Newb questions about battery back construction

[Stiffler359]

Hello & thanks in advance for not being too hard on me for my ignorance!

I've been helping a friend of mine out with his ebike company, where I've been focussing on the electrics (I have a pretty good understanding of low voltage because of the industries I've been in) while he focuses on the 'look'. Recently he's asked me to swap out the BMS's on a few of his packs because they are apparently fried, but after disassembling them I've found several damaged (burnt) cells. In an effort to salvage the cells that were not damaged, I completely disassembled the pack.

The packs in question are Calibike 20s4p packs using Model 3 2170 cells. After nearly burning the shop down (use your imagination) I got the cells all separated & measured all of their voltages. 4 of the cells measured slightly negative, 4 of them measured about 3.7 volts, and the rest (well, except for the roman candle) measured 4.28 volts.

I'm now in the process of discharging them all on a bunch of Opus chargers, but before moving any farther I have a few questions:

Isn't 4.28 volts a little on the high side? The packs have been sitting around for months, so they were probably even a little higher voltage before. Could this be why some of the cells fried?

While pulling the cells apart the shrink wrap came off (the cells were all touching in the original pack, and had kinda melted together). If I reuse the cells I will use cell holders, do I need to re-shrink wrap them?

What voltage should the cells be at when I assemble a new pack? What is the process of balancing a new pack?

Thanks!

 

[JackFlorey]

Isn't 4.28 volts a little on the high side?

Yes. That is destructive. Do NOT try to charge them again. Things like dendrite formation can result in fireworks if charging is attempted.

While pulling the cells apart the shrink wrap came off (the cells were all touching in the original pack, and had kinda melted together). If I reuse the cells I will use cell holders, do I need to re-shrink wrap them?

You need to isolate any cell groups from each other. Cells within a group can touch since they are at the same potential.

It's a little worrying that you are "focussing on the electrics" for an ebike company and asking this.

What voltage should the cells be at when I assemble a new pack? What is the process of balancing a new pack?

They should all be similar. Most batch cells are shipped at the same SoC; you will likely see 3.6V or something. As long as they are all similar you can go ahead with the pack build. If they are divergent it can make sense to parallel them all and let them equalize themselves. (Do this with thin wire so radically different voltage cells will not try to equalize at 40 amps or something.)

Once the pack is assembled you should do a few 'equalize' cycles. Same voltage does NOT equal same SoC or same capacity; cells vary in total capacity. Thus, when you charge, you'll see some cells hit 4.2V (cutoff voltage) before others even if they started at the same voltage. A resistive balancer, which is the most common, will use a resistor to 'pull them down' while letting the other cells fully charge. It will take a few charge/discharge cycles to get them to all terminate at about the same time.

 

[Stiffler359]

Thanks for the info!

I should have clarified my role in the company: I only help out occasionally, and when I do I run wires between components; I don't build batteries. My day job is as a low voltage lighting specifier/installer, so I understand the ins & outs of wiring.

Cheers