BCI Group Sizes Explained¶
TL;DR — BCI (Battery Council International) group sizes standardize the physical footprint and terminal layout of automotive and truck batteries. For heavy-duty trucks, Group 31 is the modern standard; Group 8D and 4D are large legacy sizes.
What Is a BCI Group Size?¶
The BCI group size is a code that defines a battery's physical dimensions, terminal type and terminal orientation. It does not define the battery's electrical performance (CCA or capacity) — two Group 31 batteries can have very different CCA ratings.
Heavy-Duty Truck Group Sizes¶
| Group size | Approx. dimensions (L×W×H) | Typical use | Notes |
|---|---|---|---|
| Group 31 | ~330 × 173 × 240 mm | Class 7–8 trucks, off-highway | Modern heavy-duty standard |
| Group 8D | ~527 × 283 × 250 mm | Large trucks, generators | Large, heavy legacy size |
| Group 4D | ~527 × 222 × 250 mm | Large trucks, marine | Large legacy size |
| Group 65 | ~306 × 192 × 192 mm | Light/medium trucks | Top-post automotive |
Approximate dimensions
Dimensions vary slightly between manufacturers. Always confirm the exact footprint against your battery tray and hold-down.
Group 8D vs Group 31
Group 8D is being increasingly replaced by Group 31, which delivers comparable CCA in a smaller, lighter package. Check your battery tray before switching sizes.
Why Group Size Matters¶
- Fit — the battery must physically fit the tray and hold-down
- Terminals — the terminal type and position (left/right positive) must match your cables
- Replacement — using the wrong size can damage cables or fail to secure properly
CCA vs Group Size¶
A common mistake is assuming a larger group size means more power. Group size is about dimensions; CCA is about starting power. Always verify both independently.