For the everyday installation, and even most custom jobs, the automobile battery under your hood will work just fine. Should you decide to spend a lot of time at car or auto-sound competitions with your engine off and sound system blasting away, you might want to get a second battery to run the entertainment system so you can still drive off the lot at the end of the day. For systems that seriously tax the vehicle's charging system, a more capable alternator might be in order as well. Another useful power component is the capacitor. It stores an electrical charge and, when the demand is greatest from an amplifier, rapidly discharges power into the entertainment system and recharges again with very little loss of voltage. This keeps the system running smoothly during peak output. Capacitors are rated in farads, with most being 1 farad.
A Word on FM ModulatorsThese allow a media player changer in a trunk or truck bed to connect without wires to a head unit. They are useful when laying wire is not an option or when you can't plug your portable MP3 or CD player into your head unit, but there is a drawback: The signal transmitted by radio is not as strong as that transmitted via cable or wiring.
InstallerLast, but certainly not least, is the technician who installs all of this equipment into your vehicle. An installer who really knows their stuff will be able to tune the sound system to fit a vehicle amazingly well. We've sat in the front seat of a BMW with an aftermarket sound system that gave the impression that the band playing the music stood on the hood of the car with the vocalist closest to us, the drummer furthest away, and the guitarists somewhere in between. This was done without a surround-sound processor's electronic wizardry. Chances are, if you have spent years becoming a shade-tree mechanic extraordinaire in regard to suspension or engine mods, you might not have had enough time left over to become a mobile electronics expert. If that's the case, go to a good local mobile electronics dealer. Ask if the shop's installers are MECP certified (the mobile electronics equivalent of ASE certification). Find out how long they have been in the business, and ask to look at their portfolio of recent installations. Many installers started as hobbyists, just like you, and were eventually hired onto an installation staff. The right people love this stuff, and will work into the wee hours to get a job done.