Aged 20S battery out of balance

Mar 14, 2022

I've owned this 20s custom battery (made with boston power cells) for 3 years and it's older than that by a few years, I've only given it 180 cycles. I've never had issues with the battery itself, but last night the BMS cut off 3 times and after inspecting it noticed group 1 was at 4v, group 2 at 3.18v (is that big of a disparity even safe?) and the remaining 18 groups were all around the same balance 3.6ish. I'm using xiaolang blutooth BMS to find all this out and the voltage disparity score was 64 with a big red symbol when you first boot up the app, it showed .74. Guessing this was the voltage disparity.

Obviously this is an old battery, I'm mostly wondering is it safe to use. I've had another custom DIY battery have an 'incident' let's just say and I'm very weary of these. No shop would ever try to repair it nor do I because I lack the expertise, I'm wondering what the issue is (dead cells or just not charging right/unbalanced), and should I stop using the battery immediately? And I guess side question, safe vendors to by a replacement battery that I can trust?
With that big volt difference and cells both high and cells low i think there is an issue with the bms readings or connections. I’ve had similar issues with this type of bms when a balancing lead had only intermittent contact which makes the other cell readings to error. You can confirm it if you measure on the cells and find different voltage values there.

First: if all cells have good voltages when measuring on the cells then it’s just a bms measurement error, check all connections to possibly find a root cause, you might need to swap to a new bms.

If cells really have these large differences then i’d try to charge them so that they’re properly balanced, and monitor what happens if you use it for some cycles. If they continue to go out of balance then it’s time to retire the pack

There’s a fair chance that the battery is damaged, and we don’t know how long it might have been going on.
Be aware that there could be fire risks while continuing to use this pack.
Appreciate the information. Do you know of any reputable battery builders that can make a battery like this safely? I'm located in the tri state area (NYC) and would prefer to get something locally, but if I would need to buy one shipped that would help. I've heard of lite speed in New Jersey but I've also seen many reviews talking about scams and never receiving their product.
em3ev which has been on the forum for a while with good reviews:

Grin is high end, western prices, but worth it in many cases:
larsb said:
em3ev which has been on the forum for a while with good reviews:

Grin is high end, western prices, but worth it in many cases:

I emailed EM3EV who said they no longer make custom batteries and the highest voltage they go up to is 14S.

I've heard about, I guess despite the price, they don't offer the amount of parallels, I'm looking for at least 20amph, probably around 25amph.
update: charged to 100%, same issue has occurred, group 1 pretty much untouched at 4v, 2nd group way below the rest, and groups 3-20 all balanced between eachother. I'm pretty sure something is wrong with the pack and am seeking a new one immediately, most likely not even worth it to try to repair to do its age, and I'm positive the person that built this wasn't very good or experienced.

EDIT: If anyone wants an update, the battery was even more dangerous than I knew. Previous owner (who lurks here btw) lied, he put in a BMS bypass that wasn't doing literally anything. Thing was a walking bomb, scary to think about. Person said he was trying to look out for me at one point, sold me something that was a serious fire hazard and a BMS that did absolutely nothing except showed cell levels. Battery literally soldered, zero insulation no cell holders, literally taped up with duct tape. I went to a shop in NJ named powerful lithium that built me a new one with everything I wanted. I reccomend everyone go there and dodge a liar and a thief ebike fraud guru named Matt from the Bronx.
Last edited: