I guess there are two ways to get a flat battery. One is to leave something switched on, which could - I guess - happen to anybody. The other way is for the battery to fail due to old age (or, rarely, a reliability failure during its normal life span). It's this latter case I've been thinking about.
When I was a reliability engineer, systems with "fall back" or "standby" facilities were common. The portable starter box by Dagoke is a good example of this: it works as a standby option when the main system (the battery) has failed.
Now, reliability engineers know that