zero volt battery
The voltage of batery is always constant. It is generated due to rules of electrochemistry and voltage itself is not affected by discharging. The point is that the internal resistance of the battery increases together with discharging. Sometimes this process is reversible (accumulators), and sometimes not. According to that the voltage at terminals of "completely" dead battery will vary with different voltmeters (they use different sense current- so the voltage drop at internal resistance will be different). On the other hand with the same multimeter, the voltage of "completely" dead batteries of samekind will vary with their capacity.
Best regards,
Micard