you shouldnt need one, i had a stock honda alternator at 130 amp with the optimal yellow top with two JL Audio 500 watt amps. The alternator was DEFINATELY not charging my battery so for weeks we could not figure what was wrong and finnally my car set on fire and it was because the wire I had from my deck to my amps burned completely off. It wasnt a secure connection, then the person that put my alarm in didnt have good connections, same thing with the capacator and my speakers even, so i rewired EVERYTHING and now my alternator charges the battery fine and bass hits EVERY time and my lights dont dim at all. It could also be that your alternator is bad. You should first check all the connections make sure they are absolutely PERFECT and then check your alternator to see if it is charging the battery at the right voltage and the right amps.
Crappy panasonic head unit, boston Pro Series 6.53, Boston RM9, 3 JL Audio 12 W3v2, JL Audio 500/1, JL Audio 500/5, optima battery, dynamat bulk kit, custom box, 1 fared capacator.