Umm spare batts are great to have when the car is turned off to listen to music..
His car is dimming when he is driving when the ALT should be putting out its full potential, so there is basically no need for a battery which acts as a small buffer..
If the alt is not producing who gives a shit about how many batteries you have, they will all run dry with out the alt to charge them..
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i cant run my amp unless my car is on....it just doesnt turn on....i need aother new batt then?