NiMH cells emit smell of gas

I have a rapid charger with a fold down cover. When I charge a couple eof AA or AAA cells there is a smell of gas in the chamber under the lid.

Seems to me that the cells are getting overcharge and are venting.

This is a charger with a sensor for "negative delta-V" to edermine the end of the charging. The cells are about 10 to 15C above room temperatre.

Is the charger cooking my cells? Or is the smell noticeable because of the enclosed battery compartment?

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Jon D
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Possibly the cells had vented previously and the seal is now open. I would throw them away, then wonder why they'd gotten hot enough to vent previously. Some Delta-V chargers only have a Delta-V detection but not temp monitoring, so if a cell were fully charged and an event like AC power flicker occured at long enough duration to reset the charger, it might then start charging at the fast-charge rate again, but there will be no Delta-V to detect since the cells were past that stage already... so if they get too hot, they vent which was better than an explosion.

At the very least if you continue to use that charger, you might not want to fold down the cover to increase passive cooling. There are other things you could do like pointing a fan at the cells but it seems a hassle instead of just seeking a different charger, IF it seems the charger doesn't have the temp cutoff feature.

No with undamaged cells and proper charging (not overcharging) there is no gas escaping to be building up in the compartment.

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You need a better charger, like a Maha MH-C401FS.

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deltaV doesn't work very well with NiMH cells because NiMH voltage either doesn't drop or drops only a very small amount when full charge approaches.

Some chargers are much worse than others. The best use not only deltaV but also temperature, temperature difference, rate of temperature rise, and, as a last resort, time.

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