Help! My ebike battery doesn't fully discharge!

[Abneer Ejaz]

Hello everyone, (first post)
I have an electric bike battery that has a peak voltage of 42v, nominal voltage of 36v and it should discharge down to 30v. My problem is that the battery will not work after it reaches a voltage of 40.5v. I don't really know why that is or how to fix it.
Any help would be much appreciated!

 

[Nealh]

30v may be considered low but as you are not getting near it something to ponder later for better battery life/ balancing.

For how long has it been stopping at 40.5v ?
Does it charge to 42v ?
And assume you may have a voltage meter as you know it is 40.5v !!!

 

[Abneer Ejaz]

30v may be considered low but as you are not getting near it something to ponder later for better battery life/ balancing.

For how long has it been stopping at 40.5v ?
Does it charge to 42v ?
And assume you may have a voltage meter as you know it is 40.5v !!!

Thank you for replying
I recently noticed it stopped at 32v but I didn't pay mind to it but it jumped to 37v and I just charged it,went for a ride and it cut out within a few minutes. I brought it home and it read 40.5v (yes I do have a multimeter, 2 in fact just to check it wasn't a faulty meter)
Fully charged it it 42v

 

[Andy-Mat]

Thank you for replying
I recently noticed it stopped at 32v but I didn't pay mind to it but it jumped to 37v and I just charged it,went for a ride and it cut out within a few minutes. I brought it home and it read 40.5v (yes I do have a multimeter, 2 in fact just to check it wasn't a faulty meter)
Fully charged it it 42v

The voltages you quote, how are you measuring them? If a bike display, they might be wrong. If you are using a voltmeter, check its battery is still good, and then compare it to a known good meter, both on the same voltage source, just in case yours is bad.
If everything is accurate, then as someone else mentioned, maybe the BMS is bad,or some cells have gone tits up!. This is a job for an expert then.
I personally have never heard of such a problem, thankfully!
I wish you well.
Andy

 

[Nealh]

Distance fault finding isn't easy and cause could be one of many to fault find check thru.

How old is said battery ?
Any idea of what cells are used ?
What bike or kit ?

 

[Abneer Ejaz]

The voltages you quote, how are you measuring them? If a bike display, they might be wrong. If you are using a voltmeter, check its battery is still good, and then compare it to a known good meter, both on the same voltage source, just in case yours is bad.
If everything is accurate, then as someone else mentioned, maybe the BMS is bad,or some cells have gone tits up!. This is a job for an expert then.
I personally have never heard of such a problem, thankfully!
I wish you well.
Andy

Thank you so much for your advice, I will definitely consult an expert!
I wish you well too,

Abneer

 

[Deleted member 25121]

Are you able to measure the battery voltage under load, for example by turning the pedals with some light braking and the driven wheel raised, eg on a bike stand?

It's possible that the battery voltage had dropped very low under load but recovered by the time you measured it which would suggest that there's a problem with the battery pack.

 

[vfr400]

It sounds to me like you're using a 48v controller or have your controller set to a 48v battery with a 40v lvc instead of a 36v battery with a 30v lvc.

 

[Abneer Ejaz]

It sounds to me like you're using a 48v controller or have your controller set to a 48v battery with a 40v lvc instead of a 36v battery with a 30v lvc.

Thanks for your suggestion but the battery worked with the controller before under 40v and so I think everytim I charged, a cell or 2 got worse and the bms is stopping over discharge.

 

[Andy-Mat]

Are you able to measure the battery voltage under load, for example by turning the pedals with some light braking and the driven wheel raised, eg on a bike stand?

It's possible that the battery voltage had dropped very low under load but recovered by the time you measured it which would suggest that there's a problem with the battery pack.

Sounds very possible.

 

[Andy-Mat]

Thanks for your suggestion but the battery worked with the controller before under 40v and so I think everytim I charged, a cell or 2 got worse and the bms is stopping over discharge.

You are right, as some controllers are "programmable" for both voltage and maximum current, it might have accidentally changed itself! A check would be a great idea.
regards
Andy

 

[Nealh]

40v lvc entered my mind originally but as Op mentioned it once hit lvc at 30/32v, dismissed that line. Some times posters omit details which can be significant.

I have had a couple of similar issues like that over time.
One was a controller on it's way out with component failure, I did get lvc error code though both batteries in use were good.
Another time a battery I welded up had 2 or 3 series strips lose contact forcing the lvc shut down with battery at high voltage.
The fault could be simple or more in depth.

 

[vfr400]

40.5V is too high for the battery's BMS to cut off. The BMS will cut when the first cell reaches 2.9V. That would mean that the highest voltage that could happen would be 9 x 4.2V and 1 x 2.9V, i.e hot off the charger at 40.7V ( but OP has confirmed 42V hot off the charger). As soon as the bike was used when you start to pedal or use the throttle, the voltage sag would drop the voltage below 40.5v and cause a cut-off, so the bike wouldn't be able to go anywhere. You're barking up the wrong tree or it's not cutting off at 40.5V.