Battery Charger

These are great.
http://www.progressivedyn.com/power_converters_9200.html

Can be used as a charger and/or a clean electronics quality 12 volt power supply.
They are fully overvolt and polarity protected, and also you can get the "Charge Wizard" connector that shows state of charge and will let you override the automatic charging when needed.
They back down the output as the battery is charging until the battery is at about 13.6 volt completely automatic and have a storage mode that bumps the charge rate for 15 min every 24 hours. (Sulfation) protection.
A lot of show people use them to run thier stereo's and lighting at the shows.
They are originally intended for RV's but they sure do work for cars also, as you can see by the Amp outputs.
You can leave these on a battery any time the care is just sitting and it will maintain the battery without overcharging it.
Might want to do a little reading on them as they are darn fine units for the price.