Battery Monitoring to detect low power state?
Use a battery protection IC, such as those manufactured by Seiko. This is not intended to be an endorsement of a specific company. I just happen to be familiar with Seiko battery protection IC's.

http://www.sii-ic.com/en/semicon/products/power-management-ic/lithium-ion-battery-protection-ic/

Some of them have very low quiescent current.