I have the identical problem which started yesterday when my supplier switched the supply off to change the meter - alarm sounded as expected. The difference is that our alarm was in use every day and night and working right up to the morning the guy turned off the power. I don't understand how this can be the battery. Today I followed your instructions and once I had worked out that the tamper light shows with the lid off the alarm CPU (as Accenta call it) I got it working with no battery. As the batter showed a voltage of 11.55V when disconnected I thought I would try reconnecting it. It all works OK. I know 11.55 is less than 12V but I think my meter under-reads anyway (e.g. new 1.5 V cells read 1.3 - 1.4V). Please let me know whether you still think this is a battery problem (and why?). The battery is 20 years old so that might be reason to change it. I used to mend electrical appliances so understand electrics - but have no knowledge about alarms!!
Thanks
Richard