Shop AtBatt.com for a BCI group size 31 battery below by leading manufacturers like AtBatt.com, Optima, US Battery, Odyssey, Trojan, Northstar, Interstate and more. The BCI (Battery Council International) Group Number does not designate a battery's capacity; rather it defines the following characteristics, maximum dimensions (L x W x H), voltage (6 volt or 12 volt), Post configuration, and the type of terminal used (top, side, etc.).