View Full Version : Interesting NiMH Charge Phenomenon
09-05-2005, 02:29 PM
In cycling a new 17-cell 3300 mah pack I used,for the first time, my new cooling box, which is capable of keeping the pack at room temperature during charge at low charge rates. I was doing a 0.2C (700 ma) cycle regime for three cycles. For each charge cycle the Triton charger I was using failed to detect a peak and shut off at the overcharge limit I had set of 3700 mah.
When these packs are charged on the bench they heat moderately to about 110 - 120F at 0.2C and the Triton always detects a peak at about 3300-3350 mah. The failure of the Triton to detect the peak (set at 8 Mv) must be correlated wit the failure of the pack to heat at all. In other words the extra 400 mah put in was simply blown off as heat without the pack rising in temperature at all. Interesting.
12-28-2005, 05:10 AM
:) Some things make you go "hmmmmmmmm."
12-28-2005, 03:43 PM
I think that peak detect chargers have a hard time detecting a peak at such a low C rate. If you had use say .5 amp it probably would have shut off at the proper charge time. Also, NiMH batteries will take some overcharge on a low C rate. I don't think I have ever had my charger stop charge on a lowC rate untill it had gone past the battery capacity by about 5 - 10 percent.
12-28-2005, 05:01 PM
I agree. I think you have a perfectly predictable combination of circumstances. The dV/dT change that the charger uses to detect end of charge is very much reduced or even non-existent at very low charging currents (below about 0.5C). In addition the change is indeed temperature related so artificially preventing the normal temperature rise with force cooling reduces it still further. It's no wonder the charger is getting confused.
Personally I'd never trust a delta peak charger for that. I usually use a simple timed charge for anything lower than 0.5C charge rate.
12-28-2005, 06:33 PM
From Harding Energy http://www.hardingenergy.com/pdfs/NiMH.pdf
"In order for a ĖdV to be seen on a NiMH cell it must be subjected to a small margin of overcharge. This overcharge condition forces the voltage depression. It occurs when all of the active material in the cell is chemically reacted, and oxygen evolves and then diffuses into the negative electrode. The negative electrode is designed to be larger than the positive so it accepts a portion of this oxygen diffusion, however pressure is still generated within the cell, which generates heat. The heat then causes the cell voltage to drop. A direct comparison of the energy input during charge and output during discharge cannot be made because a portion of the input energy was wasted on oxygen evolution and was never recombined. In order to derive a % of rated capacity, we must calculate the energy received during discharge of our test, with that cellís IEC rated capacity"
Interesting - No temperature rise, no (or less) delta V
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