would be that one of both terminals have opened up, so isn't/aren't making contact with the pin.

Fully charge the battery, then measure the voltage again as soon as you take it off the charger. That will give an indication of cell problems.

 

[Topcat33]

Yes I thought that was rather strange so went to take another reading and the top battery indicator actually worked and showed full charge so I placed it on bike but sadly nothing and the indicator is no longer working.
I tested bottom terminals again and am getting a reading fluctuating between 1.3 and .7 volts
It will not charge
Also I see there is a small fuse on top as well as larger one, replaced both but still no joy
Thanks

 

[Deleted member 4366]

I need actual values to help you. What's the voltage on the charge socket immediately after you disconnect the charger?

The readings on the terminals would normally indicate that the BMS is switched off. The question is why?

There's another strong contender as the cause of your problem. That's the key-switch. I've repaired a few that had bad soldering on the wires at the back of the switch. The joints looked OK but they weren't actually attached to the little strips. It happens when they solder too fast, which doesn't give enough time to allow the strips to heat up and let the solder fuse. You can test that by removing the top (4 screws, then remove the key switch (2 screws and lever out). Measure the voltage of each side of the switch to ground. It would be intermittent, so wiggle wires and try it on the bike if the voltage appears on both sides. Often it makes contact, so you get the 40v, but as soon as you connect it to the controller or try and get power to the motor, the connection breaks and the voltage on the outside of the switch goes to zero.

 

[Topcat33]

Thank you D8veh I have just taken the switch out and one solder joint is loose, as in photo for the benefit of others
When I make the connection it shows full charge on top panel
Off to borrow a soldering iron,will keep posted but I am hopeful
Thanks

 

[Topcat33]

Success, all back together, not the neatest soldering in the world but it’s held fast and it’s back on the bike showing everything it should on the screen.
Thank you so much for your help, it is really appreciated
All the best