It is the way the cells are designed. You could use a car battery just a couple of times. But after that, they would not take a charge any longer. You can use one for the cranking battery, but again, if you use accessories a lot the steady draining and charging will kill it. New marine batteries are designed to be recharged over and over again. And unlike those from just a few years back, they no longer have to be totaly drained to recharge them and not worry about memory. It is worth the $75 investment to get a good marine battery. If nothing else, just for the peace of mind.