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Group 31 battery: match fit and electrical duty

By the Plain Prepper editors. Updated 2026-10-14.

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Quick answer

A Group 31 battery label is not enough to choose a replacement. Check exact height including terminals, hold-down, voltage, chemistry and the equipment requirements. ODYSSEY ODX-AGM31 lists 1,150 CCA and 103 Ah at the 20-hour rate. Trojan 31-AES has a Group 31 version rated 104 Ah at that rate. LiTime markets its 12V 100Ah TM as Group 31 size, but explicitly prohibits using that model as a starting battery. Physical fit does not make these batteries interchangeable.

Start with the job, then read the group designation

For an engine-starting replacement, use the equipment maker's approved battery specification and starting requirement. For a component backup bank or house loads, inspect the deep-cycle capacity, current limit and charging system. A battery that occupies the available space may still be wrong for the electrical duty.

Battery Council International says group classifications consider voltage, maximum overall dimensions, terminal arrangement and other fit features. Its explanation also discusses performance and chemistry designations. Do not reduce the complete selection to the number on the case.

BCI's published dimensional table lists 31A and 31T at 13 in long, 6 13/16 in wide and 9 7/16 in high, with different terminal references. These are reference group dimensions, not a measurement of every maker's current battery. The actual product drawing, terminal clearance and equipment approval remain necessary.

In particular, a lithium battery advertised as fitting Group 31 space does not inherit a lead-acid battery's charging profile or engine-start approval. Check the exact lithium model rather than applying a claim from another product in the same brand.

Compare documented examples without mixing their roles

Manufacturer example	Capacity / role	Published size L x W x H	Mass
ODYSSEY ODX-AGM31	12 V AGM; 103 Ah at 20 h; 1,150 CCA	13 x 6.8 x 9.5 in, terminals included	77.8 lb
Trojan 31-AES, Group 31 version	12 V AES AGM; 104 Ah at 20 h	12.80 x 6.81 x 9.37 in, current table	69 lb
LiTime 12V 100Ah TM	12.8 V nominal LiFePO4; 100 Ah; deep cycle only	13 x 6.77 x 8.43 in, spec table	Approx. 23.4 lb

Trojan's current page offers both Group 29 and Group 31 versions under the 31-AES name. The name alone therefore does not establish which size is being offered. Its quick facts and linked Group 31 datasheet give 9.43 in height, differing from the current table's 9.37 in. Confirm the terminal configuration and actual overall height.

LiTime has a similar discrepancy: feature text says 8.5 in high, while its specification table says 8.43 in. Do not promise a tight lid clearance from the smaller value.

ODYSSEY distinguishes terminal-inclusive height from the container alone, listed at 8.5 in. Use the overall height when checking the compartment. The different reference and maker dimensions do not justify assuming every item marked Group 31 fits the same closed box.

CCA, amp-hours and BMS current answer different questions

Trojan defines CCA as the discharge current a new, fully charged battery can maintain for 30 seconds at 0°F above a specified voltage per cell. Its linked Group 31 AES datasheet lists 540 CCA. That starting-performance figure is distinct from capacity in amp-hours or a short electronic current limit.

LiTime lists a 100 A continuous discharge limit and a 500 A discharge allowance for one second on its TM product page. Neither is a CCA rating. The same page explicitly says not to use this model to start devices. Do not turn its brief surge allowance into permission to replace a starter battery.

Amp-hours also need the rating conditions. Trojan lists 83 Ah at the five-hour rate and 104 Ah at the 20-hour rate. The same battery has different published capacity under different discharge conditions. Its numbers are not a guaranteed runtime at an arbitrary inverter load.

For nominal energy arithmetic, 12 V multiplied by 104 Ah gives 1,248 Wh. LiTime's 12.8 V times 100 Ah gives 1,280 Wh, matching its named energy figure. These calculations do not establish identical usable AC energy after discharge limits, wiring and inverter losses.

Trojan's quick facts list 1.38 kWh, its current Group 31 table lists 1.33 kWh in an energy column labeled 100-hour rate, and the linked sheet gives 1.38 kWh at that rate with 115 Ah. The sheet also gives 178 minutes at 25 A, versus 177 in the Group 31 table and 180 in quick facts. Those discrepancies are explicit; do not substitute one value silently for a load-runtime promise.

Check terminals, retention and ventilation before replacement

ODYSSEY identifies ODX-AGM31 with threaded stud terminals. LiTime lists M8 bolts. Trojan offers terminal configurations and warns that dimensions depend on the handle or terminal. Confirm the actual delivered variant and appropriate hardware; a familiar group number does not establish that existing cable ends or hold-down parts match.

