Sorry I know this thread is old, but I'm looking into buying a couple for my machines too. Do you guys run the chargers with the battery in the ski and connected? I have traditionally disconnected the ground when charging to be safe, because I know the mpem does not like to see more than 12v before the engine is started, but if I was careful not to power the mpem up while the tender is connected, is that an issue? It would be convenient to wire these up to plug in and switch around quickly.