3 min read ·
Group 31 Battery Dimensions: Check the Case and Terminals
Group 31 batteries have a 13 × 6 13/16 × 9 7/16-inch maximum envelope. Compare actual dimensions, 31A/31T terminals and replacement fit checks.
BCI lists Group 31 maximum overall dimensions as 13 × 6 13/16 × 9 7/16 inches, or 330 × 173 × 240 mm, in length × width × height order. These limits apply to both Group 31A and Group 31T—not a guarantee that every product measures exactly that size. BCI’s dimensional chart lists both under 12-volt heavy-duty commercial batteries.
| Dimension | Inches | Decimal inches, rounded | BCI millimeters |
|---|---|---|---|
| Length | 13 | 13.00 | 330 |
| Width | 6 13/16 | 6.81 | 173 |
| Height | 9 7/16 | 9.44 | 240 |
Use this envelope for an initial space check. Before buying, compare the exact battery’s specifications with your tray, hold-down and available overhead clearance.
Actual Group 31 batteries can measure differently
The manufacturer’s listed dimensions matter, especially when a battery box or retaining bracket is a close fit.
| Battery model | Listed dimensions, L × W × H | Technology |
|---|---|---|
| Interstate 31-MHD, Group 31T | 13 × 6 13/16 × 9 7/16 in | Flooded lead-acid |
| OPTIMA YELLOWTOP D31T | 12.81 × 6.56 × 9.38 in | AGM |
| OPTIMA YELLOWTOP D31A | 12.81 × 6.56 × 9.38 in | AGM |
OPTIMA publishes its measurements in height × width × length order; the table above rearranges them for consistent comparison. A smaller case still needs a hold-down that secures it correctly—it should not be left free to move in a larger tray.
Group 31A versus 31T: check the connection
The two groups share the same BCI maximum dimensions, but the terminal distinction can determine whether your existing cables connect:
- 31A: SAE-style top posts are a common configuration. OPTIMA specifies “SAE – Type A” for its D31A.
- 31T: Threaded posts are a common configuration. OPTIMA specifies “Threaded Post – Type T” for its D31T.
BCI also lists marine-terminal configurations within these groups, so inspect the exact product rather than relying on the suffix alone. Its terminal reference chart distinguishes SAE Type A, threaded Type T and marine terminal designs.
Match the positive and negative terminal positions to the existing installation. Do not assume that matching case dimensions means matching cable reach or connectors.
Fit checks before ordering
- Confirm the application. Check the vehicle’s year, model, engine and battery specification in the manufacturer’s fitment catalogue. Group 31 dimensions alone do not establish vehicle compatibility.
- Measure the usable tray and restraint. Check the space inside retaining edges and brackets, not just the tray’s outside dimensions. See our battery tray fitment guide for the hold-down checks.
- Check total installed height. Measure from the battery’s support surface to its highest point, then allow for the connected clamps or cable lugs, terminal covers and clearance from the hood or box lid. Do not treat the case lid as the top of the installation.
- Match the electrical specification. Verify voltage, approved chemistry, required cold-cranking amps and capacity—not just “Group 31.” For example, Interstate’s 31-MHD lists 950 CCA and 195 minutes of reserve capacity, while OPTIMA’s D31T lists 900 CCA and 155 minutes. Those are different specifications despite similar dimensions. Interstate specifications; OPTIMA specifications.
Stop before purchase if the battery cannot be secured, the terminals do not match, or overhead clearance is uncertain. The right replacement must fit the mounting hardware and cables as well as the available space.