What is the capacity of a Group 31 AGM battery?

Group 31 AGM batteries typically range from 80Ah to 120Ah at the C20 rate. The exact capacity depends on the manufacturer and whether the battery is designed as dual-purpose (starting + cycling) or dedicated deep cycle. Naradex Group 31 AGM batteries are rated at 100Ah (C20) with 800+ cycles at 50% DOD.

How much does a Group 31 battery weigh?

A Group 31 AGM battery weighs between 26-31 kg (57-68 lbs), depending on the manufacturer and internal design. Higher-capacity models tend to weigh more due to thicker plates and more active material. This is heavy enough to require careful handling but still manageable by one person, unlike larger Group 4D (55-65 kg) or 8D (68-75 kg) batteries.

Can I replace a Group 27 battery with a Group 31?

Only if your battery compartment can accommodate the larger dimensions. Group 31 is 24mm longer (330mm vs 306mm) and 15mm taller (240mm vs 225mm) than Group 27, with the same width (173mm). Measure your available space carefully before upgrading. If it fits, Group 31 provides 15-20% more capacity at a better cost per amp-hour.

How long does a Group 31 AGM battery last?

In float standby applications (UPS, backup power), a quality Group 31 AGM battery lasts 8-12 years at 25°C. In cyclic applications (solar, marine, electric vehicles), lifespan depends on depth of discharge: expect 4-6 years at 50% DOD with 800+ cycle batteries. Higher operating temperatures significantly reduce lifespan — every 8-10°C above 25°C approximately halves the battery life.

What is the difference between Group 31 and Group 31T batteries?

Group 31T has the same external dimensions as standard Group 31 but features threaded stud terminals instead of standard automotive posts. The T designation indicates the terminal type only — it does not affect capacity, voltage, or performance. Choose Group 31T when your cables use ring terminals that bolt onto threaded studs, which is common in marine and commercial vehicle installations.

Group 31 AGM Battery: Specs, Applications & Sizing Guide

What Is a Group 31 Battery?

Group 31 is a Battery Council International (BCI) standard that defines a battery's physical dimensions — specifically 13 inches long × 6.8 inches wide × 9.4 inches tall (330 × 173 × 240 mm). These standardized dimensions ensure that Group 31 batteries fit into equipment designed for this size class, regardless of manufacturer.

Group 31 is one of the largest commonly available battery sizes, making it popular for heavy-duty applications that demand high capacity in a single unit. When built with AGM (Absorbed Glass Mat) technology, Group 31 batteries deliver 80-120 Ah of capacity with excellent deep cycle performance and maintenance-free operation.

Group 31 AGM Battery Specifications

ParameterSpecification **BCI Group Size**31 **Length**330 mm (13.0 in) **Width**173 mm (6.8 in) **Height**240 mm (9.4 in) **Nominal Voltage**12V **Typical Capacity**80-120 Ah (C20 rate) **Weight**26-31 kg (57-68 lbs) **Terminal Type**SAE automotive post, dual terminal, or stud **Cycle Life (50% DOD)**500-800 cycles **Float Design Life**8-12 years at 25°C **Cold Cranking Amps**800-1,050 CCA (dual-purpose models) **Reserve Capacity**180-230 minutes at 25A

Naradex Group 31 AGM batteries (ND12-100G31 series) deliver 100Ah rated capacity with 800+ cycles at 50% DOD — exceeding the industry standard by 30-60%.

Group 31 Battery Size Comparison

Understanding how Group 31 compares to other common BCI sizes helps you determine if it is the right choice for your application.

BCI GroupLength (mm)Width (mm)Height (mm)Typical AhWeight (kg)Best For Group 2426017322570-8518-23Marine, RV, small UPS Group 2730617322585-10522-27Marine, RV, medium UPS **Group 31****330****173****240****80-120****26-31****Trucks, large UPS, solar, commercial** Group 4D527222250160-22055-65Large trucks, telecom, data centers Group 8D527283250225-26068-75Military, heavy equipment, large telecom

Group 31 hits the sweet spot between capacity and portability. At 26-31 kg, it can still be handled by one person (unlike Group 4D at 55-65 kg), while delivering significantly more capacity than Group 24 or 27.

Group 31 vs Group 27: Which Should You Choose?

This is the most common comparison because both sizes fit similar applications. Here is how they differ:

Capacity: Group 31 typically delivers 15-20% more capacity than Group 27 (100Ah vs 85Ah). This translates directly to longer runtime in backup power and deeper energy reserves in solar systems.

Physical size: Group 31 is only 24mm longer and 15mm taller than Group 27, but check your battery compartment carefully — those millimeters matter in tight installations like boats and RVs.

