Group 31 Batteries - Dimensions, Features, and Recommendations

Group 31 batteries are relatively large and powerful batteries, featuring 20h capacity in 75-125 Ah range, providing 750-1250 CCA, 150-250 minutes RC, etc.

Their physical dimensions are approximately 13 x 6 13/16 x 9 7/16 inches (~13 x 6.8 x 9.44 inches, 330 x 173 x 240 mm), while their features depend on the battery design, intended use, chemistry, and similar.

Group 31 batteries are very common in marine, automotive, off-the-grid and similar applications, making them one of the most popular battery groups.

Updated: October 6, 2022.

On This Page:

• Group 31 Batteries Comparison Chart
• Group 31 Batteries Dimensions, Weight, Chemistry, Features
• Group 31 Batteries - Recommendations
• Frequently Asked Questions
• Lead-Acid vs Lithium Group 31 Batteries 

 Group 31 Batteries Comparison Chart

The following comparison chart lists the most popular group 31 batteries, with their most important features:

Group 31 Batteries Dimensions, Weight, Chemistry, and Features

The physical dimensions of BCI Group 31 batteries are approximately 13 x 6 13/16 x 9 7/16 inches (~13 x 6.8 x 9.44 inches, 330 x 173 x 240 mm), and they weigh anywhere from just under 30 pounds (lithium batteries) to almost 80 pounds (lead-acid batteries).

The most common Group 31 batteries are lead-acid batteries (AGM, flooded cells, and gel cells) and Lithium Iron Phosphate (LiFePO₄) batteries.

Group 31 Lithium Batteries

Lithium group 31 batteries are becoming more and more common thanks to advancements in chemistry and electronics.

Lithium Iron Phosphate (LiFePO₄) group 31 batteries with protective Battery Management Systems (BMS) that monitor battery conditions, including voltage, charging/discharging current, temperature, etc., prevent undesired events, often can directly replace lead-acid batteries in marine, automotive, off-the-grid, and other deep cycle applications.

When compared with lead-acid batteries, lithium batteries are 2-3x lighter, can withstand 5-10x more charging/discharging cycles, can provide almost fade-free power regardless of the charge condition, etc., but they are also more expensive than equivalent lead-acid batteries.

Also, lithium batteries must be recharged using dedicated lithium battery chargers or advanced AGM battery chargers that feature dedicated lithium battery charging modes.

Due to their price, lithium Group 31 batteries are used where weight savings are important and when their features justify the price difference - they are commonly used as RV house batteries, as lightweight trolling motor batteries, in the off-the-grid, medical and security applications, etc.

Group 31 AGM Batteries

Group 31 Absorbent Glass Mat (AGM) Sealed Lead-Acid (SLA) batteries feature an electrolyte (diluted sulfuric acid) absorbed in the glass mat between lead plates.

AGM group 31 batteries are maintenance-free, spill-proof batteries that can be mounted in any position except upside-down.

Despite being heavier than lithium batteries, group 31 AGM batteries are very popular regardless of the application and are offered as dual-purpose and deep-cycle AGM batteries. They are also very resistant to vibrations and mechanical impacts, and thanks to their low internal resistance and large plate size, they can easily provide very strong currents - hence the dual-purpose group 31 batteries are often used as starting batteries.

Since they don't have built-in Battery Management Systems (BMS) like lithium batteries, they are often used for mission-critical applications where the battery must provide reliable power even in the event of power surges and spikes, which can effectively shut off lithium batteries.

With the advancements in technology, some group 31 lithium batteries are becoming even cheaper than premium AGM group 31 batteries, but they are still very popular.

Group 31 Gel Batteries

Group 31 Gel-Cell Lead-Acid batteries feature an electrolyte suspended in the silica gel between the lead plates.

Just like AGM group 31 batteries, Gel-cell group 31 batteries are also maintenance-free, spill-proof batteries that can be mounted in any position except upside-down.

The main difference between AGM and Gel-cell batteries is that AGM batteries generally feature better discharge characteristics, while Gel-cell batteries generally feature lower self-discharge rates, support a larger number of charging/discharging cycles, etc.

