Rechargeable NiMH cells have a voltage of 1.2v, and stay pretty much at that voltage until they're flat. Alkaline (and other disposable) cells start at 1.5v (ac

Rechargeable NiMH cells have a voltage of 1.2v, and stay pretty much at that voltage until they're flat. Alkaline (and other disposable) cells start at 1.5v (actually a little higher), and gradually diminish in voltage over they're life. If the appliance requires more that 1.2v, then a NiMH cell will simply work. In any case, clocks tend to draw a very small amount of power over a longer time, which doesn't tend to suit NiMH cells due to a higher rate of self discharge (eneloops and other similar 'ready to use' rechargeables are much better though)