As I mentioned a couple of months ago, I have a big and expensive Shorai lithium battery in my BMW R1200RS and another one in my F650GS. They both stay charged just fine using the bike's on-board charging system. I also have an Optimate lithium battery charger and sometimes plug it in after a ride. Its green light, indicating a full charge, typically comes on after about 10 minutes of charging. If you want to go cheap, purchase a small Battery Tender lithium battery charger, which I use to charge the lithium battery that is in my 2011 Bullet. It has been keeping that little battery in good health for the past 7 years. I really don't think you need to pay big bucks for the Shorai charger.