Please help me! Battery reading at 4.2v

I built my board and it ran, did everything it needed to so I started designing the enclosure to 3d print it. The board sat for a month (with bluetooth bms and vesc plugged in) and now I am getting everything set up to put together and the battery is outputting insanely low voltage. I made a 10s5p battery out of samsung 30q’s so it was originally max charge at 42v. A couple of weeks ago I checked the voltage and it was at 38v. Now for some reason it is at 4.2v which shouldn’t be possible. Did the bms bluetooth module or vesc slowly drain the battery? Should I make an anti-spark switch to unplug everything in the case the board ever needs to sit again?
Any advice on where to go from here or why this is happening?? I was so excited to have the board together by next week…


There’s a button on the back of the display, make sure it’s set to the right series count.


Yeh, I just included the picture to show roughly what it is reading and how my wiring is set up. I have a multimeter and that is what read the 4.2v.

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Plug in a charger while measuring voltage on the battery side (pre-BMS). If the voltage spikes to 40+ or goes rapidly up from 4.2, then unfortunately your battery is probably toast and you’ll need a new pack.


^and thats where I would have cut the battery lead. Cause this mofo is getting drained.

If you were at full charge and assuming a “couple of weeks ago” is 3 weeks, then you let it sit for 1-2 weeks. Lets for simplicity say that 38V=50% battery capacity on 10S. that means your pack drained roughly 1000mAh a day… Not sure what you thought when you checked the voltage but your reaction should have been: Holy Moly Batteryman, This isn’t very good!


I forgot to mention that it sat for a week in between checking and it didn’t drain really at all so I just assumed that it wasn’t full charge by the time I got all the electronics all set up. It still probably wasn’t right move but at the time the rate that it was going down while sitting wasn’t enough to set off any flags.

Smart bms? What are the p group voltages?

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If the battery is at 4.2V then charging rapidly might damage it more than it already could be

I would discover as much information as possible first, including the P-group voltages as @deucesdown suggested


I get you there, but best case senario here is one P group is at 4.2 and the rest 0, or all the P groups are at 0.42v individually, either of which I cannot see how this pack could be any more damaged than trash already unfortunately.

Edit: in my head the rapid charging would simply be to see if the battery goes out to the recycling bin, or the problem lies not in the pack.


Put a meter on it them batry testeres are not the most reliable thing could be giving you false information

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So it looks like most of my p-groups are at 0.65 to 0.7v, but there are two that are at 0.15v. If I could save as much as I can from this pack that would be great, this whole build has ran me up a pretty penny and if I had to rebuild the entire battery pack then I don’t know when I would be able to save that up… Should’t the bms have stopped this from happening?

Yes, depending on how it was wired up.

Show us how you wired it.

Idk if you can see it in the picture but my wiring set up is that the negative lead goes from the battery to the BMS, then the negative goes from the BMS to the VESC and the positive goes straight from the battery to the VESC. I also have a positive to a charge port and the negative of the charge port to the BMS.


The BMS may not be free of blame here. It could’ve had a malfunction and caused the discharge itself. Hard to say without testing it with a fresh pack. As far as recovering the pack, only way you can try to recover it is putting on a charger at a low amperage and see what the cells do. Since none of the cells reached absolute 0v, they might be damaged but still usable. Attempt to charge it and keep a close eye on all cell temperatures for the first hour. if any cells heat up dramatically, pull the charge immediately.

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You meter is hooked straight to the battery no? If so, there’s your problem. Your meter drained the battery.

As @BluPenguin said, you can charge them back up to see if they’re usable. Just use very low amperage. Less than 1a per cell. .5 is better. High amperage charging an over discharged battery can cause shunts (I believe is the term) in the battery. Which can lead to catastrophic failure.

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No I just plugged the meter in for the picture, it had been unplugged. How do you advise I charge in since it is all soldered up in a pack? I only have the one charger that fits the plug I wired on there.
Thank you for your help

Without an adjustable CC-CV power supply or charger, your only option might be plugging in your regular charger and hope for the best. how many amps is your charger? You have a 5p pack so anything close to a 2 to 4A charger should be fine for this purpose

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So I have a 12s5p that had a bms fail on me and packs went wonky. Bad. In 12 groups, I have voltages ranging from 3.27-4.04v. Its apart in groups now.

I also have these:

Which I’ll use to charge each group. You’re a 5p so after sag each cell may get .20a of charge. Mostly like less so you’ll be fine.

Charging them is a long process. The waiting for them to charge part that is lol.


Unless you take it apart, it’s a tedious process with the charge boards. I’m assuming you don’t those means?

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On ok, yeh my charger is a 2a one so that would put out .4 amps to each cell right?