i have a battery charger and it only has a 12v plug for the car... i want to be able to use it inside the house... i have an old computer power supply... the 12v rail of the power supply has an output of 13 amps...

the battery charger i have has an input rating of 10.5-15v and 3amps fused...

if i were to use this computer PSU to power this charger, is it ok that its output is rated at 13amps and would the charger just take what ever amount of current it needs...?

A car batt can output several 100amps, so your charger is taking care of that already and will easily handle the PSU. So yes, the charger will take what it needs.

Yes the pc supply works great converted into a power supply. I just recently converted an old one into a charger supply myself. Cheap and easy.

turned out it was really easy... i bought a blank 12v socket, like you'll find in a car, from Radio Shack for $7... i just straight wired it to a male molex plug so it can connect to the power supply...