12V in/out battery charger?

I have a 12V, 35ah, deep-cycle AGM battery.

I'd like to keep it in the car, and charged, to operate a laptop for long periods when the car is not running.

My google-fu fails me when I try to figure out how to recharge it using 12V when the car is running.

I certainly don't think I need a full-on battery isolator system.

Any help out there?

run wires to your existing battery and let your alternator do the work.. put a switch on one of the wires to disconnect it from the car's electrical system once it's charged, or set up a relay to switch it "on" when the key is turned on or when you flip a switch on the dash or something.

I would be worried about overcharging it if you connect directly to the car's 12V system.

It shouldn't overcharge on the regular system. You would be better off with a stand alone charger circuit, but I don't know if there's enough voltage difference to make it work. I would do like derrik sez and hook a relay into the circuit so it only kicks in when you're running the motor. And maybe an extra switch to isolate it whenever I wanted to. Just 'cause. And a fuse.

Charging circuit on a car is ~14.8 vdc, so should not overcharge a battery. The relay/ switch is still a good idea.