Bi-Color LED as Power Indicator
Use a single bi-color LED to display power status.
A single (bi-color) LED is used to represent both power ON and undervoltage conditions. LED glows green when power ON. LED glows red when battery goes under voltage. Total current consumption was minimized. Three versions of circuits offered, both analog and digital - Arduino based.
Update: new version further reduces current consumed. A version with blinking low voltage indicator added.