So 18 is baaaaaad. Yes, I believe it's called the voltage regulator that keeps the charge where it should be. I'm ASSuming you've checked the ground and alternator harness? Your battery cell(s) are also probably fried/bad due to the overcharging. PepBoys, Autozone, etc will simply put a volt-meter on your battery while car is running and all accessory light are on. If it is OVER 14.5 they will say your alternator and/or regulator are fried.