I am interested in performing the typical which battery lasts longest experiment with some AA and AAA alkaline batteries. The idea was to simply put maybe a 20ohm resistor in circuit with it then measure the voltage. 1.5v / 20ohms = 75mA so my current in mA = analogRead / (1024 / 5 * 1.5) * 75... Or something like that, just a general idea.

My question is, when the voltage of the battery drops, is this equation still valid or will it not account for a dying battery? Because obviously that's the point.

Secondary question, I might not even worry about it, but can I drop the vRef to 3.3v by simply wiring it on the breadboard or PCB and connecting vRef to a 3.3v source? Then obviously the equation would change.

Thanks.