RV Battery Group Sizes (BCI)
Last updated: September 3, 2026
Quick answer
BCI group size is the box (dimensions + terminals) — not the capacity. A Group 24 holds
70–85 Ah, a 27 holds 85–105, a 31 holds 95–130. The same Group 31 slot with a lithium battery delivers
roughly double the usable energy of a lead-acid one.
The chart
| Group | Dimensions (L×W×H) | Typical Ah | Flooded weight | Lithium in same box | Notes |
|---|---|---|---|---|---|
| Group 24 | 10.25 × 6.81 × 8.88 in 26.0 × 17.3 × 22.5 cm | 70–85 Ah | 42–54 lb | ≈ 19–26 lb (100 Ah class) | Smallest common factory-fit RV battery |
| Group 27 | 12.06 × 6.81 × 8.88 in 30.6 × 17.3 × 22.5 cm | 85–105 Ah | 50–60 lb | ≈ 24–31 lb (100–200 Ah) | Mid-size step up; common boondocking starter |
| Group 31 | 13.0 × 6.81 × 9.44 in 33.0 × 17.3 × 24.0 cm | 95–130 Ah | 60–70 lb | ≈ 26–31 lb (100–200 Ah) | The big deep-cycle group; tallest of the three |
| Group 4D | ≈ 20.75 × 8.75 × 9.88 in 52.7 × 22.2 × 25.1 cm | 150–220 Ah | 95–130 lb | varies | Large motorhome banks |
| Group 8D | ≈ 20.75 × 11.13 × 9.88 in 52.7 × 28.3 × 25.1 cm | 200–255 Ah | 120–160 lb | varies | Maximum flooded capacity per box |
Capacity ranges are typical published values across manufacturers (see sources); a specific model may sit anywhere in its range. Measure your compartment before ordering — group size is a hard fit rule.
Group size vs usable amp-hours — the part that matters
A battery's group number tells you what fits. Chemistry tells you what you can actually use:
| Battery | Rated Ah | Usable (planning rule) | Usable Wh @ 12 V |
|---|---|---|---|
| Group 24 flooded | 80 | ≈ 40 Ah (50%) | ≈ 480 Wh |
| Group 27 AGM | 95 | ≈ 48 Ah (50%) | ≈ 570 Wh |
| Group 31 AGM | 110 | ≈ 55 Ah (50%) | ≈ 660 Wh |
| 100 Ah LiFePO4 (Group 31 box) | 100 | ≈ 90 Ah (90%) | ≈ 1,080 Wh |
Run your own loads through the battery runtime calculator — it applies these same usable-capacity rules. The full chemistry tradeoffs are in lead-acid vs lithium.
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Pairing and wiring notes
- Two 6 V golf-cart batteries in series (Group GC2: 10.3 × 7.1 × 11.6 in) make one 12 V bank — typically 200–235 Ah at 12 V equivalent. The classic flooded deep-cycle answer.
- Parallel banks add Ah at the same voltage; keep cables equal length so current shares evenly.
- Weight adds up: four Group 31 flooded batteries ≈ 260 lb; four 100 Ah lithiums ≈ 110 lb — with roughly double the usable energy.
- Terminal types differ (automotive post vs dual stud); check before buying an exact-fit replacement.
Sources:
- BCI group dimensions and capacity ranges: BatteryStuff BCI group size chart (batterystuff.com) and manufacturer spec sheets for Group 24/27/31 deep-cycle models.
- 50% / 90% usable-capacity planning rules: standard battery-industry guidance (see our guide for the reasoning).
Related tools & guides
guide
Lead-acid vs lithium
reference