Sounds as though you need to clean your battery terminals. In particular you'll notice and want to pay attention to the one or two smaller wires coming off the positive post of the battery. When (inevitably) those corrode and stop conducting it'll kill power to everything in the car. At that point the starter motor will not engage even if the main positive clamp is still in good contact with the battery because there's going to be nothing going to the starter solenoid when you turn the key.