Advantage zx6 alarm

We recently had a power cut and all was well when it came back on. however when i went to set the alarm the keypad won't work, no beeps nothing. i took the cover off and the alarm went off, the only way i could stop it was to pull the battery and disconect the mains. all the fuses or ok. anyone have any ideas

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Money says it's the battery.

Ever fitted a new battery?
How old is it?
5yrs (or even less sometimes) or more it needs replacing.
12v / whatever with a meter means nothing, replace it.