As a general rule, it takes a lot of radio use to drain a battery because they are solid state devices and use very little current to function. Of course, the louder you play, the less time you play. If, however, you are like me, and you tend to run your lights (both headlights and KC's) while your truck is off, then you may have need for a secound battery.
I haven't done the job yet, but I've done the research and it seems that you can add a switch that allows you to charge two batterys but use one for running stuff (radio, lights, TV, Stove, whatever) and the other for starting your Truck after you left your lights on for too long becuase your drunk friend ran his car off the road and called you at 2am to help him get it out but it was dark and he needed to be able to see how to hook the chain up to his frame and you were half asleep so you forgot a flashlight and while he was laying on the ground hooking your logging chain up, he passed out and you were too tired to notice it so you ran your lights for longer then needed to the point your battery was dead...
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