But, this is very model-dependent and must be checked for each model.

Group 31 Wet/Flooded Batteries

Group 31 wet/flooded batteries feature an electrolyte in the form of liquid between the lead plates, hence the names "wet" or "flooded."

Group 31 wet/flooded batteries are not spill-proof, and they are not maintenance-free batteries - it is up to the user to periodically check the electrolyte level and, if required, to add distilled water.

As such, group 31 wet/flooded batteries are often used in permanent installations with easy access to the batteries for maintenance and with good air ventilation to prevent the buildup of hydrogen gas.

Regardless of their chemistry, group 31 batteries are used as starting, dual-purpose, and deep-cycle batteries in various applications and installations.

Group 31 Starting Batteries

True starting lead-acid Group 31 batteries are rare - some manufacturers even label some of their models as 'starting' or 'cranking' batteries, but in essence, these batteries are designed to be dual purpose, with slightly emphasized 'starting' characteristics.

A good 'starting' Group 31 battery should be able to produce at least 1000 CCA and 1200 MCA.

Note: CCA (Cold Cranking Amps) is the maximum current that a new, fully charged, 12V battery can deliver for 30 seconds, with the voltage NOT dropping below 7.2V, at 0°F (-18°C). CA (Cranking Amps) or MCA (Marine Cranking Amps) is the maximum current that a new, fully charged, 12V battery can deliver for 30 seconds, with the voltage NOT dropping below 7.2V at 32°F (0°C). HCA (Hot Cranking Amps) is the maximum current that a new, fully charged, 12V battery can deliver for 30 seconds, with the voltage NOT dropping below 7.2V, at 80°F (27°C).

Again, some manufacturers label/describe their batteries as having, for example, 1000 CCA at 32°F or 68°F or even 80°F, and then they state the actual temperature - this is incorrect labeling. CCA is a value given at 0°F, MCA at 32°F, and HCA at 80°F.

For more about these values, please check our Battery Terms and Abbreviations - Glossary article.

Group 31 Dual-Purpose Batteries

Dual-purpose Group 31 batteries are mostly lead-acid batteries - they feature strong starting currents (better than deep cycle, worse than true starting batteries), and they also feature good deep discharge and deep cycle operations (better than starting, worse than true deep cycle batteries).

Good dual-purpose lead-acid batteries should be able to provide at least 900 CCA, 200 minutes RC, and at least 100 Ah.

However, there are some Group 31 models of rather specific design that are much lighter than others (Optima D31M, for example) and thus are not as 'strong' as heavier batteries from this Group, but pound-for-pound are as good as, or even better than heavier batteries.

Group 31 Deep Cycle Batteries

Deep cycle group 31 batteries are designed for deep discharge and cycle operations in electric vehicles, off-the-grid applications, marine applications, etc.

Although they can't provide strong currents, they feature excellent characteristics when being discharged with low currents.

Some of the best deep cycle lead-acid and LiFePO₄ Group 31 batteries have 20h capacity in the 120-125 Ah range and RC value in the 240-250+ range.

Group 31 Marine Batteries

Group 31 marine batteries are commonly dual-purpose group 31 batteries with both SAE and "marine" (commonly 3/8” Stud (positive) and 5/16” Stud (negative)) battery terminals allowing them to be used as both "marine" and "automotive" batteries.

Many brands offer their group 31 batteries in several versions, differing only in battery terminals and perhaps outside battery casing - internally, these batteries are practically the same.

Group 31 Batteries Voltage

Most group 31 batteries feature a nominal voltage of 12V (lead-acid batteries) or 12.8V (lithium batteries), but some brands offer group 31 lithium batteries with even higher voltage, including 24V (actually 25.6V) and 36V (actually 38.4V).

Such batteries are intended for powering 24V and 36V systems, including trolling motors, RVs, power inverters, and similar, decreasing the number of required batteries.

Group 31 Batteries - Recommendations

It is really sometimes hard to pick 'the best' battery even though there are few clear candidates and the main reasons are simple - we all have different requirements and the table itself doesn't state other important features like warranty, number and depth of recommended charge and discharge cycles, number, and type of terminals, not to mention the price, etc.

