How hot is too hot for NiMH AAs?

[y_p_w]

I was just kind of playing around with some older Sanyo Eneloop (HR-3UTGB date code says 2013) batteries trying to kind od drain them in a Coleman 4D inflator using AA to D adapters. I wasn't sure how long it would last, but when it started getting weak I figured that it was probably time to stop. It wasn't fully charged when I started, but it got to that point after about 6-7 minutes. So it was probably putting out several amps. And then I took them out and the batteries were too hot to handle comfortably.

What kind of damage could this do? I get that NiMH battery packs in power tools can get pretty hot, but then again those battery packs aren't rated for 1800 cycles like these Eneloops. I thought that one reason for lithium-ion besides their capacity is that they don't tend to heat up as much from high output.

 

[ekrampitzjr]

Might have been too fast a charge rate. I remember the era of NiCads that took overnight to charge.

 

[blufeb95]

I think power tools are still using NiCad in a lot of cases and they can take higher drain rates, 9 year old nimh batteries probably have a lower capacity and higher internal resistance than when they were new, for a high continuous current draw application a primary battery or true d cell nimh is probably a better choice

 

[y_p_w]

I think power tools are still using NiCad in a lot of cases and they can take higher drain rates, 9 year old nimh batteries probably have a lower capacity and higher internal resistance than when they were new, for a high continuous current draw application a primary battery or true d cell nimh is probably a better choice

These have barely been used. I've kept my sets of 4 together and cycle them through assorted devices like radios. But for the most part I'd be surprised if they've gone through more than maybe 30 cycles in those 9 years. It's my understanding that the Sanyo (and successor) low self-discharge batteries are pretty good when it comes to aging. I have some that are still strong after 15 years. However, I've never gotten any this hot before.

I just waited for them to cool before tossing them in a Panasonic 300 mA charger to charge overnight. At least none of them showed a failure.

 

[930.engineering]

What kind of damage could this do?

Aging. Batteries are low in power while cold, but they age when hot.
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[y_p_w]

Aging. Batteries are low in power while cold, but they age when hot.
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I remember the temp was similar (maybe a bit hotter) to some "fast" chargers that I had that charged at 1000 mA. I think it was from Maha. It had two modes, but I stopped using the fast mode because the temperature seemed way too high. I pretty much only use the Panasonic charger now.

However, I've never had a NiMH battery get that hot. I used to drain NiCads with a paper clip before recharging them in a timed/dumb charger, and they'd get warm but never too hot to handle.

 

[SubLGT]

From Google:

https://eu.industrial.panasonic.com/sites/default/pidseu/files/downloads/files/id_ni-mh_1104_e.pdf

Absolute max discharge temp is 55degC, but for best service life keep it below 40- 45degC