Thanks Todd, I should have been more concise, I wasn't thinking of making the battery itself portable, more that the system, mainly the solar panel, would be. Instead of being wired into the electrical system of the car. What I'm envisioning is putting the battery into some sort of container with a wiring hookup for the panel and then some sort of output system.

You bring up a good point about battery size. Maybe (2) smaller batteries would make more sense.

John E