The MCP100-XXX family of integrated circuits is designed to monitor a voltage source for drops below a hard-coded voltage. The chips are intended to reset a microcontroller whenever the voltage drops. However, the output can be directly routed to an LED (low-battery indicator) or sent to an input pin of a microcontroller for monitoring as described in Electronic Design.

The program simply monitors RB0. If there’s a logic 0 on that input, the program flashes pin 17 (LED), indicating to the user that the battery voltage is below 3.15 V dc.

Block Diagram of MCP100 chip

Check out the MCP100/101 datasheet for details. In small quanties, these chips can be had for 33 cents.