Joy had some new NMh batteries recently and they only lasted two hours - compared to the 25 of the originals when new.

I have to look at them, the radio, and the charger. Measuring voltage does not tell you what charge they have. The voltage/charge left curve is flat for most of the charge level and only falls at the last minute, (or rises at full charge).

If the batteries are high impedance they will cause the fault described. Depending on the duty level of the devices powered.

High impedance can occur if the charger is faulty or if the trickle charge is too high for too long. And if the batteries are not fully discharged when charged. (YES even NMh and Lion if you want to be pedantic, it is a question of amount) but it takes a while to do that and is not as bad as NiCad.

I hope this makes it clearer.