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.
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
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.
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
What type of thermal transducer are you measuring the temperature
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
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.
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. :-]
As Roy pointed out, the real answer is in the cell manufacturer's
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.
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
Thank in advance and best regards
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