On a sunny day (Sun, 24 Feb 2013 00:06:18 -0800) it happened Mike Cook
wrote in
:

A friend has a battery powered Ryobi lawn trimmer ("weed whacker") which
doesn't work. The 12vdc wall wart puts out only ~ 12.4v.

I charged the lead-acid gel cell using a car charger. The charge rate
(selectable) was 0.5A. Afterward the trimmer seemed to work as new.

Can the car charger be used to charge this battery? He can install a timer to
limit charge time if that's something required.

Thanks.

Old trick was to put a car headlight or some other low voltage bulb (break light)
in series as current limiter.
Maybe modern car has only LEDs, dunno, :-)