The chemical system used in most rechargeable cells gives a nominal 1.2V, in contrast to the chemical reaction in carbon-zinc or alkaline batteries which produces nominal 1.5V. The nominal voltage is under load, freshly charged and under light load, the voltage will be higher than 1.2
If you have an application that will not handle the lower voltage supplied by rechargeable cells like NiCad or NiMH, rechargeable alkalines do offer a higher nominal voltage, but it will drop with heavy use. If they are always used to no less than 50% then recharged, they can maintain 1.3V to 1.4V, but will give less duty cycles than other rechargeables.
Edited by Platypus, 07 March 2014 - 10:49 PM.