Acceptable temp for rapid charging NiMH cells?

What might be an approximate acceptable temperature over and above room temperature at 20C for NiMH cells to be at whilst being rapidly charged?
By "rapid" I mean a charge current of approx 700 mA which is applied to an AA cell that might be rated at maybe 1000 to 1800 mAh.
This is the charger: http://www.vanson.com.hk/details/p3001.html
I get some gas smells (as I mention in another thread) and wonder if the cells are overheating during charge and venting out gas.
The extra temp I see is about 12C by measuring inside the cell "enclosure" with a probe thermometer.
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Jax wrote:

Many manufacturers give the maximum charge temperature for their cells on the cell data sheets at their web sites.
Roy Lewallen
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Hmmm, that air temp reading is quite differemt to the reading I get at home when I measure the cells surface.
The surface temp can be a whopping 110C (or 90C above ambient).
But my technique is to blame as I have only a probe thermomenter which does not measure surface temp so easily. Therefore I wrap the newly charged cells in aluminium foil and then to insulate against heat loss I wrap them in a towel. The probe then takes the temperature inside the foil wrap.
I suspect the chemical reaction inside the cell continues for some time as the cells were most distinctly much hotter after 3 or 4 minutes of this insulation than the hottest I have felt them in the charger and that is when I took the 100C reading.
Anyway 110C felt far too hot for comfort so I placed the calls into the fridge to cool them before they suffered any more damage.
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Jax wrote:

I can see what you are trying to do but wouldn't that increase the surface temperature ( the heat being normally radiated into the air) in a similar way to body surface temp increasing under blankets or in clothes?
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What type of thermal transducer are you measuring the temperature with?
BTW, that is far too hot for any rechargeable cell.

So. What configuration is the "probe element" in? A bead? A stick (wand)? And what type is it? Is it calibrated or have you created a chart showing the offsets from a calibrated instrument?

So. Even perfectly blanketed batteries are suitable. The absolute temperature is what you want anyway. Your soak time through the battery medium will be quicker blanketed, and you will actually be able to control the system better.

You should tape it against the battery surface with a thin masking tape, like that blue 3M stuff everyone loves now. It is a two inch wide roll of masking tape at home depot for a couple bucks. Great stuff for electronics proto work. With 1/8 wide strips, you can lash it down against the battery on its ends, and insure good contact. Then place the foil, air pocketed shell over it. Gingerly lash it down as well. That is, don't put the foil right against the probe, make a quarter or half inch air gap (foil boat) all the way around the probe's sense area. Seal that off so little chilled whisps of air don't slide in at the base of the shell. That way, you will read just the battery(ies). Same reason we place the thermometer under our tongue. We blanket it in a single temperature medium. Other locations in the mouth leave the thermometer bulb exposed, allowing air currents to chill the bulb and give a low reading. So, you *want* your probe isolated from air currents and in as much contact as possible with the test item.

Charger? These (those) cells need managed charging cycles. Why would you let your cells get that hot? Also, once they have, they will always charge hotter, and deplete faster as they are likely damaged.

I get images of some of the creatures that were driving the funny cars on packs of "ball cards" that we bought back in the sixties. Slightly melted...
I wouldn't take any battery like those over 70C. I would target 60C as a max in general. If temp grows even after turn off, I would target lower so that the max ends up at 60C.
You could likely pulse them at higher charge rates (incorporate a duty cycle) and get better results thermally speaking (and charge time). Like a TENS unit for batteries. :-]
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As Roy pointed out, the real answer is in the cell manufacturer's recommendations.
In designing NiMH chargers, I usually set 45degC as the safety cutoff point unless the client wants a lower figure - 40degC is sometimes used.
A 12Cdeg rise at ambient 20degC is fine, assuming steady state (thermal equilibrium) and - noting your later post - the cells are in free air rather than thermally enclosed.
Obviously the temperature rises much more rpaidly as the cells reach full charge. While the SOC is low, the elevation should be fairly inconsequential even in a one-hour charger.
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On Thu, 25 May 2006 14:29:26 +0100, Jax wrote:

Ahem, Stand Back coz I am going to shout this:
YOU CROSS-POSTED TO FOUR NEWS GROUPS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Gave us:

Stand back 'cause I'm gonna shout this.
YOU'RE A GODDAMNED RETARD. YOU'LL NEVER BE A NETKOPP.
SO SHUT THE FUCK UP, DIPSHIT!
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On Sat, 27 May 2006 16:45:07 +0000, Roy L. Fuchs wrote:

Me a netcop? You stupid moron.

Eat my turd you moron
--
HERETICS IV. Mr. Bernard Shaw. By Gilbert K. Chesterton

In the glad old days, before the rise of modern morbidities, when genial old
  Click to see the full signature.
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Sir, Sir !
Obviously, your internal temp exceeded the threshold. First, may I suggest you to put your head in the fridge for 2 or 3 hours in order everything comes cooler before you attempt a new charge ? Second even though you disagree with somebody, you should keep polite. For example writing your insults using only one letter over three should have led to the same result and made the post more funny to read. You might also have used the ROT13 code.
We'd appreciate you correct the sentences with the suggestions above and repost.
Thank in advance and best regards
[...]

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On Mon, 29 May 2006 21:24:04 +0200, "frischmoutt"
Do NOT top post in Usenet!
http://en.wikipedia.org/wiki/Top-posting
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On Mon, 29 May 2006 21:24:04 +0200, "frischmoutt"
Based on your first retarded remark... Fuck you, boy!
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This exactly the answer I was expecting. It confirms that you should put your entire body in the fridge and remain here at least one night. You'll have fresh ideas the day after.
Gave us:

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On Tue, 30 May 2006 22:55:07 +0200, "frischmoutt"

Dude, your retarded, top posting ass is worse than Jeffery Dahmer.
At least he didn't advertise on the net. Get help, son.
I wouldn't remain at your place one second, much less one day.
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