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.

 Group 31 Batteries Comparison Table

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

Group 31 Batteries Dimensions, Weight, Chemistry, Features

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).

Most common Group 31 batteries are lead-acid batteries: AGM, flooded cells and gel cells.

Lithium batteries are not so common due to the price and sensitive charging, however, lithium-ion 12V 'drop-in-replacements' of lead-acid batteries are more and more common.

Such batteries are lithium batteries (often LiFePO4) with protective electronics that monitor battery condition including voltage, charging/discharging current, temperature, and similar and can directly replace lead-acid batteries in marine, automotive, off-the-grid, and other applications.

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

Due to their price, lithium Group 31 batteries are used where weight savings are important and when their features justify the price difference.

True starting 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.

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 not correct labeling. CCA is a value given at 0°F, MCA at 32°F, and HCA at 80°F.

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

Dual purpose Group 31 batteries are the most numerous - 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 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.

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 Group 31 batteries have 20h capacity in 120-125 Ah range and RC value in 240-250+ range.

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.

For short:

The best deep cycle Group 31 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, as dual-purpose and especially as starting battery, it is a waste of money :)

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).

The best lightweight, multipurpose SLA (AGM or Gel-Cell) Group 31 battery is 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). Although its/their nominal capacity is 75 Ah, they weigh 'just' ~60 pounds and are able to provide 900 CCA and 1125 MCA. RC of 155 minuts could be better, but they are dual-purpose batteries with slight incline toward starting performances. Also, it is 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).

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

- 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 20h capacity of 103 Ah while being discharged down to 10.5 volts (or 1.75 volts per cell), while Odyssey 31M-PC2150 features 20h capacity of 100 Ah while being discharged down to 10.05 volts (or 1.675 volts per cell). This is a 'plus' for NorthStar NSB-AGM31M, but it just shows '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 very low self-discharge rate and generally it 'feels' somewhat more rugged battery.

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

If you need Group 31 battery and you don't mind paying some extra money, but want to get top-of-the-line battery and you can get NorthStar NSB-AGM31M for 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).