A Group 31 battery is a standardized BCI case size, roughly 13 × 6.8 × 9.4 inches, not a specific chemistry or power rating.
For the full breakdown, see our best Group 31 Lithium Battery guide.
Group 31 is stamped on batteries that deliver anywhere from 75 to 130 amp-hours, so the number clearly is not a power rating. It’s a physical footprint: a standardized case size and terminal layout defined by the Battery Council International (BCI), found under the hoods of trucks, RVs, boats, and industrial gear.
On a spec sheet, what “Group 31 battery” means is a standardized box — about 13 inches long, 6.8 inches wide, and 9.4 inches tall. What’s inside that box, from flooded lead-acid to lithium, and how much power it holds, is up to the manufacturer. The table below shows how actual products land on that footprint.
The Group 31 Footprint: Size, Voltage, And Capacity
The Group 31 footprint measures roughly 13.0 × 6.8 × 9.4 inches (330 × 173 × 240 mm), and nearly every consumer Group 31 battery runs 12 volts. Capacity and cranking amps are where the group splits apart.
| Battery | Dimensions | Typical Specs |
|---|---|---|
| BCI Group 31 standard | 13.0 × 6.8 × 9.4 in (330 × 173 × 240 mm) | 12V; 75–130 Ah across the group |
| US AGM 31 | 13 × 6.85 × 9.37 in | 100 Ah, 800 CCA, 1050 MCA |
| West Marine Group 31 AGM | 12 15/16 × 6 3/4 × 9 3/8 in | 105 Ah, 800 CCA, 1000 MCA |
| Crown 12CRV110 AGM | Standard Group 31 tray | 110 Ah @ 20Hr; 95 Ah @ 5Hr |
| Flooded deep-cycle Group 31 | Standard Group 31 case | 120 Ah, 885 CCA, 225 reserve min |
Across the category, capacity typically spans 75–130 Ah, with many batteries landing near 100–110 Ah. Cranking performance varies just as much: 750–1250 CCA and 150–250 reserve minutes are common depending on whether the design favors starting or deep cycling. Case dimensions drift a few tenths of an inch between manufacturers, so the footprint is a starting point, not a guarantee.
Fitting the footprint is only half the story. The U.S. Army’s