Diagnosing Battery, Bms, Charging issues

Didn’t find much in the way of discussion with this topic. So I figured we should start one up because power issues seem to be pretty common.

Any ways, my charger says my battery pack is charged. I go for a ride and I notice diminished range. What the hell is up? I get home charge the pack, I pull out my multimeter and measure some voltages. Results? Hold your horses.

12s Li-ion battery, Bestech D140 charge BMS, 12s 5a charger. Fused charge port(8a) and charger(10a).

First we need to check the voltage on the main terminals of the pack. My readings show 47.6v at the main terminals on the battery pack. Then leaving the positive probe of the multimeter at the main terminal and taking the negative probe to the B- on the bms. Readings show as expected 47.6v. Continuing to the P-(or C-) of the bms with the negative probe I read 45.6v. A -2 drop in voltage. Yet my charger says pack charged.

Connections including physical inspection of cells? Good.

Fuses? Good.

Temp readings on each cell with laser temp gun? 74-75°F, ambient 75-80°F.

Individual series voltage? S1 4.01v, S2 3.96v, S3 3.97v, S4 3.89v, S5 3.96v, S6 3.95v, S7 3.97v, S8 3.95, S9 3.95v, S10 4.1v, S11 3.95v, S12 3.79v. Seems ok, nothing a bestech D140 can’t charge.

Conclusion, faulty BMS because of -2v difference?

Anyone able to chime in on anything I might have missed?

PS, clear heatshrink is nice for visual confirmation of good connections.


Yeah it kinda sounds like a faulty bms, a few of the cells are a bit lower than I would expect after charging too. 0.2v difference is more than I ever see on my BMS packs, granted they are lipo, idk how much of a difference that makes but I think it may be the BMS as well that is faulty in your case.

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It is bypassed. The p- is going directly to the negative of the charge port only. The vesc has it’s own set of main leads from the battery.

I took the p- reading with the charger unplugged.

Bestech d140 p- and c- are same port.

i concur here… fairly affordable for single/double stack “single” rows… but finding double wide can cast a hair more.

no info on your issue at hand =(

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yeah id say 4 and 12 are the biggest anchors possibly keeping things from balancing out…


Thanks, this seems to be more accurate than my original diagnosis. How ever, when I made the pack the cells were off by more than .05v, (closer to .2v off) I hooked everything up, charged the pack to full, checked each series and all were within .02v.

I will try your solution to root out that issue. Thanks for the quick response.

Ive done this in the past with builds, being eager to finish and ride… whats best IMO is to try and group your cells by voltage during your build, and then charge up those groups to meet the other groups with highest V. AND/OR if lets say 3 of 4 in a parallel group are similar Ill let that group rest a bit to balance out the one cell…then go back and self balance the whole whole…

but those previous build were really hard to balance out when I just slapped everything together and hoped for the best…

my last 3 batteries all groups are exactly the same which makes me feel good lol