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I have the Battery Tender Plus from Deltran. I have mine hooked up to the auxilliary power supply in the center console. No where in the instructions (for this model anyway) does it say that that "long term" use would be harmful. Sheesh, it's a battery maintainer, it's meant to be hooked up for long periods of time.
It's got a "brain" inside that maintains a "float" charge on the battery. Keeps the battery charged at maximum volts and never over-charges.
This is what I have plugged into my aux power supply:
Interesting, I never thought of charging through the cigarette lighter.
I bought an 8 foot length of 13 amp 2 wire, just a standard replacement cord and added the same connections and slipped it down the inside of the engine compartment. This way my Battery Tender sits on the lift ramp and the hood says closed. I just didn't like the idea of having this device active in the engine compartment in case it shorts out or something.