Puzzled by battery numbers

I have a pack of 18650cells, which is made up of 2 6s blocks with a fuse inbetween. Each block has balance lead that I hook up to a CellLog. One wire broke off, so that rendered one of the balance leads useless, but I didn't think it mattered for the most part.

I bulk charge the 12s pack and stop when the CellLog beeps at the programmed 4.20V. So, in the end I'd have 50V.

I decided I'd resolder the lead back, and when I checked that pack, all the cells were at 4.18V after my ride home. However, on the other pack, the cells were at 3.87V

Is it normal for one pack to be so different than the other?

It's impossible for one pack to be at 4.18v after your ride home. They'd drop to that within 100 meters of starting. You must have measured after charging or your meter needs a new battery.

I would look at that voltmeters accuracy.

One explanation would be one pack started out at 5v.

Bring them both to full charge, then check the voltage of each pack. I think both of us are assuming you run 12s. If you run 6s, (24v) then you had a bad connection, and only one pack discharged.

I bulk charge the 12s pack and stop when the CellLog beeps at the programmed 4.20V

Click to expand...

Once you've verified that the measuring instruments are accurate...I would strongly suggest that you begin a regimen where you charge to a max of 4.1V per cell (and 4.0V is better). If you charge to 4.1V per cell, it might double the life of your pack.

Tesla has an eight year warranty on their 18650-cell lithium battery pack. This is not a typo. Recently a Tesla-S taxi that was two years old passed the 300K mile mark, and the pack still provides over 90% of the original range....

4.2V is bad. Doing that is actively throwing away half the life of the pack...

running 4.1V to 3.0V is better

4.0V to 3.3V is best (if possible)

since the blocks are definitely unbalanced, and I was only measuring the lower one, it's very likely that I did overcharge the other block. I've probably ridden like that for a while.

I checked the "resistance" of the cells, and the values for the overcharged block are definitely higher, probably from the abuse.

OK, I will change the alert value on the CellLog. Now that I resoldered the balance lead, I'll be more aware of balance issues.

I doubt that you over-charged it. Once the voltage reaches 4.1v it rises rapidly as further charge is added. There's very little charge between 4.1v and 5v. The reverse happens when you discharge. Say you did charge them to 5v, they'd still be down to 4.1v by the time you reached the end of the first street.

Good point,, maybe the cells that have shitty capacity are the ones that got overcharged, or damaged in some other way, like over discharge. Or just that set has a crappy cell or two in it.

Best way to skunk it out would be a resistance test on each cell group.

The cells that were at 4.18 have a range of mOhm: 014, 011, 09, 11, 11, 10. after sitting, these are all around 4.19
The other cells that were lower are: 09, 05, 07, 05, 06, 06. after sitting, these are all around 4.14, 4.15V

Today was my first ride since balancing the packs. Before I left, one side was 4.19V, and the other was 4.14V. Upon arriving, both sides are pretty much at 3.87V. I'll bring them both up to 4V before going home and test again.