Group 31 Battery: Dimensions, Capacity, CCA and Why Stud Terminals Matter
Group 31 is the heavy-duty battery that shows up everywhere — trucks, buses, boats, RVs, off-grid solar banks and medical carts. It is a physical size standard (about 330 × 173 × 240 mm), and most Group 31 batteries cluster around ~100 Ah capacity and ~900 CCA under SAE J537, with stud terminals instead of the automotive post.
What is a Group 31 battery?
Group 31 is one of the large-format groups defined by the Battery Council International (BCI). The group number describes the case dimensions and terminal layout, not the performance — two Group 31 batteries from different makers fit the same tray but can differ widely in capacity, CCA and construction. The Group 31 reference page in our data section lists the cross-verified typical specification: 330 × 173 × 240 mm, ~100 Ah, ~900 CCA (SAE J537, −18°C), stud or stud/post combination terminals.
The format grew out of North American heavy-duty trucking, where it became the standard electrical workhorse. Its blend of size and output has since spread it to marine, RV, off-grid and industrial duty — which is why the same group number appears on batteries with very different internal builds.
Group 31 at a glance
| Attribute | Typical Group 31 value | Notes |
|---|---|---|
| Voltage | 12V (6 cells) | Nominal lead-acid voltage |
| Dimensions | ~330 × 173 × 240 mm | Confirm height & terminal style per brand |
| Capacity | ~100 Ah (20-hour rate) | Varies ~90–110 Ah by maker and duty |
| CCA | ~900 A | SAE J537 (−18°C); state the standard when comparing |
| Terminals | Stud, or stud + post | 3/8″-16 stud common; SAE automotive post on some |
Values are cross-verified typical ranges from the World Battery Hub data section, not a substitute for a specific battery's datasheet. CCA is standard-dependent — a figure without its test standard is not comparable (see what CCA is).
Why stud terminals define this group
The Group 31 terminal is the group's most practical feature. Where passenger cars use tapered SAE posts, heavy-duty installations bolt ring lugs onto a threaded stud — usually 3/8″-16 — with a nut and washer. A bolted connection survives vibration, resists loosening, and carries the continuous currents of winches, inverters and hydraulic pumps that a spring-clamped post connection handles poorly.
The catch is the layout variants: stud-only, stud-plus-post, and different stud positions. Before ordering, check which terminal pattern your cables and accessories expect — the dual-purpose and marine variants are the ones that most often add the automotive post for clamp-on accessories.
Starting, deep-cycle or dual-purpose?
Group 31 is a case, and the same case holds three very different batteries:
- Starting Group 31 — many thin plates for maximum plate surface area, high CCA for cranking large diesels. Built to give short, hard bursts, not deep discharges. Compare deep-cycle vs starting construction.
- Deep-cycle Group 31 — fewer, thicker plates that tolerate repeated discharge to 50% and recharging, at the cost of some cranking punch. The choice for house banks, trolling motors and solar storage.
- Dual-purpose Group 31 — plate design between the two, good enough to crank and able to cycle modestly. Convenient where one battery must do both, with neither done as well as a purpose-built unit.
The nameplate alone will not tell you the duty. The datasheet will: a deep-cycle unit quotes cycle life and reserve capacity rather than a headline CCA figure, and its CCA is usually lower than a starting unit's. Read the datasheet, not the sticker.
AGM vs flooded Group 31
Group 31 exists in both flooded and AGM construction. AGM versions dominate in marine, RV and off-grid duty because they are sealed, position-tolerant, vibration-resistant and charge faster; flooded versions remain common in heavy trucks where cost per Ah matters and maintenance is routine. The trade-offs mirror the general AGM vs EFB vs flooded comparison — with one Group 31-specific note: the AGM version of the same group is often slightly shorter than the flooded one, so check height against the hold-down.
Where Group 31 is used
- Heavy trucks and buses — often as the main cranking battery or one of a bank; see the truck battery guide.
- Marine — house banks, trolling motors and dual-purpose duty; see the marine battery guide.
- RV and camper — house battery for lighting, pumps and inverters; see the RV battery guide.
- Off-grid solar — small battery banks where the 100 Ah format is a building block; see the solar battery guide.
- Industrial and mobility — lift gates, pallet movers, scrubbers, medical equipment.
Key takeaways
- Group 31 is a size standard (~330 × 173 × 240 mm), not a performance spec — verify Ah, CCA and duty per maker.
- Typical values are ~100 Ah and ~900 CCA (SAE J537); CCA must always carry its test standard.
- Stud terminals are the group's signature — check stud-only vs stud+post before ordering.
- Choose the duty deliberately: starting, deep-cycle or dual-purpose are different batteries in the same case.
What procurement buyers should check
- Is the CCA figure stated with its test standard (SAE J537 vs EN vs JIS), and does the quoted Ah carry a discharge rate?
- Which terminal layout is specified — stud-only or stud+post — and does it match the fleet's cable lugs?
- Is the duty stated (starting / deep-cycle / dual-purpose), and does the plate construction support it?
- Does the supplier's drawing match the tray, including height and hold-down, for both flooded and AGM versions?
- For bank purchases, are matched units (same maker, lot and age) specified, since series or parallel banks degrade unevenly otherwise?
- Does the quotation separate the battery spec from accessories, and is a like-for-like comparison possible across suppliers?
Frequently asked questions
What is a Group 31 battery?
A BCI-standardized 12V battery group measuring about 330 × 173 × 240 mm, typically rated around 100 Ah and 900 CCA under SAE J537, with stud or stud/post terminals. It is the most common heavy-duty format across trucks, marine, RV and off-grid use.
What are the dimensions of a Group 31 battery?
Approximately 330 mm long × 173 mm wide × 240 mm high (about 13 × 6.8 × 9.4 inches). Height and terminal style vary between brands and between flooded, AGM and lithium drop-in versions, so always confirm against the tray and the manufacturer drawing.
Is a Group 31 battery deep-cycle or starting?
Both variants exist. Starting versions maximize CCA with many thin plates; deep-cycle versions use thicker plates for repeated discharge; dual-purpose versions compromise between the two. Check the duty in the datasheet before buying.
What is the difference between Group 31 and Group 27?
Group 31 is larger — about 330 mm long versus about 306 mm for Group 27 — and typically carries roughly 100 Ah and 900 CCA versus roughly 90–95 Ah and 800 CCA. They are not interchangeable unless the tray and cables accept the bigger case.
Can I replace a Group 31 with a lithium battery of the same group?
Lithium drop-in batteries are sold in Group 31 case format, but the charging system, temperature limits and battery management requirements differ from lead-acid. The vehicle or system must be verified for lithium compatibility, including the alternator and charge profile.
Sources
Cross-verified from the World Battery Hub entity database and public references; where sources differ, the difference is flagged rather than silently resolved. Values quoted are typical ranges, not guarantees.
- World Battery Hub data section — Group 31 reference page (dimensions, capacity, CCA basis, terminal).
- Battery Council International (BCI) — battery group size definitions.
- SAE J537 — cranking-ampere test method used for North American CCA ratings.
- Manufacturer datasheets (flooded and AGM Group 31, starting / deep-cycle / dual-purpose variants) — dimensional and rating cross-checks.
Last reviewed: 2026-09-22