18650 cells getting hot … what's the cause?

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wshyang
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18650 cells getting hot … what's the cause?

I’ve been going through some harvested 18650 cells, and the first test I run on them is to completely drain them down to 3V and charge them back up again, discarding those that gets hot during charge.

It seems like these hot cells show up at random. Sometimes they are just that odd cell that came out of a battery pack that gets hot, while the rest of the cells from that pack are perfectly fine. Other times they are that overdischarged cell. And yet other times they are that cell that came out of the pack with a good voltage.

In short, you generally can’t quite predict what cell will get hot until you actually charge it.

What causes these cells to get hot?

The biggest irony is that some of these do actually hold a better charge than others that charged cool!

Edited by: wshyang on 08/16/2012 - 21:16
scaru
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What charger are you using?

wshyang
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scaru wrote:

What charger are you using?

A Xtar SP2 on 1A.

kramer5150
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Are you certain its the cell heating up, and not the charger circuit getting warm and warming the cell?

I ask simply because my WF-139 gets pretty warm when I charge up an 18650 from a completely discharged state. For the longest time I thought my cells were the ones getting warm, but its been the charger circuit heating up the cells all along. I was not able to confirm this until I made separate wired cradles to charge cells outside of the WF-139.

I have split open at least 25-30 laptop packs over the past ~5-6 years, and have only encountered a hot cell condition maybe 2-3 times. Hot cells are a sign of over-charging. More likely in this senario you have a cell thats been chemically damaged and can’t peak at ~4.15-4.20. In the latter scenario the cell peaks at (say) 3.98V, the “plate” reaches its electron capacity, and it can no longer store. I think the charger keeps on passing current, trying to load the “plate” with electrons, and the cell heats up as a result of the free flowing current.

I have worded this in simpleton terms, allow others to correct me on my wrong points… but in a nutshell I think this is what is happening. FWIW I almost never charge cells unattended, I am ALWAYS at home at least… especially freshly harvested cells on a first charge.

Do you have a way to meter the voltage as they charge? Even an inexpensive DMM can serve you well if all you are doing is monitoring battery voltage.

budynabuick
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kramer5150 wrote:

I ask simply because my WF-139 gets pretty warm when I charge up an 18650 from a completely discharged state.

Kramer, what do you consider ‘pretty warm?’ My 139 never gets over 95F (35C) which feels ‘slightly’ warm..

Keith

wshyang
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kramer5150 wrote:
Are you certain its the cell heating up, and not the charger circuit getting warm and warming the cell?

I ask simply because my WF-139 gets pretty warm when I charge up an 18650 from a completely discharged state. For the longest time I thought my cells were the ones getting warm, but its been the charger circuit heating up the cells all along. I was not able to confirm this until I made separate wired cradles to charge cells outside of the WF-139.

I have split open at least 25-30 laptop packs over the past ~5-6 years, and have only encountered a hot cell condition maybe 2-3 times. Hot cells are a sign of over-charging. More likely in this senario you have a cell thats been chemically damaged and can’t peak at ~4.15-4.20. In the latter scenario the cell peaks at (say) 3.98V, the “plate” reaches its electron capacity, and it can no longer store. I think the charger keeps on passing current, trying to load the “plate” with electrons, and the cell heats up as a result of the free flowing current.

I have worded this in simpleton terms, allow others to correct me on my wrong points… but in a nutshell I think this is what is happening. FWIW I almost never charge cells unattended, I am ALWAYS at home at least… especially freshly harvested cells on a first charge.

Do you have a way to meter the voltage as they charge? Even an inexpensive DMM can serve you well if all you are doing is monitoring battery voltage.

Thanks for the long detailed reply Cramer Smile

Yes, I am quite sure the cells are getting hot. Usually the telltale hint for me is that the adjacent cell is relatively cool (although warmed up thanks to the hot cell), and that the hot cell is actually REALLY hot to touch.

I am quite puzzled by what causes these, I suspect it is the overcharging thing as you said, but my luck definitely has not been as good as yours. In some batches of laptop cells I might end up having to catch at least 1 out of 3 cells for the hot issue.

Really peeved. If only they can be saved somehow.