The on-chip NiCd charger uses an internal gas gauge to minimize fast recharge time and prevent overcharging of the backup cell, thereby improving system efficiency and extending the life of the backup cell. The LTC1559 also provides a user programmable trickle charge current to compensate for self-discharge losses in the backup cell. The LTC1559's automatic backup switching scheme requires minimum intervention from the host system and provides feedback to the host to minimize system loading in the backup state. Its internal VCC fault detector and reset generator eliminate the need for a separate microprocessor supervisory chip in most applications.