What you want is a float charger.
I had a trickle charger (100 milliamps.) Left it on my motorcycle and after a few months, the battery measured almost 16 volts and was out of electrolyte. It never recovered.
Fully charged, a battery will read about 13.1 volts. If you connect a battery to a 13.1V power source it will charge up fully but get exponentially slow. Will charge to 75% in say 2 hours, 93% in another 2 hours, 95% 2 hours later and so on. Eventually it will draw almost no current.
A "battery tender" or "float charger" costs as little as $10 (at places like harbor freight).
Using a wall wart may work but it may also overcharge.--
Embedded Systems Consultant,
SHDesigns home - DIY Welder