I have used my trickle charger on my motorcycle battery, but not for long hours. Yes, you could damage the battery by using a large garage type charger. There is a "Battery maintainer" available that you can leave on all of the time. It is great for motorcycle batteries in storage over the winter, or for tractor batteries, too. Keeps them fully charged, assuming the battery isn't way past its "prime of life!"
for my full size lawn tractor batteries, i use my floor type charger, and it works ok for car and truck batteries, or for starting a lawn tractor with a dead bettery.

There is some good information at this site.

http://www.accumate.co.uk/MFBatteries.htm

This place should have you covered.

http://www.batterytender.com/

Thanks den for the link to batterytender. I have seen that on "My classic Car". Probably a good product.

Anybody know how you could hook up a batterytender with a small solar panel? (for not too much money) Must be available. They have such systems for electric fences.

Just to add, it's for an old tractor - 6 volt.