Redoing the whole group is the simplest option, if more expensive.
If all the cells read the same voltage, then you’d want to measure the capacity of each cell and find the troublesome one, which could be done by de-grouping and testing them individually with a capacity checker (they can be had from the usual sites for a few bucks), or by stringing them all in series and discharging them, then checking to see which cell drops more then the rest.
Either way you’d need to break down the cell group, test, and then replace the bad cell and reassemble.
It is less work to just make up a new p-group from all new cells and swap it out, and then you could use the “bad” P-group for something else like a large power bank!
Sorry but how did u arrive at the conclusion that it’s a faulty cell @vegr0? More than likely just some broken welds aka a bad connection. Before you pull apart the group and start testing capacity and the like, add a bunch of welds to both sides, charge the group individually to bring into balance with the rest of the pack and cycle it a couple of times while keeping a eye on charge and discharge rate. If the suspect group is still not conforming to the other groups then look at the more invasive option. IMO you always start w the theory of broken welds in a cas like thus before the prospect of a faulty cell which is a) rare with legit cell manufacturers and b) requires you to deconstruct the group.
Absolutely. Bad welds and a bad cell look much the same when looking at charge/discharge conformity but the former is much more common and also, easier to address. Accordingly, add some welds to ensure a good connection and observe. If the group continues to behave differently than the rest, it would then be logical to assume you’ve got a bum cell and to either pull apart the group and test IR and capacity (time consuming) or just replace the entire group if you’ve got ~10 cycles or less (potentially expensive).
Bad welds charge slow because as the cell goes in and out of contact the capacity varies and it’s less efficient to charge the cell from the rest of the parallel group, bad cells charge fast because reduced capacity means more change in voltage per amp hour charged/discharged
If we were talking about one cell then I’d agree. But we’re talking about a group of cells. In the case of broken welds, the group would charge and discharge faster bc one of the cells is in intermittent contact (or possibly no contact) with the rest of the group and is essentially creating a situation where, for e.g. a 4p group is now a 3p thereby significantly reducing the group’s capacity and causing the group to charge and discharge more quickly. Similarly, if a cell in a particular group is faulty and is either drastically lower in capacity or completely dead, it would have much the same effect, lowering the groups capacity sufficiently so it would have a similar charge/discharge pattern.
really i would just go straight to replacing as im not sure how to do any of the capacity testings. the 6 cells would be a $40 fix. I only have 3/6 cells i need on hand atm, so i’ll try adding some more welds.
All other packs ive made (12s) have been perfect, all cells bought from 18650batterystore
Cheapest and fastest fix is rewelds so definitely reasonable to weld over and hope it works, just don’t obliterate the nickel
If you can’t get at all the cans because of a solder connection or something then I suppose you’ll have to disassemble the group and grind the cells down before rewelding
Its my 5th p-group. Just want to confirm that I count from my negative side where i started the BMS harness wire. That group lines up with “5” on the actual BMS unit so im 90% sure i know which one it is.
just wanna make sure its not on the other side of the pack so im not fixing the incorrect group
I’m also trying unplug as little as possible and leave the battery in the enclosure if i can. Do i have to unplug the XT90 from the ESC, or the harness wires from the BMS for this ‘operation’?
I think LiPo cells have a tolerance of +0.05V for a full charge, so I THINK this would be within spec for those. You either have a BMS for LiPo cells or your end-of-charge voltage is set too high.
Its definitely a smart bms for Li-ion. Is there a way to find those settings? The bms app i have doesnt have acess to alter or even read deeper data like that.
Most Li-Ion cells I’ve seen, including most LiPo’s, are rated at 4.20V, +/-0.05V, so technically 4.22V is okay. Though it does stress the cell more than a lower voltage would.
There are li-ion cells that are rated 4.30V-4.35V but they’re not common in the DIY communities. Some R/C packs use them though and some phones and tablets too.
No, it was my first solder attempt. In my defense I think it used to look a lot better, but after opening my enclosure I noticed some serious tugging happening on a few of my wires… I think that made it worse. Shitty job though, I know
