On Tue, 14 Mar 2006 17:01:07 +0200, "Bart Bervoets" wrote:

I want to charge a 12V lead cell as used in security systems
in my car to use with an inverter.
I want to feed it a constant charge to allow the battery to
be full all the time.
Do i need to build a special charge circuit or will a
diode in front be sufficient to stop reverse flow to my car
battery?

There is a kit available in Australia for an automotive charger for sealed lead
acid batteries. It uses a switchmode DC-DC converter to step up the output so
that an adequate (and controlled) charge will result for variable input voltage.
Highly recommend you go this route.