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Check all your connections. A bad or corroded connection can cause voltage drops. What gauge wire is going from your alternator to the battery. Cobra wiring harnesses are a lot of times woefully inadequate for high power consuming accessories.

Although it's most likely not your problem, I would NEVER run a one wire alternator. They don't charge worth a crap at low RPMs. I had one... I know. After much research I rewired the same alternator for a 3 wire setup. After that, with the same alternator and under full load, my output voltage jumped over a full volt. I've never had a battery problem since.