You can have a battery cell that is 3v, 100mAh. And another cell that is 6v, 100mAh. The second cell has double the energy storage as the first cell. Really m

You can have a battery cell that is 3v, 100mAh. And another cell that is 6v, 100mAh. The second cell has double the energy storage as the first cell. Really mAh is only useful when comparing cells of the same voltage. It became a common metric since so many battery cells in electronics are 3.7v, so you could assume a constant voltage and just compare mAh. But not all cells are 3.7v, so sometimes that mAh comparison falls down completely and you should really be using watt-hours (milliamp-hours * volts).