Do i need to replace my back up battery or is it rechargeable?

We had a power cut over the weekend and the back up battery in our accenta G3 alarm ran down. The alarm then kind of started to sound outside. (is there a battery in the box on the outside wall?). The power is now back on, but will the back up battery recharge (in case the electric goes off again), or do i have to change it?
Thanks for any help!