Group 31 Batteries: Dimensions, Features, and Prices (2026 Guide)
Everything you need to know about the group 31 battery dimensions, fitment, and why it's the gold standard for RVs and boats.
Table of Contents
If you are looking to replace the battery in your RV, boat, or heavy-duty truck, you have likely come across the term "Group 31." It is one of the most popular battery sizes in North America, known for offering maximum power while still fitting in standard battery boxes.
But will it fit your specific setup? In this guide, we break down the exact group 31 battery dimensions, compare the features of Lead-Acid vs. Lithium, and analyze the real-world pricing to help you make the right choice.
1. What is a Group 31 Battery?
A Group 31 battery is a standardized size (approx. 13"L x 6.8"W x 9.4"H) for high-capacity batteries, common in heavy-duty vehicles, large RVs, marine setups, and off-grid solar systems, known for strong starting power (CCA) and deep-cycle performance, available in Lead-Acid (AGM, Gel) and lightweight Lithium (LiFePO4) types, offering reliability for demanding electrical needs.
2. Exact Group 31 Battery Dimensions
Before you buy, you must measure your battery box. While BCI standards exist, slight variations occur between manufacturers. Here are the standard group 31 battery dimensions:
| Measurement | Inches (Approx) | Millimeters (Approx) |
|---|---|---|
| Length | 13.0 inches | 330 mm |
| Width | 6.75 inches | 173 mm |
| Height (Case Only) | 9.25 inches | 235 mm |
| Total Height (With Terminals) | 9.5" - 10.0" | 240 mm - 254 mm |
⚠️ Fitment Warning:
Most installation failures happen because of the Total Height. If your battery box has a tight lid, you must account for the height of the terminals and the cable lugs connected to them. Always allow an extra 0.5 to 1 inch of clearance on top.
3. Don't Forget the Terminals: SAE vs. Stud
Even if the box dimensions are correct, the wrong terminal type can ruin your installation. Group 31 batteries typically come in two styles:
- SAE Posts (Automotive): These are the smooth, tapered lead posts you see on car batteries. They require a clamp-style connector.
- Threaded Studs (Marine/RV): These are screws (usually 5/16" or M8) sticking out of the battery. They are perfect for attaching ring terminals and are preferred for deep-cycle applications.
UPNRG Lithium batteries typically use M8 terminals, which are universally compatible with most RV and Marine ring terminals.
4. Key Features & Applications
Why is Group 31 so popular? It hits the "Sweet Spot" of size and power.
High Capacity
Standard Group 31 batteries offer 100Ah to 125Ah. This is significantly more than Group 24 (75Ah) or Group 27 (90Ah).
Heavy Duty
Designed for commercial use, they often feature thicker plates and better vibration resistance, making them perfect for bumpy RV travel.
5. The Lithium Upgrade: More Power in the Same Box
This is where the game changes. You can buy a Group 31 battery in old-school Lead-Acid (AGM) or modern Lithium (LiFePO4). While the group 31 battery dimensions remain the same, the performance is night and day.
| Feature | Group 31 AGM (Lead-Acid) | Group 31 LiFePO4 (UPNRG) |
|---|---|---|
| Weight | 65 - 75 lbs (Heavy!) | 22 - 25 lbs (Lightweight) |
| Usable Capacity | ~50% (50Ah) | 100% (100Ah) |
| Lifespan | 300 - 500 Cycles | 4000+ Cycles |
The takeaway: By switching to UPNRG Lithium, you fit 2x the usable power into the exact same space, while removing 40+ lbs of weight.
6. Price Analysis: Is it Worth the Cost?
A standard Group 31 AGM battery typically has a lower sticker price upfront. A Group 31 Lithium battery commands a higher initial investment.
At first glance, Lithium seems more expensive. But let's look at the long-term value over 5 years:
💰 The 5-Year Cost Reality
-
AGM Option: You buy 1 battery. It dies in 2-3 years. You have to buy another one.
Total Cost: Multiple purchases (plus the hassle of replacement). -
UPNRG Lithium Option: You buy 1 battery. It lasts 10 years or more.
Total Cost: One time purchase (often cheaper in the long run).
Ready to Upgrade Your Group 31 Battery?
Get double the power, half the weight, and zero maintenance with UPNRG's Group 31 compatible Lithium batteries.
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