I use a 6v sealed lead acid setup on my 3 speed. I run 2 halogen headlights and a incandescent tail light. The lights are adequate for my nite rides. I want to be seen more than see. I find that if you discharge and charge every couple of weeks while the batt is not being used, it will hold up just fine.I have been doing this for over a year with good success. When in use, I get about a 3 hour run on the battery and then recharge it.
I have the same battery in a coleman lantern and do the same thing, discharge and charge. These batteries are relatively inexpensive but do last. They are the same batteries that are in alarm systems and emergency exit lights.
I would not leave on trickle charge when not in use because it is possible to overcharge a fully charged battery and possibly cause damage. It may be a different story with a FLOAT charger, which shuts off when batt is at full capacity and will start up again when the voltage drops.
My set up works just fine for me.