Overcharged 18650 battery pack - now what

[flyingbeekeeper]

Whoops, used the wrong charger on my homemade 18650 pack. About 17ah 13s9p made from recycled 18650s.

The cells accidentally charged to 4.59v. Likely spent 2-4 hours overcharged. The entire pack got warm which is unusual for it. My first instincts was to put a 1amp load on it and bring it down which Im' doing now.

The question for the forum: What effect will this have?

My guess is the pack now will have a much shorter lifespan. Maybe another 100-200 cycles instead of 500-800. I'd not be surprised if it only makes 50 cycles.

 

[Ykick]

If you don’t know/understand how to do IR (internal resistance) load testing this is probably a good opportunity to learn.

Even better of course would’ve been logging mOhm IR cell values while building the pack for comparison following an unfortunate event such as this.

Using RC Lipo I’ve overcharged to 4.35V/cell or thereabouts for an extra mile or two but it’s never sat there for very long. Personally running several 2-4 year old packs abused in this manner and showing no discernible differences and/or problems. Any guesses about compromised lifespan are just guesses. Means nothing, IMO.

I do however take IR measurements when new and have used those values to chart gradual (in most cases) degradation of cells over a long period of time.

Useful info, never regretted learning about IR - plenty of youtube RC Lipo videos, etc.

 

[dogman dan]

They might be fine, if they did not vent. But a very close look at them as suggested may reveal any individual cells that did.

The good news is if they seem ok now, not stinking, or lost a lot of capacity already.

Isn't the main danger that they might have run away while they were charging, or sitting there that high? If they didn't vent or get crazy hot, they may be relatively ok.

 

[999zip999]

Have you had watt meter and tested capacity, ah before overcharging ? So to compare ah now ? Or just go for a ride, you don't have to drain to lvc to feel a difference. Do you have sense wires on the pack for ez checking with a cellog ? I'm guessing you are not using a bms.
What's the amp draw on the pack ?

 

[flyingbeekeeper]

They might be fine, if they did not vent. But a very close look at them as suggested may reveal any individual cells that did.

The good news is if they seem ok now, not stinking, or lost a lot of capacity already.

Isn't the main danger that they might have run away while they were charging, or sitting there that high? If they didn't vent or get crazy hot, they may be relatively ok.

What signs am I looking for on venting? Do they expand? Do they vent out the positive tip side? Or at the seam on the side? Will I see a white goo like alalines?

 

[flyingbeekeeper]

Have you had watt meter and tested capacity, ah before overcharging ? So to compare ah now ? Or just go for a ride, you don't have to drain to lvc to feel a difference. Do you have sense wires on the pack for ez checking with a cellog ? I'm guessing you are not using a bms.
What's the amp draw on the pack ?

Yes, I do have watt meter specs for that pack. The 17ah pack is normally run at 0.75C-1C, and peaks of 1.4C which is a tad higher than it should be run. (recycled 18650 packs should be run 0.5C cont, 1C peak) I'll be using my misfortune to document what happens and post what I find here. Since no other member would intentionally overcharge the piss out of a 18650 pack to educate newbies on it, I guess I got volunteered. I'll post my findings soon.

 

[Skrzypas]

peaks of 1.4C which is a tad higher than it should be run. (recycled 18650 packs should be run 0.5C cont, 1C peak)

You can still discharge recycled cells according to their specs (which is usually 2C) if the cell holds capacity. Important is heat, so if you peak-load a recycled pack at 2C for a minute, let it cool down later to avoid temperature cumulation leading to thermal runaway = fireball.

Yes: lower currents, longer life. But recycled cells are here mainly because they are simply cheap...

Did you measure all voltages in the pack at the overcharged state?

 

[flyingbeekeeper]

peaks of 1.4C which is a tad higher than it should be run. (recycled 18650 packs should be run 0.5C cont, 1C peak)

Did you measure all voltages in the pack at the overcharged state?

Yep, all 13 cells were at or about 4.59. Except bank #3 that is a little weak, it was 4.54.

I just got back from a 9 mile ride on the battery, there was a hell of alot of capacity over 4.4v. Far more than I expected. My beliefs were that there was going to be alot of surface charge, but that was not the case.

 

[dogman dan]

With round cells, it could be very hard to see it if the can ever puffed slightly, or vented. But if you smell something, then somebody farted for sure. Kind of a sweet acetone stink.

But later on, any cells that vented will display obvious shitty performance. Self discharge, high resistance, low capacity etc.

Not really that huge an overcharge, if they were not too hot to hold I doubt anything vented.

 

[flyingbeekeeper]

For those who want to know more of what I've found, in the event they also have overcharged a pack-

After draining the pack to 50% SOC and then charging/balancing, I then drained to 90% SOC and balanced twice. I charge/balance to 4.15v. It now becomes apparent that one bank will not stay charged at 4.15v with the others as it did before the OE(overcharge event). I will remove the bank, break it up and test each.

My #3 bank was weak before the OE, and still is. Doesn't appear worse, but as long as I'll be cutting into the pack, I'll remove the oldest 18650 cells from bank #3 to test.

Overall, I'm rather impressed how well this survived the OE.

 

[Rutiger]

How did you go with bank #3? Was it only a couple of cells that had issues, were the rest OK?

 

[sonnetg]

Overall, I'm rather impressed how well this survived the OE.

The overcharge probably have damaged the batteries internally or reduced it's capacity. You probably wont notice any difference in the short term, but really curious to see how it behaves in the long term. Do keep us posted on this.

FYI, this is a very good source of information for all things related to lithium.

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

 

[flyingbeekeeper]

At this point, I cracked open the pack and tested all cells in groups of 2 or 3. The very old cells that are 5+ years old took the most 'damage' meaning a reduced capacity. It became clear that certain brands and models took the worst brunt. After a while, I realized that all the light green ones were trashed. Overall 25% was damaged heavily, 25% somewhat, and 50% no apparent damage. The clear trend was chronological age, ie. more modern 18560s took no damage.

So, I had a backup 13s9p pack that I chose to take the best of the survivors and make into a 13s13p pack. The survivors where marked with double stripes so I can track them well into the future. New pack is at least 26 Amp-hours, but haven't run it far to be sure.