Nickel based cells should never be charged above the 1.45V mark.
(I know that's not what you said, but it's too important to not be really clear about it).
NiMH can (and should) be charged to somewhere between 1.40V and 1.45V per cell. Anything higher will kill them.
A 20-cell NiMH battery will be termed a 24V battery these days ((a while back, it would be termed a 25V battery as the nominal cell voltage was then seen as 1.25V - this is solely a matter of manufacturers decision).
The end-of-charge voltage for a good (i.e. undamaged) 20-cell battery should be 28..29V and after some time out of the charger, it will show something less
if the battery is given some rest after charging, it will show something less (typically around 27.5V). As soon as a modest load is applied, it will drop further, but after removal of the load, the voltage will rise a bit.
When still recently charged and properly loaded, it should show close to 1.25V/cell or 25V for the lot.