It comes with a battery already, why need more? You may wan

want to connect the charger permanently to an outlet to make sure it's always fully charged.  As you said it can start 3 times and the battery can be charged while the generator is running, right?  There shouldn't be that much of a concern.  The last resort is to manually start it.  A battery, if left alone too long, will lose capacity.  If you really want to use a car battery, you will need to consider how to charge it as well, not to mention the trouble of connecting to the generator.