The original owner of my M5 was OCD about the battery and put it on the battery tender anytime the car went more than 24 hours without being driven or if he went away and left it in the garage during vacation. His battery tender came with my car when I bought it, but I have since bought a new Battery Tender Plus (one made in the last 5 years that has the new circuit board logic). I connect mine under the hood. My M5 rarely goes more than 24 hours without being driven, but when it does, I clip the leads on under the hood. How well does it work? I'm still on the original factory battery! I also put dielectric grease on the battery terminals in the trunk every 6 months to keep corrosion away.