Car Audio on a Cart - Alternatives to Battery Power?

[DanL]

Alright, let it be known that this year, 810 has joined the list of teams who put car stereos onto their carts. Before the flaming comes, I'd just like to say that Joel J. said it best in this thread when he said something to the extent of a sound system on your cart is not a bad/illegal thing until it is being blasted in the pits - something we're not going to be doing. We're building this to increase moral on the team, give the less-advanced kids something to do that they'll enjoy working on while the robot design is being finalized, and lastly, to have something to play music in our work room. With that said, on to my question.

Are there any good ways of powering a car-audio-based system (headunit + 2 20watt 5's + 2 30watt 7x9's, no amp) other than a battery? We plan on having this going during our building phase, so we don't really want to have to charge a battery 3, 4 times a day. We also don't want to dish out $50 or $60 for a car battery. Specifically, we want something that can just plug into an outlet -- like an ac power brick, but can deliver a lot more current. It's a 120watt headunit, so does that mean it needs 10amps? I've heard that ratings for headunits are only peak readings (tricks the manufacterers play to make them seem more powerful than they are), but actual rms power is much much less. Anyone know how much less, and what would be a good current value to shoot for? Finally, does anyone know where to buy an ac adapter that can supply this much current?

Or, for teams that have done this using car batteries, how often do you have to recharge your battery?