App note from Maxim Integrated about low current and voltage battery monitors. Link here
Many of today’s portable consumer electronic devices are powered by small button or coin-cell batteries. Users, of course, expect long battery life and reliable charge-level information. However, it can be quite challenging to efficiently monitor the health and state-of-charge (SOC) of these batteries without significantly affecting said SOC. In this application note, learn how simple, low-power monitoring circuits for small batteries can address this challenge.