Scooter Therapy Inc. sell many types of battery chargers. Battery Tender is a good unit that can be left on your machine for extended periods, to keep the battery fully charged when in storage without overcharging. Includes quick attach leads to permanently install on your motorcycle. 
12v 1.25 amps or 6v   60$  monitor battery for years without overcharging 
6 or 12 volt     Junior model  40$ small enough to carry on trips 
charge 10 at once! no damage if hooked on battery wrong     Battery managment system for shop 660$ 
               Schauer Charger 6 and 12 volt  35$
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