Do i need to change the battery on my accenta G3 burglar alarm?

We had a power cut over the weekend and my accenta G3 burglar alarm went on to back-up battery power. As the power was off for a significant amount of time the battery ran down and the alarm switched off. The box on the wall outside then started to sort of alarm (it was much quieter than the full alarm and wurry, is there another battery in the box on the wall outside?). The power then came on and the full alarm sounded. I inputted the code and it turned the alarm off. Is the back-up battery in the alarm rechargeable or do i need to replace it? If so how? If the power goes off again will the wurring sound from the box outside start again? Is there some way to stop it? (Neighbours quite annoyed!) or will the back up battery have recharged and come in to use? Thankyou so much for any help you can give me.