I think mine was about 8 years old when it failed like that. It was the original battery, so 8 years is a lot to ask from an SLA battery.

It was in a closet where a return air duct was, when the hvac turned on it blew nasty smells out with a little smoke. I spent like an hour tearing the furnace apart trying to figure out then eventually traced the smell back. Never tripped the circuit breaker just sat there smoldering when the battery pack went dead short internally (pretty common for me).

I was quite impressed.