Ha, I'm doing a small SQ system. I am going to install everything but not even attempt to run it loud until I get a H/O alternator. There's no way my car will handle it. Hell, I turn my steering wheel and my dang lights dim a little. Even though I have 2 xp750 batteries, I don't think they'll help. Sure, they may prevent the dimming, but the alternator will slowly kill itself and the batteries. I had a dB drive okur a7 2000.1 amp hooked up to my old car. Tried to hook a huge d31 battery up, did the big three upgrade and everything. Voltage was dipping like crazy. Was not dimming anything, but it was horrible. Ended up buying an excessive amperage alternator for that buick, didn't have any voltage drop less than 14.4 //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif Anything above 1,000 watt, I'd get a H/O alternator.
What my advice for you is, Follow these guy's ideas. Do a trail and error. See if you really need to do it. Although you probably would; it would be highly recommended that you do get a secondary battery, do the big three upgrade and get a nice H/O alternator. Trust me, you don't want to be buying all these nice subs and amplifiers and damage them because it's not getting enough voltage...or damage anything in your car. P.S. Adding a sound system in your car will void your warranty on some parts of the vehicle. (Not trying to scare you or anything, but I asked my dealership that and he said if anything was caused because of improper installation, it will void out the warranty and the repairs would have to come directly out of my pocket)
