Accenta 6 RKP no power, but numbers lit after powercut?

I have an accenta 6 alarm system, after a power cut the RKP is not working - the numbers are lit but the red power led is out. Searching various forums suggests that my battery probably needs replacing. I dont know the correct size both physical and amphours, so need to look inside the control unit. How do I stop the system alarming when I take off the cover of the control unit if I cannot enter a code ??