Group 31 Dual Purpose and Deep Cycle Batteries

Group 31 batteries are very popular marine, automotive and industrial batteries - they are mid-size batteries offering good capacity and high starting currents at an acceptable weight.

Group 31 are mostly lead-acid batteries optimized for deep cycle or dual-purpose applications, but lithium-ion batteries are also present, especially as LiFePO₄ deep-cycle batteries.

Published: March 11, 2023.

 

Group 31 Battery Dimensions and Features

Physical dimensions of group 31 batteries are (L x W x H) 13 x 6.8125 x 9.4375 inches (330 x 173 x 240) - actually, according to standards, battery group 31 battery must fit in the battery compartment of these dimensions.

Group 31 batteries on average feature nominal capacity of 75 - 125 Ah, nominal Cold Cranking Amps (CCA) of 600 - 1150 Amps (if stated by the manufacturer) and Reserve Capacity (RC) of 155 - 240 minutes.

Group 31 batteries are designed as either dual-purpose batteries or as deep cycle batteries - starting group 31 batteries are very rare since these batteries are large enough to crank large gas and diesel engines even when designed as dual-purpose batteries.

Like many similar batteries, group 31 batteries are made as maintenance-free Sealed Lead-Acid (SLA) batteries, either Gel-Cell batteries or Absorbent Glass Mat (AGM) batteries.

For most applications wet/flooded lead-acid batteries are simply phased out.

Lithium group 31 batteries are more and more common, with deep cycle models offering a large number of charging/discharging cycles (10-15x more cycles), lighter weight (2-3x lighter), but with a much higher initial price. For safety reasons, lithium batteries are almost exclusively made as Lithium Iron Phosphate (LiFePO₄) with protective Battery Management Systems (BMS).

Group 29 vs Group 30 vs Group 31 Battery

Group 31 batteries are very popular batteries. However, some battery groups feature dimensions very similar to this group:

- Group 29H: 13 1/8 x 6 3/4 x 9 1/8 inches (334 x 171 x 232 mm),

- Group 29NF: 13 x 5 1/2 x 8 15/16 inches (330 x 140 x 227 mm),

- Group 30H: 13 1/2 x 6 13/16 x 9 1/4 inches (343 x 173 x 235 mm),

- Group 31: 13 x 6 13/16 x 9 7/16 inches (330 x 173 x 240 mm).

As one can see, groups 29H, 29NF, and 30H are very similar in size to group 31 - always, but really always before getting a new battery check the size of your battery compartment and the actual size of the battery you are interested in.

Group 31 Deep Cycle Battery

Practically all the most popular group 31 batteries are intended for deep cycle applications, with some models being designed as dual-purpose batteries.

Deep cycle group 31 batteries are commonly used in off-the-grid applications as solar/wind batteries, as RV house batteries, as trolling motor batteries, in medical and security backup systems, and similar standby/float and deep cycle applications.

Dual-purpose group 31 batteries are used as strong marine, automotive and industrial batteries for powering both high-demanding loads and as deep discharge batteries in single-battery systems.

Most popular battery group 31 batteries are given in the following comparison chart:

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Most notable Group 31 batteries include:

Optima 8052-161 D31M is the lightest AGM group 31 battery, designed as a dual-purpose battery.

It has the lowest 20h capacity due to its spiral-wound cells. But, despite its 75Ah 20h capacity, it features 900 CCA and 1125 MCA Amps rating and 155 minutes RC.

It weighs 'only' 59.8 pounds (27.1 kg).

Optima 8052-161 D31M is a strong and durable battery, very resistant to vibrations and mechanical impacts. It is very often used where both starting and cycling features, at an acceptable weight, are important. Also, it is a very popular trolling motor battery, especially for mid-range trolling motors (30-40 Amps max).

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VMAXTANKS deep cycle AGM group 31 batteries include three models: SLR125, MR137-120, and XTR31-135 batteries.

VMAXTANKS SLR125 is intended for deep cycle off-the-grid applications, but it can also be used as an excellent trolling motor battery, sump pump battery, RV house battery, and similar.

VMAXTANKS SLR125 features 125 Ah nominal capacity, 260 minutes RC, and weighs 75 pounds (34 kg). It should not be used as a starting/cranking battery.

