I lost a battery in my motorcycle last winter.
This winter I had a Battery Tender Jr. (The small battery model, rather than a non-"Jr." version) on the bike's battery, and it's lit off every time I went out to the garage to crank it up, including having had a 3 month stretch where I wasn't able to do so.

I know Napa auto parts also sells a float charger for storage of vehicles which claims to be safe to leave plugged in the entire time it is on the car.
Using one of those, I had a car parked 2 years while I was out of the area, and other than a couple of connections that were a little oxidized, I was able to fire the car up when I got back.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...