I am not sure how accurate measuring a battery voltage under load to figure out capacity remaining. (All battery checkers I have seen seem to do this, but the battery disconnected from the system and is only plugged into the battery checker and is not under a load).
If you want a simple voltmeter to measure the battery voltage (or whatever else), maybe use a voltage regulator and run the regulated voltage through a resistisve dividers with several resistors to give several different reference voltages. Use these voltages as the reference for some op-amp comparators driving LEDs. Then run the battery voltage through a resistive divder that will reduce the full battery voltage so that it is equal to the voltage regulator output. Then feed this reduced battery voltage to the op-amp comparators.
This should make a kind of "LED bar voltmeter" which I guess could be used as a rough battery capacity indicator.