Short answer: a Group 31 battery is a large 12-volt battery about 13 inches long, 6.8 inches wide and 9.4 inches tall (330 x 173 x 240 mm). It weighs roughly 55 to 75 pounds depending on construction. Heavy duty Group 31s are rated around 900 to 1,150 cold cranking amps and about 100 amp-hours. They are the standard battery in Class 8 trucks, buses and a lot of heavy equipment, and they are also common in boats and RVs.
We sell Group 31 batteries to fleets, contractors and marine suppliers across Houston every day. Here is what you need to know to buy the right one.
Group 31 battery size and dimensions
Group 31 is a BCI (Battery Council International) size. Every brand's Group 31 fits the same tray and battery box, within a fraction of an inch.
| Measurement | Inches | Millimeters |
|---|---|---|
| Length | about 13.0 | about 330 |
| Width | about 6.8 | about 173 |
| Height (to top of terminals) | about 9.4 to 9.5 | about 240 |
Example: our XHD31A measures 12.99 x 6.77 x 9.46 in.
How much does a Group 31 battery weigh?
Most Group 31 batteries weigh between 55 and 75 pounds. AGM batteries are usually heavier than flooded batteries of the same size because they pack more lead into the case.
- XHD31A maintenance-free wet cell: about 57 lb
- OPTIMA YellowTop D31T AGM: about 60 lb
- RDX31T-925 AGM: about 68 lb
Group 31 terminal types: 31A vs 31T
The letter after 31 tells you the terminal style. Getting this right matters more than anything else, because the battery has to match your cables.
| Label | Terminal | Common use |
|---|---|---|
| 31A or 31P | SAE tapered posts, like a car battery | Equipment, trucks and vehicles with clamp-style cable ends |
| 31T or 31S | 3/8 inch threaded studs with nuts | Class 8 trucks, buses and fleets with ring-terminal cables |
| Dual terminal | Both posts and studs | Marine and mixed use, or when you are not sure |
Tip: look at your old battery. If the cables clamp onto round posts, you need posts. If they bolt down with ring terminals, you need studs.
Group 31 specs: CCA, reserve capacity and amp-hours
- Cold cranking amps (CCA): starting power at 0 degrees F. Heavy duty Group 31s run about 900 to 1,150 CCA.
- Reserve capacity (RC): how many minutes the battery can supply 25 amps. Commercial 31s are usually 160 to 200+ minutes.
- Amp-hours (Ah): total stored energy, usually about 75 to 105 Ah for a Group 31.
Example: the XHD31C is rated 1,000 CCA, 200 minutes reserve capacity and 100 Ah.
Starting, deep cycle or dual purpose?
- Starting (cranking): built for big bursts of current to turn over a diesel. Best for trucks and equipment.
- Deep cycle: built to be run down and recharged repeatedly. Best for trolling motors, RV house banks and solar.
- Dual purpose: a balance of both. A good fit for boats with one battery, and for trucks with heavy sleeper or liftgate loads.
AGM or flooded Group 31?
A flooded (wet cell) Group 31 costs less and works well in most trucks and equipment. An AGM Group 31 is sealed and spill-proof, handles vibration better, recovers faster from deep drains and can be mounted in tighter spaces. AGM is worth the extra cost for trucks with high accessory loads, liftgates, APUs and marine use. Whichever you choose, use the same type across the whole bank.
What uses a Group 31 battery?
- Class 8 trucks, which usually run three or four Group 31s together
- Buses and fleet vehicles
- Construction and heavy equipment
- Generators and standby power
- Boats and work vessels (marine Group 31M)
- RVs and off-grid house banks
Should you replace all Group 31s in a truck at once?
Yes. Batteries in a truck bank are wired together and share the load. A new battery next to old ones gets dragged down and wears out early. Replace the whole bank at the same time, with the same model.
Where to buy Group 31 batteries in Houston
Republic Battery stocks Group 31 batteries in posts and studs, wet cell and AGM, at 6130 Long Drive, Houston, TX 77087. We are open Monday to Friday 7:30 AM to 4:30 PM and Saturday 9 AM to 2 PM.
- Shop Group 31 batteries
- All commercial and heavy duty batteries
- Open a commercial account for fleet pricing and route delivery across greater Houston
A $27 core charge applies to each Group 31 unless you bring the old battery back. Questions? Call (713) 645-2141.