Measure the compartment and inspect the cable route, polarity and clearance over exposed terminals. Have the mounting and wiring assessed for the actual installation. Avoid stretching cables or improvising adapters to conceal a terminal mismatch.

Trojan says its AES battery can mount horizontally or vertically, but that is model-specific permission. Its Group 31 datasheet also says not to install or charge in a sealed or unventilated compartment. Non-spillable AGM does not mean the charging enclosure can ignore ventilation.

The Trojan user guide calls for protective clothing, eye protection, acid-resistant gloves and insulated tools. It prohibits metal jewelry and warns against placing tools or metal objects across batteries. The weight labels also warrant a safe lifting plan rather than an assumption that a handle makes every installation manageable.

Match the complete charging profile and cold-weather limits

Trojan's linked Group 31 AES sheet lists 14.40 V absorption and 13.50 V float at 77°F, with temperature compensation. Its maximum charge current is 50% of the 20-hour capacity. For the listed 104 Ah, that calculates to 52 A as a maximum, not a required charging current. These are settings for the specified battery and conditions, not a universal AGM charger program.

LiTime recommends constant-current/constant-voltage charging at 14.4 V plus or minus 0.2 V, with 20 A recommended charging current for this TM model. Although an absorption voltage can look similar, the full charge behavior and chemistry requirements differ. Confirm a compatible charger, controller and any alternator-charging arrangement before changing chemistry.

The LiTime page says charging automatically stops below 32°F and recovers at or above 41°F. It does not list self-heating for this version. Low-temperature cutoff is protection, not a promise that charging remains available during a cold outage.

Trojan advises against charging a frozen battery and against applying equalization to AES, AGM or gel batteries. Follow the current exact battery instructions and temperature limits. Do not carry a flooded-battery maintenance routine into a sealed or lithium replacement.

Inspect the exact Amazon variant and installation plan

Use the searches below to identify manufacturer-documented examples. Match the model, group version, terminal configuration and chemistry in the offer with the documentation. A shared capacity label does not justify replacing a required starter battery with a storage-only pack.

• Confirm engine-starting or deep-cycle duty with the equipment maker.
• Check overall dimensions, polarity, terminal hardware and hold-down.
• Compare capacity at the stated discharge rate and current limits.
• Resolve conflicting dimensions and energy figures for the exact variant.
• Verify charging, cold-weather operation, ventilation and safe wiring.

A component battery also needs the supported equipment around it. Do not treat the case size as an entire backup system or assume a built-in BMS replaces installation protections. Have cable and circuit protection requirements checked for the actual bank and load.

The deep-cycle battery guide explains storage-duty planning, while the AGM guide covers that lead-acid design. For Group 31 selection, keep fit and electrical role visible together.

What to look for when you buy

Questions people ask

What size is a Group 31 battery?

BCI lists reference 31A and 31T maximum dimensions of 13 x 6 13/16 x 9 7/16 in. Verify the exact maker dimensions, terminal-inclusive height, retention and cable layout; current examples differ in their published measurements.

How many amp-hours does a Group 31 battery have?

The number alone is not a capacity guarantee. The examples here include Trojan 104 Ah and ODYSSEY 103 Ah at the 20-hour rate, and LiTime 100 Ah. Keep voltage, chemistry, current limits and rating conditions visible.

Can the LiTime Group 31 TM replace an engine-starting battery?

No. Its current maker page expressly prohibits starting use. The continuous BMS limit and brief surge allowance are not a CCA rating or an engine-start approval.

Can I charge every Group 31 battery with the same charger?

Check the complete chemistry-specific profile, current and temperature requirements. This lithium example has a low-temperature charging cutoff, and Trojan AES has its own absorption, float and temperature-compensation instructions.

Sources

• BCI Group Sizes: classification and fit, Battery Council International
• BCI Group Numbers, Dimensional Specifications and Ratings, Battery Council International
• ODX-AGM31 (31-PC2150S), current specifications, ODYSSEY Battery
• 31-AES 12V AGM Battery, current Group 29 and 31 table, Trojan Battery
• GR31-AES datasheet, revision July 2023, Trojan Battery
• 12V 100Ah TM Deep Cycle Trolling Motor Battery, current specifications, LiTime
• Battery User Guide: flooded, AES, AGM and gel, Trojan Battery

Sources checked 2026-10-14.