Why the hissing/sizziling sound from my NiMh packs? Fried? - RC Groups
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May 09, 2006, 10:54 AM
magic612's Avatar

Why the hissing/sizziling sound from my NiMh packs? Fried?

I charged my 8.4V, 750 mAh GWS battery packs last night. Normally, I charge them at about 2/3 C (that's the lowest setting I can manage on my charger), but last night, I wanted to get out and fly after supper and before dark, so I charged one pack at 2C to charge it faster. (I'm using an MRC 959 Peak Charger.)

When the first pack was finished, it was making a hissing/sizzling kind of sound. I thought, "Batteries aren't supposed to make noise..." so I brought it outside and stuck it in a clay pot, just to be safe. So I went to charge the second pack, and this time charged it at a lower amperage, at about 1.3 C. It too, when finished, was making kind of a sizzling/hissing sound - not loud at all, pretty quiet actually, but noticeable when I stuck my ear next to it. Brought it outside too. Neither were any warmer than normal, and maybe I've just never noticed the noise before, since it is rather faint.

Looked at both this morning, didn't see any liquid that had seeped out or any visible damage externally, but I'm still wondering: Did I fry my battery packs?
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May 09, 2006, 11:01 AM
Use the Force!
LBMiller5's Avatar
Ni-MH cells have a safety vent in them that prevents them from exploding violently in the event of an over charge. If you charge them hard enough to cause the electrolyte to begin to boil, then gas pressure builds up in the cell. When it gets to a certain point, the safety vent cracks open to release the pressure before it gets to a dangerous level.

What you were hearing was the gas pressure bleeding out of one or more of the cells through the safety vent. In almost all cases, once this happens, the cell is no longer useable.

Lesson learned: Don't charge your batteries too fast, they don't like it at all.

May 09, 2006, 11:45 AM
Space Coast USA
hoppy's Avatar
IMO, the smaller the battery capacity, the less tolerant the cell is of high C rate charging. Some high capacity AA cells need 0.5C or less to fully charge. A lot depends on the brand and capacity.

More than you probably want to know about NiMH cells
May 09, 2006, 03:22 PM
magic612's Avatar
Thanks for the info guys. I'll discharge them and toss 'em.

Yes, lesson learned, the hard way. Grrrrr..... hate when I do stupid things.

Hey, at least they weren't LiPo's, right?
May 09, 2006, 04:12 PM
McClintock on rcu
jaeder's Avatar
Don't toss them yet, some of the best packs i have made a hissing sound once and have been working fine ever since. Just be suspicious of them for a while.. you may still need to toss them, but they may work still.
May 09, 2006, 04:37 PM
magic612's Avatar
Originally Posted by jaeder
Don't toss them yet, some of the best packs i have made a hissing sound once and have been working fine ever since. Just be suspicious of them for a while.. you may still need to toss them, but they may work still.
Ummm.... care to elaborate on that at all? What should I look for (or listen for)? Check for reduced capacity? Obviously I'll be careful charging / using them for a while, but any additional insight would be appreciated.
May 09, 2006, 05:25 PM
McClintock on rcu
jaeder's Avatar
My hissy packs were 3000mah nimh 7.2v car packs, and I like you thought-'dangit' and was reading up on how to toss them, then I thought, well what exactly happened..

The hiss comes when the absorber catylist thing gets over whelmed with gasses and it vents, spewing a little fluid. Maybe the capacity of that cell is deminished, or maybe that cell had too much in it to begin with.. Nothing to loose now by taking a chance, at worst the motor will cut out and then you land.

I didn't form charge at c/10 and instead treated them more like my tried and true nicads, charged on my mrc 969 at something like 3 amps. I heard ppppttttsssss and smelled something, felt the pack, it was hot, took it out to the concrete in fear. The other pack didnt make a noise, i only noticed it lost something fluid-wise later...

Anyway, I tested the voltage after the scary hissy pack was cool, and it was fine.. Drained it on the 969 and the capacity was ok for the first charge ever, now having read about forming I did that with a wall wart, then carefully charged it at .5 c a few times, draining it by using it in the car, from then on I charge those packs made from that batch of cells at 2amps. They may take 1c now but I'm scared.. The capacity as read by the mrc shows all three packs about the same, over 3000mah, one pack was never abused like the other two- I knew about forming for that one's first charge.

The one that went hissy always takes a little more mah than the others.. maybe tough love? It may crap out before the rest, but its still hanging fine for now.

In a plane I'd ground run it once or twice to be sure it's behaving predictiably, and just see what happens.

Long story short, just compare the way they heat on charge and the capacity to known good similar cells, and if they work, use 'em! Oh and maybe clean that stuff off that came out if you can get to it.
May 10, 2006, 08:06 AM
Forever TMWT Pilot #11
LcJ's Avatar
But nimh batteries can burn and produce a lot of smoke. Not the intense lipo type, but enough to make you feel stupid. Watched my heli crash in a ditch and then heard some hissing and saw the smoke......had bad thoughts......pulled the plug and all that was damaged was the batter pack.

Lucky I guess....only lost 5 of seven cells!

Latest blog entry: RC Logger's Eye One Xtreme Quad
May 11, 2006, 09:35 AM
magic612's Avatar
Well, I ran both packs down on my plane last night (not flying, just sitting there holding the plane with the throttle wide open), and had time to monitor and recharge one of them. No problems, no hissing/fizzling. Used to peak out at about 10.24V, this time peaked out slightly lower than that, about 10.12V. So I suppose there's been a small dimishing of capacity, but this morning there were no burned out battery carcasses in the clay pot - they were both still sitting there.

I'll be very careful with them, and if there are any signs of significant degradation, I'll just toss them. Thanks for the feedback everyone - much appreciated.
May 15, 2006, 11:54 AM
magic612's Avatar
Flew both packs this weekend - no problems. Similar flight times as in the past, about 10-12 minutes or so. Recharged both and flew them again later on in the day, same performance. No hissing/fizzling at the end of the charge, barely got warm towards the end of the cycle, as per previous charges. I'm thinking they're going to be okay.

Although, for safety's sake, I do charge them out on our breezeway now, and ordered two more from Tower (since they were marked down to $5.51 on Saturday, and are usually $9.25 apiece). So now I'll have four....which means a whole lot more flying when I do go out to fly!

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