[alanw]

It's my opinion you should use a proper small battery charger, preferably a smart charger. A car battery charger puts out way too many amps and can overcharge/destroy your battery. Max charge amps for a battery is around 1/10 of it's AH rating, so a 12ah battery should be charged at 1.2-1.5 amps using a regulated charger. Also, the SLA batteries we normally use require a slightly different charging sceme than normal car batteries. Google/Bing SLA battery charging