Best Deep Cycle Lithium Group 31 Battery

Lithium group 31 batteries are becoming more and more popular - today, practically, there are more lithium batteries offered on the market than lead-acid batteries.

When looking for "the best," one has to balance many variables, including output voltage and current, safety features, capacity, weight, number of charging and discharging cycles, support of parallel and series connections, etc.

Thus, the best three (3) deep cycle lithium group 3 batteries are (alphabetic order - Amazon links, links open in the new windows):

- Ampere Time 12V 100Ah

- Battle Born 100Ah 12V LiFePO4

- Chins 12V100Ah

All three batteries feature a nominal capacity of 100Ah, can provide 100A continuously, and support connecting the exactly same batteries in parallel and series up to 4S4P.

Note: when connecting lithium batteries in parallel and/or series, connect them only as recommended by their manufacturer!

Of course, there are other good lithium group 31 batteries on the market as well, and if they fit your needs and You can find them at a more affordable price, go for the model that best suits your needs.

For more about this topic, feel free to check our articles:

• What Is The Best 12 Volt Lithium Battery?
• 100Ah Lithium Battery - 12V, 24V, 36V, and 48V Batteries
• 200Ah Lithium Battery - 12V, 24V, 36V and 48V Models
• Best 12V 300Ah and 400Ah Lithium Deep Cycle Batteries

Best Deep Cycle AGM Group 31 Battery

The best deep cycle Group 31 AGM battery is probably VMAXTANKS SLR125 - it is one of the heaviest Group 31 batteries (75 lbs, 34 kg), but it also features a 20h capacity of 125 Ah and an RC value of 260 minutes.

However, it is not suitable to be used as a dual-purpose and especially as starting battery.

But, it is designed for off-the-grid applications, and it should be used as such.

For more about this battery, check Vmaxtanks SLR125 Amazon link (link opens in the new window).

Best Lightweight Dual-Purpose AGM Group 31 Battery

The best lightweight, multipurpose SLA (AGM or Gel-Cell) Group 31 batteries are IMHO Optima D31M (marine applications, BlueTop, Dual SAE & 5/16 Stainless Stud), Optima D31A (YellowTop, SAE Post), and Optima D31T (YellowTop, 3/8 Stainless Steel Stud) batteries.

Although their nominal capacity is 75 Ah, they weigh 'just' ~60 pounds and are able to provide 900 CCA and 1125 MCA.

RC of 155 minutes could be better, but they are dual-purpose batteries with a slight incline toward starting performances. Also, it is a rather reliable battery.

For more about these batteries, check Optima D31M BlueTop, Optima D31A YellowTop, and Optima D31T YellowTop Amazon links (links open in the new windows).

Best AGM Group 31 Battery

Although NorthStar NSB-AGM31 and its marine version NorthStar NSB-AGM31M are great batteries, THE BEST Group 31 AGM battery title goes to the Odyssey 31M-PC2150 battery, which is available in several versions, including (Amazon links, links open in the new windows):

- Odyssey 31-PC2150S Heavy Duty Commercial Battery

- Odyssey 31M-PC2150ST-M TROLLING Thunder Marine Dual Purpose Battery

- Odyssey 31-PC2150T Heavy Duty Commercial Battery, etc.

All Odyssey 31-PC2150 batteries are basically the same batteries internally, they just differ in type, number, and orientation of their terminals.

Why Odyssey 31M-PC2150 and not NorthStar NSB-AGM31(M)?

First of all, NorthStar NSB-AGM31(M) batteries are excellent batteries, in many aspects being even better than Odyssey 31M-PC2150: 20h capacity (103 vs. 100 Ah), RC value (220 vs. 205 minutes), weight (75 vs. 77 pounds).

Note: NorthStar NSB-AGM31M features a 20h capacity of 103 Ah while being discharged down to 10.5 volts (or 1.75 volts per cell), while Odyssey 31M-PC2150 features a 20h capacity of 100 Ah while being discharged down to 10.05 volts (or 1.675 volts per cell).