VMAXTANKS MR137-120 is also intended for deep cycle applications, but it can also be used for starting smaller gas and diesel engines in single-battery systems.

VMAXTANKS XTR31-135 is labeled as a deep cycle battery, but it also features a decent starting/cranking rating. It can be used for starting smaller gas and diesel engines (few hundred Amps at most), but it is best used as a deep cycle battery since it features a nominal capacity of 135 Ah and 265 minutes RC.

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NorthStar NSB-AGM31 and Odyssey 31M-PC2150 are very similar dual-purpose group 31 batteries. They feature very similar 20h capacity (NorthStar NSB-AGM31 103 Ah, Odyssey 31M-PC2150 100 Ah), very similar RC value (NorthStar NSB-AGM31 220 min, Odyssey 31M-PC2150 205 min), and exactly the same cranking values of 1150 CCA and 1370 MCA Amps - and that is plenty of power.

Also, NorthStar NSB-AGM31 weighs 75 pounds (34 kg) and is somewhat lighter than Odyssey 31M-PC2150 (77.8 pounds, 35.3 kg).

Both batteries are strong, durable, and reliable batteries - but, which one is better?

Personally, these batteries are so similar that one should go for the battery that is cheaper at that moment. Also, keep in mind that NorthStar NSB-AGM31 is a somewhat better deep-cycle performer, and Odyssey 31M-PC2150 is an excellent performer at very low temperatures.

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Group 31 Gel vs AGM Batteries

Both Gel-cell and AGM batteries are designed as low-maintenance spill-proof batteries.

Generally, gel-cell batteries feature lower charging/discharging currents, but tolerate a larger number of charging/discharging cycles.

But, this highly depends on the battery model and manufacturer.

A typical example of such batteries are Renogy RNG-BATT-AGM12-100, Renogy RNG-BATT-GEL12-100 ('Pure Gel'), and Renogy RBT100GEL12-G1 ('Hybrid Gel') batteries.

Renogy RNG-BATT-AGM12-100 is AGM battery that is able to withstand 600 cycles down to 50% DoD, while Renogy RNG-BATT-GEL12-100 ('Pure Gel') is able to withstand some 1100 cycles down to 50% DoD.

The maximum charging current of AGM model is 40 Amps, and only 15 Amps for the 'Pure Gel' model.

'Hybrid Gel' battery is a newer gel battery that may be recharged with 30 Amps max and features 1000 Amps 5s maximum current, while AGM version features 1200 Amps 5s maximum current.

'Pure Gel' model and maximum current? This value is not even labeled - 'Pure Gel' model is an excellent battery for solar, wind, and other similar applications where a large number of slow charging/discharging cycles are required.

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Group 31 Lithium Batteries

Group 31 lithium batteries are deep cycle batteries that offer a large number of charging/discharging cycles (up to, sometimes even more than 4000-5000!), typical capacity of 100 Ah, practically negligible capacity loss due to the high discharge currents, etc.

For example, GreenLife GL100 features a nominal capacity of 100 Ah, but it is able to provide 20 Amps for 300 minutes, 25 Amps for 240 minutes or 50 Amps for 120 minutes - practically, no loss of capacity due to the faster discharge.

Also, this battery weighs 28 pounds (12.7 kg) and is at least 2x lighter than the lead-acid batteries. Considering the actual capacity of lead-acid batteries when being discharged for 1h (100Ah for 20h usually means 60-65 Ah for 1h), this weight difference is actually much larger in favor of lithium batteries.

GreenLife GL100 is designed to last 3000-5000 charging/discharging cycles, while the typical lead-acid battery can withstand 300-500 cycles down to 60-80% DoD.

Each lithium-ion battery should feature the Battery Management System (BMS) that protects the battery from unwanted events like short circuit, overcharge, over-discharge, large currents, and similar.

While lead-acid batteries may be easily connected in series and parallel (same battery models from the same batch recommended), lithium-ion batteries should be connected in series and parallel ONLY if permitted/intended by their manufacturer.

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Long Story Short: Battery Group 31 batteries are well-balanced batteries, offering large capacities and strong discharge currents, at an acceptable weight.

When looking for your next group 31 battery, choose according to your own needs and requirements.