Car and Truck Battery Group Size : This refers to the battery size that will best fit the physical dimensions, terminal locations and type required for your vehicle. The Battery Council International (BCI) assigns numbers and letters for each battery group size. Group size is typically based on your vehicle’s make, model, and engine type. Although some vehicles may accommodate a battery from more than one group size, it is important that you use a battery approved for use in your vehicle. Consult a replacement guide to find the battery group size that works for your vehicle. Also, make sure your new battery will fit and be held down properly according to the vehicle manufacturer’s specifications.