On Sat, 10 Oct 2009 08:44:32 +0100, Dave Plowman (News) wrote:

According to an expert on another group the average lead acid car
battery loses 60% of its charge per month.

Naw, thats worse than NiCd!...

I'm only quoting this FWIW as it's not my experience. I have a clever
electronic battery tester which works out the actual capacity and that
shows nearer 20% per month.

I'd say even that is high, no doubt that also includes the energy
taken by the tester. Lead acid batteries have a very low self
discharge, Battery University say 40% per year (3%/month average):

http://www.batteryuniversity.com/partone-6.htm

Of course if the battery is left connected in a modern car, the
quiescent drain may well flatten it in less than a month.

Agreed, modern cars have a "deep sleep" mode that they can be put
into. The makers use this when brand new cars are spending weeks on a
boat traveling the world or parked up on disused airfields waiting to
be sold.

--
Cheers
Dave.