Topup voltage sudden drop


New Member
I recently noticed that my maximum charge voltage went from 54v to 48.7~. I'm thinking one or two strings might have faulty cells . It still holds a reasonable charge but I'm worried that I might be damaging my battery if I continue using it. I just wanted to get some feedback before I cracked it open and measure it all.


Well-Known Member
That is a little over half charge. Since you are implying you can still ride the bike with "reasonable" performance, I wonder if your voltmeter is just wrong (low internal battery). Does your bike have a battery status display and is that correspondingly low, like half its usual value? What would the charger voltage be?

If it's real, then someone has to measure all 13 groups to start figuring out what's wrong. Be careful.

You might try a long balance charge first. You got 13 groups of cells in a 48V battery. The BMS shuts off charging when any group reach the max voltage, 4.2V. In a balanced battery, they all get there about the same time. In an older battery, one gets there first. The others are a bit slower, but if you leave it on charge, the balance circuits bleed down the first one and let the others catch up. When a battery gets really unbalanced, it make take a day or two or more on the charger to balance them.

I'd do this somewhere where it's safe, like outside. Battery charging is generally safe, but if the battery and charger both go bad, it's not good.


New Member
Thanks for the suggestion. I will definitely try a balance charge. I rarely do a full charge on my battery so it's probably overdue for one. I think it's still working reasonably well because it's a 20ah pack. 9 parallel strings of 13 like you said. I probably still have enough capacity at 48V for my daily commute.

Could it also be because I never fully discharge my pack? My cutoff for low voltage is 40+/-1 volt and I never discharged this pack lower than 47V since I bought it in March 2017.

I checked with two different and recently calibrated multimeters and both measurements agree (I work in electronics engineering and have access to calibrated equipment).

I think its time I buy a spot welder and build my own batteries anyway. Mostly so I can repair things quickly because it is my main mode of transportation.


New Member
So I left the battery to charge all night long and it topped up to 54.4V! Thanks for the tip! My other battery never did this but it had much less parallel strings so that may have been a factor.