WHen is it EVER convenient to have a battery fail? Kind of like asking where is the best place to run out of gas (Story follows)
To answer the question: When it is the STARTING battery in a motor home with fully charged house batteries and a dash mounted momentary switch which may be labeled Emer(gency) start or BOOST. Why, Well simply put what is a big honking load for my Group 78 starting battery (I think 73 amp hours at the c/20 rate) is kind of snooze time for my 230 amp DEEP CYCLE batteries.
The short story: Ran out once 3 feet from the fuel pump. flat ground, stick shift car I could push with one hand.