Group 24 vs Group 27 Battery: Size, Capacity, CCA and Which Fits
Group 24 and Group 27 are the two most common mid-size 12V groups in North America — the default batteries of boats, RVs, generators and light equipment. They share width and height; the entire difference is ~46 mm of length, which buys Group 27 roughly 20 Ah more capacity and roughly 100 A more CCA.
Side-by-side comparison
| Group 24 | Group 27 | |
|---|---|---|
| Voltage | 12V | 12V |
| Dimensions | ~260 × 173 × 225 mm | ~306 × 173 × 225 mm |
| Capacity (typical) | ~70–75 Ah (20h) | ~90–95 Ah (20h) |
| CCA (typical, SAE J537) | ~700 A | ~800 A |
| Terminals | SAE post | SAE post |
| Typical duty | Smaller boats, generators, light equipment | Boats, RVs, house banks, larger engines |
Cross-verified typical ranges from the World Battery Hub data section; a specific battery's datasheet wins. CCA figures are SAE J537 (−18°C) — compare only within one standard (see what CCA is).
The physical difference: length is everything
Both groups share the ~173 mm width and ~225 mm height of the BCI mid-size family, so terminals and hold-downs sit at familiar positions. The only dimensional change is length: Group 27 is roughly 46 mm (about 1.8 inches) longer. That extra lead — more and larger plates — is where the additional ~20 Ah and ~100 CCA come from; there is no other trick to the upgrade.
The practical rule follows directly: upgrading 24 → 27 needs ~46 mm of spare tray length. Downgrading 27 → 24 almost always fits physically but drops capacity and CCA, which matters for house loads that were sized against the bigger battery.
Capacity and CCA: what the upgrade actually buys
Capacity is the headline for deep-cycle use — the difference between ~75 Ah and ~95 Ah is roughly a quarter more runtime for the same load on a house bank, trolling motor or generator start. CCA is the headline for cranking: ~800 A instead of ~700 A makes a meaningful difference turning a cold, large engine, but is wasted headroom on a small one that cranks happily at ~500 A.
For load sizing, the reserve capacity rating matters as much as CCA here, because both groups spend more of their lives running auxiliary loads than cranking.
Which one fits your application?
- Marine starting / small house bank — Group 24 where the battery box is tight; Group 27 where the box has length to spare and the house load wants the Ah.
- RV house batteries — Group 27 is the common default; see the RV battery guide.
- Generators and light equipment — Group 24's smaller case often matches the factory tray exactly.
- Dual-purpose boat duty — Group 27 dual-purpose units give the best balance of crank and cycle.
When neither case fits the box, the broader battery size selection guide walks through measuring the tray and reading the group size labels.
AGM vs flooded within the two groups
Both groups are available in flooded, AGM and dual-purpose builds. In marine and RV duty the AGM versions dominate for the usual reasons — sealed, spill-proof, vibration-tolerant, faster charging — while flooded versions remain the budget workhorses. The general trade-offs are covered in the AGM vs EFB vs flooded comparison; nothing about the 24 vs 27 choice changes them.
Key takeaways
- Group 24 and Group 27 differ in length only — ~260 mm vs ~306 mm, same width and height.
- The extra length buys ~20 Ah and ~100 CCA (typical): a real but modest upgrade.
- Fit decides first — measure the tray before choosing the bigger group.
- Downgrading 27 → 24 fits but re-check the load against the lower capacity and CCA.
What procurement buyers should check
- Does the tray dimension drawing confirm the ~46 mm length difference is accommodated before upgrading 24 → 27?
- Are the quoted Ah and CCA figures tied to a discharge rate and a named CCA standard (SAE J537 vs EN vs JIS)?
- For fleet purchases, is the choice locked to the application's real load — house-bank Ah need vs engine cranking CCA need?
- Is the duty (starting / deep-cycle / dual-purpose) stated, or is a single "group" being quoted across incompatible duties?
- Do the two groups' quotations compare like for like — same chemistry, same duty, same terminal layout?
Frequently asked questions
What is the difference between Group 24 and Group 27 batteries?
Length and output. Both are 12V, about 173 mm wide and 225 mm tall, but Group 27 is roughly 46 mm longer — about 306 mm versus about 260 mm — which buys roughly 20 Ah more capacity (about 90–95 Ah versus 70–75 Ah) and roughly 100 A more CCA (about 800 versus 700, SAE J537).
Can I replace a Group 24 with a Group 27?
Only if the tray has room for the ~46 mm longer case and the hold-down can secure it. Going from Group 27 down to Group 24 usually fits physically but loses capacity and CCA, so the load must be re-checked.
Which is better, Group 24 or Group 27?
Group 27 offers more capacity and CCA in a larger case; Group 24 fits smaller trays. The right choice is the largest group the tray accepts while still meeting the load — fit decides first, then output.
Is Group 27 the same as Group 31?
No. Group 31 is larger still — about 330 × 173 × 240 mm with typical ratings around 100 Ah and 900 CCA, and it usually carries stud terminals rather than SAE posts.
Sources
Cross-verified from the World Battery Hub entity database and public references; where sources differ, the difference is flagged rather than silently resolved.
- World Battery Hub data section — Group 24 reference page and Group 27 reference page.
- Battery Council International (BCI) — group size definitions and dimensional data.
- SAE J537 — cranking-ampere test method for the quoted CCA figures.
- Manufacturer datasheets (flooded / AGM / dual-purpose, both groups) — rating and dimension cross-checks.
Last reviewed: 2026-09-22