This sums up what just about every battery thread on here has concluded...

Extra batteries often aren't needed, and can do more harm than good.

As for using a motorcycle battery...I wouldn't. Kinetik and Stinger both make small, sealed, AGM batteries, that are about the same size, though.

Also, for those of you who don't know who Clark and Navone are...they are probably the two greatest car audio pioneers in the last 20 years. You can thank these guys for everything from twisted pair RCA's to 12v preamps to noisegates.

Toby