just about anything will do 5A is nothing for a car or leisure battery.
I got one of these for portability and not having to worry about battery acid
http://www.rapidonline.com/Electrica...V-26Ah-18-5201
was a little bit cheaper when i bought, but this is probably still a good price.
if you look at sealed lead acid gel types make sure you get one suitable for cyclic applications not standby type