I have tried tenders, but they don't always do the job. I have finally just decided to drive them as much as I can. I just replaced the coupes with a racing battery for next week. Going to try and revive the big battery that was in it. Keep the cables clean and tight if you don't drive them much. A loose cable won't let it charge as well as it should. My big ole' zero turn mower kept running the battery down. I checked the cables and the positive had come loose. Tightened it up, no more issues. It doesn't take much. Same thing for ground. Pizza knows more than me about electrical, but that has been my latest experience.