I use my alternator for an onboard battery charger.

(I KNOW I was not the only one wanting to post this...

)
Seriously though...are you looking for a trickle charger for your battery while you are not riding? If so, I would look at the Battery tender line of products. They have a small unit that you connect a pigtail to the battery and run the plug out to the edge of the seat. Then has a wall charger that plugs into a regular outlet to provide power. Small, compact, easy to store away...it would be the way to go.
I don't have a battery charger...I ride my bike...even in the winter. (My bike sat for at most 3 weeks without being ridden this last winter.)
Joe