This is another 'plus' for NorthStar NSB-AGM31M, but it just shows the "tougher" side of the Odyssey 31M-PC2150 batteries.

Also, NorthStar NSB-AGM31M is designed to tolerate (according to the manufacturer) 900 cycles down to 50% discharge, while Odyssey 31M-PC2150 tolerates 'only' ~650 cycles down to 50% discharge.

In terms of CCA and MCA, they are exactly the same (1150 CCA and 1370 MCA). Even their pulse currents are equal (2150 Amps).

However, the Odyssey 31M-PC2150 battery is designed to operate safely in extremely cold weather, down to -40°F (-40°C). And that is rather cold, to say the least.

Also, it has a very low self-discharge rate, and generally, it 'feels' a somewhat more rugged battery.

These two batteries are an example that comparing two very similar products can be really tough.

If You need Group 31 battery and don't mind paying some extra money but want to get a top-of-the-line battery and You can get NorthStar NSB-AGM31M for a much better price than Odyssey 31M-PC2150, don't think twice - go for NorthStar NSB-AGM31M (Amazon link, opens in the new window), especially for regular cycle applications.

On the other hand, if you need a strong, durable, and reliable, even under extreme cold weather Group 31 battery, consider Odyssey 31M-PC2150 battery (Amazon link opens in the new window). 

Group 31 Batteries Frequently Asked Questions (FAQ)

Here are some of the most popular Frequently Asked Questions (FAQ) about group 31 and other similar batteries.

What does Group 31 mean on a battery?

"Group 31" means that the battery belongs to the BCI Group 31 batteries with defined maximum dimensions of 13 x 6.8 x 9.44 inches, 330 x 173 x 240 mm.

What is the difference between Group 24, Group 27, Group 29, and Group 31 batteries?

The main difference between group 24, group 27, group 27, and group 31 batteries are dimensions:

Group 24: 10.25 x 6.8125 x 8.875 inches, 260 x 173 x 225 mm,

Group 27: 12.0625 x 6.8125 x 8.875 inches, 306 x 173 x 225 mm,

Group 29: 13 x 5.5 x 8.9375 inches, 330 x 140 x 227 mm,

Group 31: 13 x 6.8 x 9.44 inches, 330 x 173 x 240 mm.

Because of different dimensions, group 31 batteries are the heaviest but also feature, on average, the largest capacity, the longest RC, and better other discharge features.

How long will a group 31 deep-cycle battery last?

In standby applications, group 31 Gel-Cell and lithium batteries may last up to 5-10 years, while AGM batteries may last up to 5-6 years.

In cycling applications, lead-acid batteries may last 300-900+ cycles, depending on the Depth of Discharge (DoD%), while lithium batteries may provide 2000-8000 cycles, depending on the DoD%, discharge current, temperature, etc.

How many Amp Hours (Ah) are group 31 batteries?

On average, group 31 batteries feature a nominal capacity of 100-110 Ah, although some models have as little as 75Ah or as much as 125Ah.

The actual capacity depends on the plates' construction, chemistry, discharge current, age of the battery, temperature, etc.

Lead-Acid vs. Lithium Group 31 Batteries

Long Story Short: Thanks to the advancements in technology, the price of lithium batteries is becoming more and more comparable to the price of premium lead-acid batteries.

So many people wonder when to go for lithium and when to go for the lead-acid group 31 battery.

Personally, if You need a good dual-purpose battery that must periodically crank medium to a large internal combustion engine, go for a good AGM group 31 battery.

For applications where a lightweight battery or battery pack is required that must/should withstand a high number of charging/discharging cycles but without extra high currents, go for lithium group 31 batteries - these are very common situations, and here is where lithium batteries fit perfectly.

For mission-critical applications where the battery must be as reliable as possible and where the battery must provide power even in the event of power glitches, go for a good AGM/Gell-Cell group 31 battery - Battery Management Systems (BMS) found in lithium batteries may disconnect the battery in the event of too high currents. And that is not what should happen when powering, for example, sump pumps, various winches, backup inverters, and similar.

And whatever You do, stay safe - group 31 batteries can provide really strong currents and can do some serious damage or worse ...