ok. i have a 1300 watt amp in the back of my car. i have one yellow top optima to run the car and a second yellow top just for the amp itself. the stereo battery is isolated with a 200amp solonoid that is only activated when the head unit is turned on. i did have a bad alternator where it would only charge up to 12 volts so i replaced the alternator. now when im listening to my music, it seems something is fuxed. the volt guage ill drop down to around 12.7 volts, than it will jump back up to 14ish. but this only happenes after listinging to music for about 10mins, than this cycle will continue 8 more times, each lasting about 15 seconds. the thing i dont like is when the voltage guage drops down to 12.7, the alternator really whines out and its producing 18 volts and i can just hear my fuel pump scream as well as the lugg on my engine. i really dont like seeing my accesories getting 18 volts. the car battery is ran off 4awg right from the starter and the stereo battery is 0awg right from the solonoid. both batteries/solonoid are located in the hatch. im thinking its a shotty battery or alt. but id like to see what other people have to say. thanks alot guys

 

Question, you say the guage reads 12.7 but there is 18 volts? The guage should be reflective give or take a little of what is at the battery. Delco alternators charge hard until either one of two things happen. 1) the battery gets to it's full terminal voltage, 2) the regulator IC in the alt gets too hot and it will throttle itself down. A consistent 18 volts suggests an issue with the alt since all regulation is internal to the alternator. Charging 2 batteries is always a problem since internal resistance from battery to battery is always different. My suggestion for simplicity is to get a 200 amp alt and use one battery. You don't mention the alt capacity, 63 or 105 amps? www.alternatorparts.com offers some very high output units in the CS series case, you might consider that. One thing you can do is measure each battery individually with a dvm. Right down to a tenth of a volt compare them. If one of them is substantially lower when they are connected to a common bus that the radio solenoid provides there could be an issue forcing the alt to run wide open. The radical voltage difference between where the guage reads and the battery has me concerned, it should not be. Precise readings from each battery at rest, connected together engine off, and the same with engine running may give me a clue. An O' scope reading of the alt output would be a help but probably not possible. Based on what you say the alt may already be bad.