When disconnecting a car battery you should remove and disconnect the negative battery terminal first.
The reason you should disconnect the negative battery terminal first is to prevent the accident of shorting out the battery against the body or ground of the vehicle.
Sometimes if you try to use a wrench to remove the battery terminal bolt on the vehicle and you try to remove the positive battery terminal first you can sometimes make contact with the body or other part of the vehicle grounding system and cause a short of the car battery.
So always use the wrench on disconnecting the negative battery terminal first and then disconnect the positive battery terminal.