Guy Fawkes wrote:

snip
...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...
snip more

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...
Then again, I may have been lucky.

Andy.