Optima XM burglar alarm needs new battery?

Hi folks

I switched down the power to my home during some electrical work and when I've powered up, my alarm keypad is not responding. The day and power lights are lit up on the panel but the tamper is not (hopefully this means I haven't interfered with alarm wiring). The light is flashing on the box outside but alarm is not sounding. Never changed the battery in 15 years so think that might solve it, can anyone confirm?