I contacted Optimate. Apparently, my Optimate chargers work with with these batteries....Amazon has some steals on chargers right now.
Amazon.com: Schumacher SEM-1562A-CA 1.5 Amp Speed Charge Battery Maintainer: Automotive
I have this unit. I put on a set of cable connectors on my riding mower, motorcycle, and UTV. Just plug it and don't worry about it. It's interesting that Yamaha will only warrantee batteries for 3 months. Our bikes put a small drain on the battery due to the electronics. If the battery sits discharged for about a month it will never hold a charge again.