I have two batteries with an isolator. I started off with the system running off the extra battery, but the voltage dropped on bass notes. I now have it running off the main starting battery that is closest to the alt and there's no voltage drop, but this defeats the purpose of the second battery because it isn't helping now. Why can't the alt keep up when the system is ran off the extra battery that is after the isolator. Do the 2 batteries have to be the same or can they be different, brand, size, etc. ?