ORIGINAL: Charlie P.
NiCads and NiMH will lose 5% to 10% or more (NiMH being a little worse than NiCad) in the first few hours off the charger and then level out to 1% to 2% per day.
My transmitters will drop from 11.6v to 11.2v in the time it takes to unplug them and drive 20 minutes to the runway.
Presently, I am working on a project using low self-discharge NiMH cells. During discharge, the cells drop from 1.48V immediately after charge to 1.3V in 5 minutes; but they then take a further 225 minutes to reach 1.05V which is complete exhaustion (for the purposes of the standard).
Immediately at end of charge (EOC) the cells are between 1.47 and 1.49V, dependant upon the cell and temperature,but if left alone will fall to 1.38V within an hour.
This voltage drop just after charge isn't anything to bother about unless it's grossly aberrant.