Maybe an example will help. Say I have a 24 volt battery that I want to run at 2.5 amps.

If I place a SMALL value resistor (so I don't waste power) under the negative side (say a 0.01 ohm resistor) I will get 0.0625 volts at 2.5 amps. if I run this voltage into an opamp set with a gain of 50, I get 3.125 volts at 2.5 amps. This voltage is easy to measure with any measuring gear;the most common is a microprocessor, but a meter works fine too.

The voltage can be divided down so that 24 volts becomes 2.4 volts. Running this through the buffer opamp, will allow stable readings without loading the divider. NOW, you can take a voltage that would have burned up the microprocessor (24 volts is BAD for micros) and monitor that voltage. So when we get to 2.0 volts we know we are at 20 volts on the battery, and stop using it until it is recharged.

You can do BOTH of these fuctions in one IC chip for about forty cents.

:)