Shuriken Batteries
Shuriken is a brand of car electronics mothership Metra, whose scions include Axxess Integrate, Raptor, Ballistic Sound Suppression, Tsunami, Install Bay installation hardware and the Metra Home Theater group, etc. They also operate the Installer Institute for car audio technicians. With this big a footprint in the car audio industry, it's no small wonder that they know what's required to manufacture a superior-performing line of auxiliary batteries.
Car audio has an enormous appetite for power. Your vehicles' 12V charging system wasn't designed to handle it. A few inefficient speakers, constrained to operate out of small enclosures need plenty of watts to make the kind of sound levels we crave. Just running a fat wire to the battery helps, providing a path to the trunk-mounted amps with little voltage drop, but that only means you can run the battery flat in less time while parked with the engine off.
A solution for these issues is a second battery, one dedicated to the audio system back near the amps. A wet-cell battery belongs under the hood, where its sloppy ways won't melt your interior. Instead of a few quarts of sulfuric acid slopping around in a plastic tub, an Absorbed Glass Mat battery has a few spoonsful soaked into a fiberglass mat separating the lead plates, too little to run out even if the battery is mounted on its side or even upside down.