This looks like a good page...

http://www.batteryuniversity.com/partone-13.htm


"Simon#" wrote in message
...
There are a couple of products around to maintain a car battery.

They work is different but similar ways, the most complicated is the
accumate (http://www.accumate.co.uk/OM3CHARGING%20ALGORITHM.htm), however
one of the most popular products is the Airflow
(http://www.airflow.uk.com/battery-conditioner.htm#2)

This seems expensive for what it does "The unit will charge a battery from
a
minimum of 9 volts to 13.8 volts and then turn off. It will then monitor
battery voltage until it drops to 12 volts, through natural drain, and
then
turn on again to re-charge and exercise the battery to 13.8 volts"

Has anyone built such a device? Can it be acheived for less the £40 / US$
70