What's wrong with my battery?

Can anyone help?

I have built my battery from used laptop batteries. It is approx 14A 48V battery with a 14s BMS, but I only have a 13s battery.

When it was built the battery was showing 43V. I have charged it up but now something really confusing has happened and I don't know what.

The power out voltage is showing 38V after being charged. Throughout charging it was showing the charge going up as it was charging. But once charged the final voltage is 38. The charge cables are showing 44V and when I put the voltmeter on the negative and positive ends of the battery it is showing the charge I would expect which is 51V.

The BMS is rated to 54V so its well within that. The only thing I can think is the BMS has gone. Can anyone help with any possible faults that I can check on the battery? I'm struggling this end.

I also cannot seem to get any power from the battery in to the my ebike conversion kit. I know its wired up correctly but I cannot get any power in to it. Any ideas on this also?

Many thanks





Well-Known Member
Walk a voltmeter up the balance wires. If it were fully charged, each cell group ought to be 4.2 volts. So the voltage ideally increases in 4.2 volt increments all the way to 54.6V for a 13S. See what you have, and write them down. See if they're reasonably balanced.

If you have one group below 2.5V, the BMS will not let any current thru its circuit board. Or the BMS could be mis-wired so make sure you terminated the un-used 14th section of the BMS properly so it's not being sensed as a bad group. I've not tried to use a BMS in that mode, so I'm not familiar with the steps, but you can't just tap off the 13th string can you?
That could be the problem. Each cell group is around 3.8v I think there is one or two which are at 3.65V. I think the problem could be the 13th or 14th BMS wire. When you say tap off, are you saying cut it off? At the moment the 13th and14th wire are welded together. It was just something I found in some other forums regarding the extra wire.


Well-Known Member
I believe you also have to disable the voltage sensor for the 14th circuit. It might allow charging, but certainly not operation if it's still active (and reading a dead short).
I think you are both right. The used laptop batteries was a test build just incase I made an error. I have other new unused batteries which have a 2500mah rating. Today I built a 14s 7p battery with them in the same way I did for the laptop batteries and it works a dream.

I think it could be the extra sensor wire coupled with the laptop batteries. However I think the actual batteries are okay as its holding charge so that leaves me to think its the BMS with the extra wire. Maybe I might have caused the problem by wiring in the 14th wire but I don't know.

Anyway I would just like to say thanks to those who responded in helping me identify the problem. Next time I will look to use an appropriate BMS to save me any worries or concerns. I think I'll get a 13s BMS and hook that up the laptop battery and see if it works. They were 90 of the best ones so fingers crossed. Right time to get on Aliexpress.

Cheers Guys thank you very much. This site is pretty cool. If I have more problems I will certainly add them on here to see if you can help.

Kind regards



Well-Known Member
Good luck Dowsy! The real experts are on the Endless Sphere website. You should post all your data and photos there. Before they jump on your use of used laptop batteries, better include data that shows they were holding charge before you built the pack. Just sayin.

A less judgemental but still helpful group is Endless Sphere Facebook group.