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Batterys sizzling???

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I juat charged a battery pack i made of 7 AA 2100mah NiMH batterys at 4 Amps with a peak detection charger and they got really hot and started sizzling.

Is 4 amps to high of a charge rate? Because the charger also has 1 & 2 amps charge rates.


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You are charging the batteries at a rate of 1.9C (1.9 times the discharge current that can be maintained for 1 hour from a fully charged cell) which is way too high for NiMH.

NiMH batteries can only be charged at a max of 1C.

C Rating (for charge or discharge) = Current/[Battery Capacity] = 4A/2100mAH = 4A/2.1Ah = 1.9C
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Get the data sheet for the batteries from the manufacturer. 4A sounds too high to me.
I'd guess that your peak detector didn't. :D


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Did you know that a Ni-Cad cell gets colder as it charges before it is fully charged?
But a Ni-MH cell gets warmer and warmer as it charges. Try feeling them when they are both are charging.

Therefore if the Ni-mH cell is charged at a current that is too high for it then it gets too hot. With too much charging current a Ni-Cad cell begins to get warm due to its resistance dissipating heat.
When both types of cells are fully charged then their rise in temperature and rise in internal pressure depends on the amount of over-charging current.

The "peak" of a Ni-Cad cell is much more than the peak of a Ni-MH cell so the Ni-Cad charger will over-charge a Ni-MH cell and will miss seeing its peak.
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