the rule isn't well worded, you can take it two ways:
1. the car can, when you are building it, be setup to take full size batteries.
2. the car has to be able to take a fullsize battery right before you put it down to run your heat race.
number 2 is a stupid interpretation and doesn't add anything to the reason for the rule: that the car can take the full size battery I own and save me money or be available to buy.