When the Council electricians visited my mother's house yesterday to 'stop the smoke detector chirping' I discovered two things.
1. I had previously replaced the battery in the downstairs smoke detector; it still chirped. Reason? The upstairs one also needed its battery replacing. They talk to each other. One has a problem, they both chirp.
2. Both batteries had expiry dates of March 2016. I said to the electrician, "They're mains powered, the batteries are backup. How come they need replacing at all?" He gave me a knowing look, "They draw power just the same."
Anyway, blessed silence till the next time.