..and just simple thermal inertia?. The heat will come from the core, but the sense point will be further away, so you would expect the heat to climb after current it removed, especially on shorter charge cycles.
Heat is generated when the gas absorption or hydrogen evolution starts to slow down. And thats only with a charge current. It sounds as if you have mismatched cells. AA's have poor internal resistance, but that maynot be true with the current chemisties.
Is the temperature consistant across the entire pack? what is the total T after charge? is it 1 or 2C/min? What is the capacity? What is your charge current. And the 64K question, What manufacturer?
I've seen some AA's not capable of taking more than C/3 when charging, and they are very poor at accepting large ripple currents.
I plan to use a 20 cell stick of 3AH Sub-C's in the future, so I am curious.
The temperature rise early in the charging cycle is due to resistve heating, and stops as soon as you stop the charging current.
The additional temperature rise you see in a fully charged cell is due to the recombination of the hydrogen gas being generated at one electrode with the nickel superoxide being generated at the other - the hydrogen gas is generated in solution and has to diffuse through the electrolyte before it can get together with the static superoxide to produce water and heat. The electrolyte layer is pretty thin, but diffusion isn't quick, so a perceptible thermal decay time after the charging current has been removed isn't implausible.
They didn't teach me about this in undergraduate chemistry back in the early 1960's so I can't give you chapter and verse - sci.chem.electrochem.battery should do better.
Without seeing the data my first thought is that it takes time for the heat to migrate out of the cells, so the surface temperature should continue to go up for a while after a fast charge is terminated.
-- is a retired battery engineer from GE/Gates. He seems to know his stuff -- he claims to have written the "Rechargeable Batteries Applications Handbook" from Gates. Certainly everything he's said about rechargeable batteries has either been right, our it's been out of my league. If you could get him interested in your problem he may help you out.
There are folks who fly large models with that many cells to get high power at only moderately high currents, so it's a known problem in the high-end RC electric-powered field.
Similar situation occures in hybrid car batteries. Usually they are trying to use larger size contact wires (in fact contact blocks) that also act as heat pipes. They should obviously lead to some wide outside area to be in contact with air. Of cause if you can provide air circulation inside it is great.
One thing to keep in mind is that battery is being heated from inside, but battery has large thermal capacity and not so good thermal conductivity to the surface. So temporary heat increase after termination might be just a delayed heat that "arrives" on the surface after shut-down, as cooling is not fast enough to dissipate it.
But at the other hand, reactions during overcharge of NiMH battery involve recombination of O2 and H2, and this recombination might well continue until charging is terminated until all the gases are used up.
The initial charging of NiMH is endothermic, so cell behavior in the early stages is atypical. [That is probably not phrased very well, but the idea is for NiMH, you don't use temperature measurement in any of the charging algorithms in the early stages.] Eventually the reaction is exothermic.
What kind of temperature changes are you seeing when you cut the charging current? Is it s few degrees? If so, maybe you can examine the temperature of the cells with one of those IR boxes. That is, I'm assuming you have some sort of thermistor scheme.
I got a smart pack returned to me because the manufacturer couldn't figure out why the pack wasn't charging like it used to. I spent some quality time with it and everything seemed kosher. [Translation: no problem and a waste of my time.] However, it turns out the assembly house (off-shore of course) was putting extra kapton tape around the pack, insulating the thermistor, causing a lag in the temperature measurement.
I have Done 11ish in a tight pack. Early in the charge cycle most of the energy is charging the cells. Later in the charge more and more of the energy is converted to heat. In my case I have seen the temperature rise 2 - 3 degrees C after charging.
This would only be possible, if the power dissipation had increased rapidly just prior to shutdown, so that the surface temperature was still rising during shutdown.
On a steady state situation with constant dissipation and constant surface temperature, the temperature starts to drop immediately after shutdown.
The situation is different e.g. in a fan cooled projector lamp, in which the fan keeps the bulb and base temperature at a reasonable level. Removing the lamp and fan power simultaneously, will cause a surface temperature increase towards the hot filament temperatures. This temperature increase is finally limited by increased radiation and increased natural convection, but the bulb or the base might melt before that.
However, if you keep the fan running after the lamp was shut down, until the internal temperatures have cooled to something slightly above the maximum surface temperature, the fan can be safely turned off, since the surface temperature can no longer increase above the maximum surface temperature.
So if there is a temperature increase in the battery pack after the charge was disconnected, there must have been a rapid internal temperature increase just prior to charge is removed, so that a thermal equilibrium was not reached or there were some chemical reactions after the charging. Paul
I have seen a lot of curves, but not a minimum charge rate. I think it would be difficult to come up with an absolute number since it's quite dependent on the heat sinking and arrangement of the batteries within the pack.
Also, unfortunately, I'm having trouble getting detailed engineering datasheet information on the specific Sanyo "Superlattice EVO" cells that are used-- less data seems to be available on cells aimed at consumer markets.
...yes I have, thanks.
Best regards, Spehro Pefhany
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Well, dT/dt depends on being able to sense T. ;-) I opened up many commercial packs to see how the thermistor is set. With round cells, most companies stick the thermistor in the groove between cells. One notebook pack I opened had the thermistor just sitting in free air. One of the more interesting schemes was used on the Rayovac 15 minute recharables. They "painted" a conductive stripe on the battery and I assume the resistance of the stripe was measured.
I think the only reason dT/dt gets praised so much is measuring cell voltage changes isn't all that easy with switching noise, while sensing the thermistor is pretty clean. When you characterize cells, it is generally with a linear current source, so measuring cell voltage is easy. Not so in the actual product.