Alkaline batteries start off at 1.5-1.6 volts and the voltage follows the charge reasonably linearly down to about 1.0 volts. So that's correct, alkaline batteries at 1.5V per cell are 'good' full stop. Where a rechargeable at 1.2V might or might not be. Who really knows if it charged all the way[1][2].

[1] Gallant is careful to put his nmh batteries in the the charger each night. Goofus got high and forgot. In the morning Goofus swapped his dead ones for the ones in Gallants charger.

[2] Power went out between 11pm and 6am while everyone was asleep.