Hi - you shouldn't need power to release the brakes. As it's an automatic and the battery is flat, the gearbox will likely be stuck in Park. The procedure for releasing it is -
New: Emergency Park Release - 9AT
When the engine cannot be started due to a dead battery, transmission failure or engine trouble, the 9AT transmission will be locked in park (P) and the vehicle
cannot be moved. In the case of a dead battery, recharging the battery and restarting the engine will allow the gear position to be changed. When you need to move the vehicle immediately in an emergency or when the engine cannot be run, you can
disengage the parking lock in the transmission by
following the procedure below.
1. Make sure the vehicle is on a firm, level, and non-slippery surface and apply the parking brake fully.
2. Leave the ignition switch in position (I) Accessory, and make sure the steering lock is not on by turning the steering wheel.
3. Open the bonnet.
4. Insert a screwdriver into the hole in the shaft of the parking release bolt (A) and swing the screwdriver anticlockwise to loosen the parking release bolt.
The parking release bolt is now loosened.
5. Insert the screwdriver in the screw head of the parking release bolt and rotate the bolt (B) until it contacts the stopper (C).The lock is now released and the gear has been shifted from (P) to (N).
6. Move the vehicle to a safe location as required.
7. When done, be sure to rotate the parking release bolt clockwise to return it back to the original position and then, swing the screwdriver inserted in the hole in the shaft of the bolt clockwise to firmly tighten the bolt so that it may not be loosened.
Once you've done this, you should be able to push/pull the car out the garage.