On Wed, 06 Jun 2012 09:43:48 +1200, Gib Bogle
wrote:

How can a 12v battery
hold 18v of charge?

Because it's grossly over-charged, plus there's a surface charge on
top of that. Normally, the max you'd see going into it would be 14.8V
(which is at the top of the charging range for calcium batteries).You
could try bringing it down with a load, like an old headlamp, but
don't even switch the ignition on until it's come down by at least 3
volts.