Hello,
That will be your battery in the panel that is faulty.
Depending on the amount of detectors you have will govern what battery will be needed. Your best bet though is to go for a 12volt 7amp SLA. Slightly dearer than the others but will last a lot longer in the event of a power cut. Readily available from Maplins etc, expect to pay between £12 - £22.
The Flying Penguin, October 2007