For a small motorcycle battery you want a low current charger and I think from memory you should charge at the 10 hour rate which is 0.8A for an 8Ah battery. Car chargers are unsuitable unless they have a low charge rate switch. I've got a CTEK charger that's supposed to pulse the current to de-sulphate the plates then charge at 0.8A until 14V is reached. It then reduces the output to maintain that Voltage. I think Bike magazine were giving one away if you took out a subsciption recently - godd value as they are about �36.
Hope this helps.