Smart meter clock battery can provide power supply to the clock chip when power-down. The undervoltage problems of smart meter clock battery affect the accurate timing seriously. Analyze the theory discharge time of lithium battery in smart meter from theory; According to the field operating fault case, analyse the reason of battery undervoltage; Propose measures to reduce the undervoltage fault, then compare the essential difference between battery passivation and battery undervoltage, put forward a new online battery monitor scheme.