ORIGINAL: ScaleME
...When I charged the battery I could only get it to 1.39 volts - I was expecting around 1.5. So I am not sure if the battery is really getting its max capacity.
ScaleME,
A Nickel-Cadmium battery and a Nickel Metal-Hydride battery have a nominal voltage of only 1.2V. When they are fully charged, their actual, no load voltage is 1.38-1.46V. Their internal resistance is very low, resulting in pretty high voltage falling on the glow element.
This no-load number becomes lower under load, so expecting 1.5 V is like waiting for a train that never comes in...
Normal carbon-zinc dry cells, or alkaline cells do have a higher nominal voltage (1.5V) and when new show 1.65-1.68V with no load, but they have a high internal impedance/resistance, so the rechargeables are preferable, giving more voltage and current to the glow element.