A system for managing a battery of a vehicle includes a first controller configured to control a power-on (IG ON) state and a power-off (IG OFF) state of a plurality of controllers in the vehicle and to be periodically woken up in the power-off (IG OFF) state to wake up at least some of the plurality of controllers, and a second controller configured to turn off a main relay connecting a first battery in the vehicle and a vehicle system when the power-off (IG OFF) state begins, to monitor a state of the first battery storing energy for generating power of the vehicle by maintaining power during a preset first reference time, to be woken up at a wake-up period of the first controller when the first reference time elapses, and to monitor the state of the first battery.