To blurt the obvious - these batteries we talk of are barely enough on their very best days.
Reserve capacity? puh puh - puh puh puh - what?
Imagine a 2 banger industrial engine of 55 or 70 HP, and a factory engineer comes out with one of our lantern batteries to fire it off.
The laughter would be enough to pierce a purty proper purple pocket protector, slide rule & all...
