Andy Champ writes:

Guy Fawkes wrote:
...so a 2 year old 100 Ah battery that has not been deep
cycled but has been used for engine starting may only have 75 Ah capacity
left...

Guy, you may be being pessimistic. My last car was scrapped complete
with original (working) battery after 8 years & 120,000 miles. If I had
lost 25% in 2 years, after 8 years I'd only have 30% left. Yet it
cranked fine - even when periodically I forgot to turn the immobiliser
off so it didn't fire...

He was talking about deep-cycle batteries. They are not designed for
extremely high current draw applications (such as engine starting).
Just like starting batteries are not designed to be drawn way down.
The difference is due to the plate design. A plate designed for
providing cranking amps isn't good for deep cycle applications,
and vice versa.

Head to your local library and see if they have a copy of
_Boatowner's Mechanical and Electrical Manual_ by Nigel
Calder. The introduction in chapter one gives an excellent
description of the difference between the two battery types.

In fact, this is a "must own" book for any cruiser. I paid
$80 (Canadian) for mine (second edition hardcover) and it was
worth every penny.

--lyndon