20V output on 12S3P battery

Rode 2 days ago and seemed fine. Went to ride yesterday and it only powered on briefly before shutting down a couple times. Now it doesn’t power up at all.

I’m getting 20V output on a 12S3P. It should be fully charged so it should read 42V or so? I believe the multimeter is good. I tested a couple new AAA batteries and got a 1.6V reading.

What you guys think? Bad BMS? I’ll be happy to open this up to take a look, just don’t know what to look for.


20v on a 12s3p is severely depleted… it has to be 50.4v. You can check if that battery’s bms is damaged. Im not sure if you have the right reading here though…

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so am I just looking for physical damage on the BMS? Burns or any sort of disconnections?

I was thinking the same thing with the multimeter… my last one was literally broken, but I tested it on some AAA batteries and it gave the right reading. Just installed fresh batteries in it as well.

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I would recommend testing the negative side and the positive side of the battery instead of the bms discharge lead to see if the battery is reading correctly. The negative side of the battery is where the b- wire is soldered.

Do be careful though. That voltage level is severely dangerous.

Adding on, is your charger a 42v charger or a 50.4v battery charger? This only happens when its being undercharged with the wrong battery charger.

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BourgeoisBastard said, “I’m getting 20V output on a 12S3P. It should be fully charged so it should read 42V or so?”

WARNING: given that the battery pack is suspect/failed/failing, I strongly advise doing any further work/evaluation outside on open ground far from any combustible materials. There is a real risk the battery could catastrophically fail at any time. This can mean fire and/or explosion. Read up on the protocols for how to handle Li-ion battery fires. You should also consider keeping it similarly isolated at all times until you determined it is working properly…example, don’t leave it inside the house/apartment unattended at any time unless it is in a verified fire safe container, preferably designed to contain Li-ion battery pack failures.

Note: for voltages, I’m assuming the battery pack uses Li-ion cells, not something like Li-FePO4 which produce a lower voltage per cell…

42V would be for a 10s configuration battery pack. You should be seeing about 50V for a properly functioning fully charged 12s3p pack. It might be the BMS, but it could also be one or more failed Li-ion cells in one or more of the 3p bundles…depending on how the pack is wired. Li-ion cells can fail without prior warning. I’ve also seen a cell spontaneously recover to work for a few more cycles, before then failing again, usually permanently.

If you do open it up, if you have access to the BMS connector that provides sense input from each 3p bundle of cells, then the voltage across each connector pin referenced to ground should be the same. If Li-ion and fully charged, then if operating properly each should read about 4.2V. Verify your sensor connector wiring and disconnect the battery power leads and then the sensor connector (if possible) before taking measurements. There would typically be 1 ground pin and 10 or 12 sense input pins depending on whether you have a 10s3p or 12s3p battery pack configuration.

Although this seems unlikely given what you have reported, you may also wish to try a different charger (meeting appropriate specifications for the battery of course) or verify your charger output under load is in fact capable of still charging to the appropriate voltage (e.g. 42V for 10s3p battery pack).


50.4 charger. I’ll definitely be careful and do on concrete outside when I check it out and follow @Exigent 's advice as well.

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wtf… it’s back to ~50V. Would you guys just advise not to use at all? … want to put it back together and go for a ride. :smile:

Examine for loose stuff before using/riding. Soneone decided to ride without checking all connections and weld joints… it didn’t end well for him.

Open the pack is a must now.

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I’ll check it out tonight and look for loose wiring, damage, etc.

On first photo multimeter was set incorrectly to 20v range and showed accordingly, on second one correctly to 200v range. End of discussion. Pack is most likely alive and well.

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Correct. But that didn’t solve the whole board not turning on part. Always gotta make sure. But if I can assume, probably wasn’t fully plugged in or loose connection. Usually is the case.

So 100% the battery is fucked up. I finally got the faulty 12S3P to power up the VESC earlier, but it just fizzles out after a few seconds. I have a 6S LIPO pack I use for soldering that I hooked up to confirm the VESC is still works. It stays on with the LIPO connected, but not the faulty12S3P.

Probably not the smartest move, but I just ordered a replacement battery from onsra. Needed to order some other parts from them anyway. Would have opted for a custom pack, but the wait times (understandable) were too long. I’m impatient af.

Going to try and repair this battery, but I need to do more research on how/what to look for and get some sort of water proof wrap to put back on after/if I get it fixed.

:joy: :rofl:

I don’t blame you. I brought my batteries from AEBOARD and they were samsung 35E cells. I just made it a 10s10p.

Best way is to just buy something for now and then swap it out later. I would look into buying a better battery while using the other one. A 12s2p p42A battery is like good replacement. You can go for a 12s3p if you like, but the cells are fatter.

The first thing is to open it. Don’t be afraid of removing the outer heat shrink wrap from it. You’re basically opening the pack, then proceed to use a multimeter to examine the voltage of each paired cells 12 in series 3 pairs that are welded together. Each one needs to measure similarly. If one p group is 3.3v and the others are 3.8, that means that p group wasnt getting charged by the bms. And if they’re all the same, that means the welding got loose on the battery and isnt giving the charge.

You must also check all solder joints while looking at it. This is usually a case as well. Do not use anything metal on the battery when checking. You just want to find the source of the issue and fix it accordingly.

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Alright so what am I supposed to measure with the multimeter on here?

Bottom end of the terminal and top end in the opposite side. It should be negative and positive. It should measure what the 12s should measure.

It looks fine to me however. The tape isn’t helping :confused: you can then measure the XT90 thats soldered on the battery discharge lead to see if you are getting the same reading. If not, the lead is probably broken and the bms no longer works as a discharge.

Then you gotta measure each p group and make sure they are consistent with eachother. Trust me, there’s a lot to do with a pack. I have to get into to battery making eventually. I gotta weld a new milwaukee 12.0 battery since i can only retain 75% charge :frowning:

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New battery came in from Onsra so I decided to put it all back together to test it out and maybe go for a ride. I plugged in the loopkey to access the VESC and holy shit! An XT-90 connector just started spitting fire and I immediately pulled the loopkey. It was hard to tell what exactly flared and it look a positive wire somehow made contact with the exposed metal on the backside of the charging port.

What do you guys think? Did the fuse blow? Maybe somehow there was a bit of exposed wire that made contact with the circular part of the charging port?

I would change it. Any form of heat will destroy the fuse. Trust me, i killed 3 of them trying to solder and forgetting to wait until it properly heated.

Silly me.

But do be careful and check all connections. This does happen.

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I fixed it last night and powered back up. Was sweating bullets when plugging in the loopkey. :joy:

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The new battery has a sticker on it that says 12S3P LG 18650, but I don’t know exact cells. The old battery has Samsung 35E or something like that so I set battery capacity at 9.000 aH in the VESC tool. How much does it matter if I get this setting incorrect?

You won’t get the right reading. It probably matters but i didn’t do a test with my 10s2p chinese batteries. I’ll check it myself and see if i find the battery. And then ill do a test to see if it matters.

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