Did an NiMH cell just vent?

[Infraviolet]

Just a quick question, I was sitting at the computer with an NiMH AA cell charge running behind me and heard a sudden "hiss" sound, fairly quiet and lasted about 1 second.

Was this a cell dying while charging? I put a finger to the cells (two charging together in a commercial charger, Ansmann branded charger and cells), both were hot but not more so than usual during charging. The charging cycle appeared to complete normally a few minutes later.

There's no visible damage to the cells at all, but I cannot think what else in the room could possibly have hissed like that.

Is there any electrical test I can easily perform on the cells to know if one has just done this? Preferably not one which needs them fully discharged again before recharging again. I'd rather not just dispose of them on suspicion of having vented if they might actually be fine and the hiss came from elsewhere, but would rather not put them back in to use if one has vented itself.

The two measure 1.455V and 1.454V right after the charge cycle ended.

I don't have any fancy internal resistance meter on hand, just several voltmeter/ammeter/ohmmeter multimeters, and an o-scope.
Thanks

[fzabkar]

Is this some elaborate ruse to cover up a fart?

[IanB]

NiMH cells can vent with a hissing sound while charging if they are old, and/or if they are being charged too fast.

In terms of charging, a 2 hour charge is about the limit, but a 4 hour charge or slower is preferable. A 1 hour charge is definitely too fast.

[Phil1977]

It easily may have been a vent, but that doesn't necessarily mean the cell is bad.

Some gas is produced during normal operation. Usually it escapes very slowly diffusing through the seals, but if the are extraordinarily good then it may escape by venting.

Best indicator if NiMH-charging goes right is the cell temperature. At end-of-charge the cells should be warm but not untouchable hot. Also 1C-charging rates are okay if temperature is healthy.

[IanB]

Best indicator if NiMH-charging goes right is the cell temperature. At end-of-charge the cells should be warm but not untouchable hot. Also 1C-charging rates are okay if temperature is healthy.

I tried a 1C charge on an AA Eneloop as an experiment (1.6 A charging current), and the cell vented with a hiss of gas while still cool. (The hydrogen recombination reaction goes faster at higher temperatures.)

If the cell vents, it represents a loss of electrolyte from inside the cell, which is not good. NiMH cells are hermetically sealed and are not meant to vent at all during charging. This is why a freshly charged cell may bulge slightly at the end, going back to its normal shape as the internal pressure subsides.

[Phil1977]

I have more than 20 AA-Eneloops in continuous usage, partially for over 15 years. I regularly charge them with 2.1A (Ansmann Energy 4 Speed). So far all the AA-cells are in well usable condition.

I didn't find a reliable datasheet of the eneloops on the fly, but what I found specifies 2A as the quick charge current. https://eneloop101.com/wp-content/uploads/2017/02/HR-3UTGB.pdf

But of course you´re right that they should not vent in normal operation, at least not frequently.

[IanB]

It is true that the Eneloop data sheets have described a fast charge as 1C.

Personally, I feel that the cells will last more cycles if you don't charge them so fast. I rarely have a need to do a fast charge, especially if I have previously charged cells standing by and ready to be used.

[Infraviolet]

Just to get back to the heart of the matter, how can I tell if this did happen? Is there an obvious way to tell post-venting?
Would I expect to see a lower, or zero, voltage on a vented cell?
Is venting always pretty destructive (atleast some amount of buckled casing), or can the cell look visually entirely normal afterwards?
And would a venting event be potentially quite quiet, or would it usually be a pretty loud pop sort of thing?

[IanB]

No way to tell, unless perhaps you examine under a magnifier and see traces of deposits around the seal.

You won't notice any obvious change in the electrical properties of the cell after one event. It will look and behave quite normally.

I have only encountered it once, and it just sounded like a short, quiet hiss.

NiMH cells have a starved electrolyte design, so they have no spare electrolyte to make up for any losses. If a cell regularly vents during charging the inside will gradually dry out, and after a while its performance will degrade. This is an irreversible change, and if it gets too bad you will have to replace the cell.

[Infraviolet]

"It will look and behave quite normally."
Ah, ok, hand't realised that. So a vented one is good to keep using as normal unless/until you do note worse performance or visible damage?

NiMH has the obvious advantage over lithium ion that it won't catch fire, but I had still always thought that as soon as something went even a little wrong with an NiMH cell the cell wouldn't be good for much for any longer.

[ConKbot]

In terms of charging, a 2 hour charge is about the limit, but a 4 hour charge or slower is preferable. A 1 hour charge is definitely too fast.

Id caution against charging too slow while fast charging NiMH cells(I.e. charging faster than a 16/24 hr trickle charge). Because charge termination is either temperature spike (if there's a sensor) or -dV where the terminal voltage dips a handful of millivolts as the heating spikes when the cell reaches full. If the charge current and temp spike is too low or slow, then the -dV signature is smaller and easier to miss and overcharge the cell. Where it will run till it hits either gross overtemp or cell over voltage to terminate the charge.

[IanB]

In terms of charging, a 2 hour charge is about the limit, but a 4 hour charge or slower is preferable. A 1 hour charge is definitely too fast.

Id caution against charging too slow while fast charging NiMH cells(I.e. charging faster than a 16/24 hr trickle charge). Because charge termination is either temperature spike (if there's a sensor) or -dV where the terminal voltage dips a handful of millivolts as the heating spikes when the cell reaches full. If the charge current and temp spike is too low or slow, then the -dV signature is smaller and easier to miss and overcharge the cell. Where it will run till it hits either gross overtemp or cell over voltage to terminate the charge.

This is absolutely true, and it depends a lot on the quality of the cell and of the charger.

I have found that Eneloop cells and compatible Panasonic chargers are a solid combination that terminate charge reliably at C/4 or C/5 rates. Good quality chargers have temperature sensing and use that to aid termination. Even at C/5 rates, cells will get measurably warm when fully charged. Some chargers have covers over the charging compartment to minimize heat loss and improve the efficiency of temperature detection. Chargers may also recommend charging in a vertical rather than horizontal orientation for similar reasons.

[thm_w]

"It will look and behave quite normally."
Ah, ok, hand't realised that. So a vented one is good to keep using as normal unless/until you do note worse performance or visible damage?

You can get a lithium/nimh smart charger for <$20 that will tell you the capacity of the cell.
Use that when charging, or do a dedicated discharge test, and you'll know when the cell is no longer useful and to recycle.