What I have done in the past, to check batteries life.
I have used a cheap battery operated large ANALOG display clock, driven by the same source I have been testing, with resistor current limiting for 3 diodes ( two silicon diodes and one Schottky diode) to keep the voltage at 1.5 volts
A circuit that turns the load, in my case a constant current load that when a defined voltage of the battery is reached the circuit drops the load and at the same time the clock stops working, giving the total hours run.
Nando