If you have a free on board A to D, it should be pretty easy. Get a silicon diode, say a 1N4148. These have a nominal voltage drop of .7 volts. The cathode goes to ground, and the anode is connected through a high value resistor, say 100K to B+. Connect the A/D input to the junction, and measure the voltage relative to the power supply voltage. As the battery drains, the number will rise, because the .7 volts becomes a larger portion of b+. Take samples at the target voltages, and tell the processor what to do at the preset trip points. You can even use an output pin from the processor as b+, to turn the circuit on or off for even more current savings. You may have to put a small bypass cap across the junction to ground as diodes can get noisy in forward bias. I'd try it without and see how it works first.

Ron