[Xack]

The common item is your wiring. Different tenders, different batteries. Chances are your first battery tender is still good. All else being the same, you should buy a disconnect and switch on the -(Negative). The switch makers suggest you switch your -(Negative) side since your switch at zero volts does not arc when switched on or off. Arcing causes oxidation and burned contacts which shorten the life of your device.