Small batteries are extremely sensitive to being left partially discharged or repeatedly cycled. They will sulfate up and die very quickly under these conditions. I've been using the Deltran Battery Tenders for over 15 years now, and have had no problems whatsoever. Currently, I'm using four on motorcycles, one on a generator, and one on a vintage Jeep. The generator battery is a big group 31 truck unit, and it's ten years old and still going strong. These tenders use pretty sophisticated circuitry to monitor the battery condition and keep it fully charged without overcharging. I regularly get 5 years life out of a bike battery because I plug the tender in if I won't be using the bike the next day. The Battery Tender Jr. is adequate for motorcycles. I bought two through Amazon at a really good price, so shop around.
10 Duc Streetfighter S (black)
67 Triumph Bonneville TT Special