I used one of those solar powered trickle chargers that plugs into the lighter socket, and it actually killed my battery.
How??
Well it turned out that the lighter socket only got power with the ignition switch in the ACC or ON positions, not when OFF. The solar charger plugged into the lighter socket had the effect of randomly turning the radio on (even though the car was off). And whenever the radio turned on or off, the power telescoping antenna (which was connected directly to the battery) would go up or down, respectively. So the antenna motor drained the battery.
I had left my car parked at a friends place while I was overseas for a couple months, and he said the car seemed possessed with the antenna randomly going up and down.