Alkaline Batteries

My Bluetooth mouse runs on two AA cells.  As the each battery drops below 1.2V, the mouse complains about low battery.  The alkaline battery does not completely drain until about 0.9V.  According to Energizer, its AA alkaline battery is about 3000mAh if discharged at 25mA (to 0.8V).   One of its diagrams shows that the battery has as much as 750mAh left at 1.2V, that is about 25%.

So how do we maximize the battery life?  The mouse draws relatively low current.  If we mix a fresh battery with a partially drained battery, that should have the voltage high enough.  A fresh alkaline battery has an internal resistance about 0.1 Ohm at the room temperature (rise to 0.4 Ohm at -20C) from the chart provided by Energizer.

I measured an alkaline battery with OCV 1.09V, when loaded with 10 Ohms, the voltage drops to 0.84V.  So the internal resistance about 2 Ohms at 84mA load.   For low current draw, it should still be able provide power.  With a fresh battery at 1.535V and a drained one at 0.992V, the mouse still works without any complaint.  The two batteries together are measured 2.47-2.51V when the mouse is on.  When the mouse finally start to complain about low battery, one battery has drained to about 0.83V and the other 1.47V.  We got about extra 4 weeks of battery life.