Is it ok For My Car Alternator and battery to be putting out 15.3 voltage I Do Have A Amplifer In My Car With 2-10S Subwoofer With aBattery?

Hi so yes as it needs to be above the battery output voltage so it can charge up the battery.

Try a different meter just to be sure of the reading. That voltage is a little high. Make sure the alternator field is energized from the battery or it will produce more voltage than it's supposed to. Load test both batteries. The amp is drawing too much current for your alternator and batteries. You're going to end up with cooked batteries that die soon from overcharging.

That is about the upper limit of an alternator output. Regulator could be failing. Engine off, check the voltage of both batteries. If they are both at about 12.6V, start car again and check output. If it is still high, replace alternator.

As long as you have a minimum of 13.7 volts and not over 18 volts you're good.

I Have 2 Batteries In My Car