I'd be more worried about the gas generator you're going to need to power it
Seriously, why do you need 45w X 4 speakers? To cancel out Road noise? Engine noise? Running full tilt, that player is going to draw 15 amps, and I have a feeling that a 50 pound battery isn't going to last more than a couple of hours. Yeah, I know, you're not going to play it THAT
Still, you'd be better off with a flea powered amp and highly efficient speakers. As an example speakers that have a 96 db sensitivity rating draw only half the power of a speaker that's rated at 93db. For every 3db change the power requirements doulble or half, depending upon which way you're going.
Here's a better example. a speaker rated at 89 db (and there's a ton of them out there) will need 4 times more power to put out the same sound as one rated at 95 db. So if you had a 10 watt amp with the 95 db rig you'd need a 40 watt amp to run the 89 db speakers at the same sound level.
More efficent equals less power, less battery and less weight.
That said...It doesn't matter how good your speakers are, they still SUCK
half the time