Simplest "battery low" indicator for a 3.7 volt lithium ion battery
Simplest “battery low” indicator for a 3.7 volt lithium ion battery?


I don't think you can get simpler than this circuit...



Here is the datasheet for the chip shown, although there are lots of under-voltage sensing chips available from numerous vendors...

http://www.microsemi.com/document-portal/doc_view/11145-sg3546-pdf

Note that the battery must obviously have enough voltage left in it to light the LED to see anything so pick an LED with a low forward drop (red is a good choice).