The problem with Nimh batteries is that if placed under an abnormally high load (stuck servo, ETC) within a few minutes a 5 cell pack can drop to below 4 volts while only having consumed a few hundred milliamps of energy. This is due to the high internal resistance if the Nimh cell chemistry.

After the battery has sat for a little while, you can recheck the voltage and it will read normal again like nothing happened. I have seen this first hand before more than once or twice.

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