Electronics > Power/Renewable Energy/EV's

Re-use old 18650 cells

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gpoint:
Hi there,
Hope to get some gurus here to have a discussion.
We have purchased a few new dyson V6/V7 battery packs to replace our old ones. Some of the old battery packs were still not "fully dead" after a few years usage, they still lasted couple of minutes to run the vacuum cleaner. An idea came to my mind, why not re-use the old battery packs for my ebike (i don't need large capacity for the bike for my short distance travelling) rather than dumping them.
Hence, I took out the battery pack, each 21v, and i put two in series to make up 42v for my 36v eBike motor (Many many thanks to tinfever from this forum who provided me the excellent information to activate the BMS by putting a magnet next to the switch!!!). I did some searching and found that my 36v ebike motor can take up to 52v input. So there it went.
I did a bit of soldering of connectors and hook it up to the bike and gave a go yesterday afternoon.
And guess what? It ran like a charm at the beginning (i could feel more powerful than my original 36v eBike battery)!
But but... After about 3-4km, I smelled burning/sulphur from the battery packs! That was really scary to me, i quickly got off bike and unplug the power cable, and felt the battery was just normal warm by hand. Then i got on the bike again for another 3-4 km, same smell again, I had to stop using for my scary heart.
I am not a lithium battery guru, any suggestion/discussion are really welcome and appreciated.

Could it due to the ebike motor withdraws much higher current than the handheld vacuum cleaner? But if that's the case, the BMS should have kicked in to cut off?

PS. i feel that if i don't aggressively pressed the throttle (i mean pressing for long time), it seemed that the current didn't go with over-drawn.

Thanks in advance.

Siwastaja:
Cells are possibly running very hot, maybe plastics / tapes / whatever around them are giving the smell. The 18650 cells Dyson uses definitely will have some passive safety built-in, but it's not a good idea to rely only on that. 18650 PTC cap tends to trigger around roughly 100degC surface temperature.

What could the BMS do? You have not added temperature sensing to the cells, after all.

Second-life use is always problematic, but it's especially problematic for relatively large discharge currents, because aging mechanisms usually hit the internal equivalent DC resistance more than it hits the capacity. For example, End-Of-Life is often defined as 30% capacity drop, or 100% DC resistance increase, whichever comes first. Old cells that have half the capacity left may easily have 2-3 times the DC resistance. In other words, the fact a vacuum cleaner is only running for say 6 minutes instead of 60, does not mean there is 10% of capacity left; it can also mean internal DC resistance is so high that low-voltage cutoff limit is met after discharging only 20% of the capacity or so, because of the U = R*I drop of the cell! But these cells could perform just fine in low-current application.

Therefore, I would not try to use aged cells for loads that need more than say 0.2C average current or 0.5C peak current.

"Small old pack is fine because I only make short trips" is actually the worst case, because that implies high current relative to the battery capacity.

And, of course, I cannot recommend doing any of this at all, so always remember there is a risk involved.

geggi1:
Diffrent battery cells are made for diffrent use.
If you look at LIPO batteries used for RC stuf you will see that they have very high discharge durrent.
For your Dyson cells this might be cells made for lower discharge current.

gpoint:
Thanks for the input.
Yeah, it could be the reason.
I checked the specs of the eBike controller, it says normal 10 amp with a maximum of 15+-1 amp. And it was a bit of slope when it had the burning smell.
Anyway, that was a good try to know that the battery packs are almost dead, although it outputs a good voltage but running out in short time.

Cheers.

thm_w:
From the photos I can find, dyson V6 is using 6 18650's in series, 1P S configuration. https://www.18650batterystore.com/en-ca/products/sony-vtc4
They are actually really high drain cells at 30A. At peak the dyson might draw around 14A. So not far off from the bike.

But as Siwastaja says it probably doesn't make sense to use old cells like this, their performance can be severely degraded. Unless you've actually verified they are still good capacity/esr/etc.

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