What exactly is the purpose of this trickle charger thing?
Do you want to install this permanently on car, or you want a thing that you can use a day before you intend to use the car?
aakhlas7 give u a good idea to use something like 15v 5A thing. Or i have found very small trickle chargers online you can import that stuff. Not sure if its available here or not.
Anything that has 15v, and even 1 A, will charge your battery in a few hours, car batteries are like 60AH so they should be normally charged at 6-7 Amp current. Though i dont think there is any harm if you charge them at 10 Amps once in a while. But nothing above that to be on safe side.