Ah capacity is only slightly related to cranking ability. A battery can be
significantly down on it's rated capacity yet still able to deliver adequate
voltage for the typical period needed for starting an engine, even with the
immobiliser.
--

"Andy Champ" wrote in message
...
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.