Weight: Group 31 weighs 4-5 kg more than Group 27. In marine applications where weight distribution matters, this difference is worth considering.

Price: Group 31 typically costs 15-25% more than Group 27. However, the cost per amp-hour is usually lower for Group 31, making it the better value if it physically fits your installation.

Recommendation: If your battery compartment can accommodate the Group 31 dimensions, choose Group 31 for better value and longer runtime. Only choose Group 27 when space or weight constraints dictate.

Applications for Group 31 AGM Batteries

Commercial Trucks and Fleet Vehicles

Group 31 is the standard battery size for Class 6-8 commercial trucks. These vehicles demand high cranking amps for large diesel engines plus deep cycle capability for cab accessories, liftgate operation, and anti-idle hotel loads. Dual-purpose Group 31 AGM batteries handle both starting and cycling duties in a single unit.

Solar Energy Storage

Group 31 AGM batteries are widely used in residential and small commercial solar installations. Four Group 31 batteries in series create a 48V battery bank with 100Ah capacity — enough for a typical off-grid cabin. Their sealed, maintenance-free design is ideal for remote solar installations where regular maintenance visits are impractical.

Marine Deep Cycle

Trolling motors, fish finders, navigation electronics, and cabin lighting all draw from the house battery bank. Group 31 AGM batteries provide the high capacity and deep cycle durability that serious anglers and cruisers demand. Their spill-proof, vibration-resistant construction handles the marine environment better than flooded alternatives.

UPS and Data Center Backup

Group 31 batteries are used in medium-capacity UPS systems (3-10 kVA) that protect servers, networking equipment, and critical infrastructure. The standardized form factor simplifies battery rack design and replacement logistics.

Emergency and Rescue Vehicles

Ambulances, fire trucks, and mobile command vehicles use Group 31 batteries to power onboard medical equipment, communications systems, and lighting while the engine is off. The dual-purpose capability supports both engine starting and extended auxiliary power.

Wheelchair and Mobility Scooters

Heavy-duty electric wheelchairs and large mobility scooters use Group 31 deep cycle batteries for maximum range. The sealed construction eliminates acid spill risk, and the maintenance-free design benefits users who may have limited ability to perform battery maintenance.

How to Choose a Group 31 AGM Battery

1. Determine Your Primary Use Case

If you need both engine starting and deep cycle capability, choose a dual-purpose Group 31 AGM with high CCA (800+) and moderate cycle life. If deep cycle is your only requirement (solar, trolling motor, UPS), choose a dedicated deep cycle model with maximum Ah capacity and 800+ cycle rating.

2. Check Capacity at the Right Rate

Battery capacity varies with discharge rate. A battery rated 100Ah at the C20 rate (5A for 20 hours) may only deliver 80Ah at the C5 rate (16A for 5 hours). For high-power applications like inverters, check the battery's performance at C5 or C3 rates.

3. Verify Terminal Type

Group 31 batteries come with automotive posts, dual terminals (post + stud), or threaded studs. Verify which terminal type your cables and connections require before ordering.

4. Consider Operating Temperature

If your batteries will operate in extreme heat (above 35°C), derate capacity by 20% and consider gel alternatives. In cold environments (below 0°C), derate by 25-40% and ensure your charging system compensates for temperature.

5. Evaluate Total Cost of Ownership

A Group 31 AGM battery costing $250 that delivers 800 cycles has a cost per cycle of $0.31. A cheaper battery at $180 delivering only 400 cycles costs $0.45 per cycle — 45% more expensive over its lifetime. Always calculate cost per cycle, not just purchase price.

Charging Group 31 AGM Batteries

Correct charging maximizes the lifespan of your Group 31 AGM battery investment.

Float Voltage: 13.5-13.8V per battery (27.0-27.6V for 24V systems) Cycle Voltage: 14.4-14.8V per battery (28.8-29.6V for 24V systems) Maximum Charge Current: 25A for a 100Ah battery (C/4 rate) Equalization: Not required for AGM batteries — equalization overcharges sealed cells and shortens life

Use a smart charger with AGM-specific settings and temperature compensation. For solar applications, ensure your charge controller is programmed with the correct AGM voltage setpoints — many controllers ship with flooded battery defaults that will undercharge AGM batteries.

Conclusion

Group 31 AGM batteries deliver the highest capacity-to-size ratio in the most versatile form factor for commercial vehicles, solar installations, marine applications, and backup power systems. With 100+ Ah capacity, 800+ cycle deep cycle performance, and sealed maintenance-free construction, they represent the best value for demanding applications.

Naradex Group 31 AGM batteries ship from our factory to distributors and OEM customers in 100+ countries. Standard models are available from stock with MOQ of 100 pieces. Contact our sales team for pricing and custom specifications.