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Best Group 31 Marine Battery (2026): Lithium vs. AGM vs. Flooded
Short answer: For most boats, the best Group 31 marine battery is a lithium dual purpose battery like the Pro-Guide 31M180-DP. It cranks an engine (1,000 CCA lead-acid equivalent), runs electronics and trolling loads from 180Ah (2,304Wh), and weighs 41.9 lb. If you want a sealed lead-acid battery, the Pro-Guide Group 31 AGM Deep Cycle (800 CCA, 205 minutes reserve capacity) is the best all-rounder. For pure cranking, choose the Group 31 AGM Starting battery (1,150 MCA).
Group 31 is the largest common marine battery size, so it is the go-to choice for big outboards, long trolling days and boats with heavy electronics loads. Below, we compare every Pro-Guide Group 31 battery and explain how to pick the right one for your boat.
Key Takeaways
- Start with the job: starting, deep cycle or dual purpose. Group 31 is only the case size.
- Lithium is lightest: 41.9 lb for the 31M180-DP, compared with 72.75 lb for the Group 31 AGM Deep Cycle.
- AGM is the sealed lead-acid choice: no watering, with up to 1,150 MCA for cranking or 205 minutes of reserve capacity for deep cycle use.
- Flooded costs the least up front but needs regular maintenance. Pro-Guide flooded batteries are sold through dealers.
- Check the height: standard Group 31 batteries are 8 1/2 to 9 1/8 in tall, while Group 31 Tall lithium models are 10.83 in tall.
Best Group 31 Marine Batteries at a Glance
| Best for | Battery | Key ratings | Weight | Warranty |
| Best overall (start + run) | 31M180-DP lithium | 180Ah / 2,304Wh, 1,000 CCA (PbEq), 180A continuous | 41.9 lb | 3-yr free replacement, 10-yr pro-rata |
| Most capacity | 31M235-DP lithium | 235Ah / 3,008Wh, 1,000 CCA (PbEq), 200A continuous | 52.9 lb | 3-yr free replacement, 10-yr pro-rata |
| Best AGM all-rounder | Group 31 AGM Deep Cycle | 800 CCA, 1,000 MCA, 205 min RC | 72.75 lb | 24-month free replacement |
| Best for cranking big engines | Group 31 AGM Starting | 925 CCA, 1,150 MCA | 60 lb | 18-month free replacement |
| Best value (dealer) | 31M225 flooded dual purpose | 675 CCA, 845 MCA, 210 min RC | 54 lb | 12-month free replacement, 30-month pro-rata |
| 24V or 36V trolling in one case | 24VM90 / 36VM60 | 2,304Wh each, 90A / 60A continuous | 41.9 / 44.1 lb | 5-yr free replacement, 10-yr pro-rata |
CCA = cold cranking amps. MCA = marine cranking amps. RC = reserve capacity in minutes at 25 amps. PbEq = lead-acid equivalent.
The Picks, Explained
Best overall: Pro-Guide 31M180-DP (lithium dual purpose)
The 31M180-DP replaces both a starting battery and a deep cycle battery. It is rated at 1,000 CCA (lead-acid equivalent) for cranking, stores 180Ah for electronics and trolling, and fits a standard-height Group 31 case (13.07 x 6.93 x 8.9 in). It adds built-in heating for cold-weather charging, Bluetooth monitoring and NMEA 2000 compatibility. At 41.9 lb, it weighs about 31 lb less than a Group 31 AGM deep cycle battery.
Most capacity: Pro-Guide 31M235-DP (lithium dual purpose)
The 31M235-DP stores 235Ah (3,008Wh) with the same 1,000 CCA (PbEq) rating and a 200A continuous rating. It uses the Group 31 footprint but stands 10.83 in tall, so measure your compartment height before you order.
Best AGM all-rounder: Group 31 AGM Deep Cycle
The Group 31 AGM Deep Cycle has 205 minutes of reserve capacity and 800 CCA / 1,000 MCA, so it can run trolling motors and house loads and still crank many engines. It is sealed and maintenance-free. The trade-off is weight: 72.75 lb.
Best for cranking big engines: Group 31 AGM Starting
The Group 31 AGM Starting battery delivers 925 CCA and 1,150 MCA, the highest cranking rating in our lead-acid lineup. Choose it for large outboards and inboards when you already have a separate deep cycle battery for your accessories.
Best value: 31M225 flooded dual purpose (dealer)
The 31M225 is a flooded dual purpose battery with 675 CCA, 845 MCA and 210 minutes of reserve capacity at 54 lb. It has the lowest up-front cost, but it needs periodic watering and checks. It is sold through Pro-Guide dealers. Call 1-800-827-3232 to find one near you.
24V or 36V trolling motors: 24VM90 or 36VM60
If your Group 31 compartment feeds a 24V or 36V trolling motor, one 24VM90 or 36VM60 replaces a two- or three-battery bank. See the best lithium battery for trolling motors guide for runtime math.
Lithium vs. AGM vs. Flooded in Group 31
| Factor | Lithium (LiFePO4) | AGM | Flooded |
| Weight (Pro-Guide Group 31) | 41.9 to 52.9 lb | 60 to 72.75 lb | 54 lb |
| Usable capacity | Nearly all of the rating | About half, to protect battery life | About half, to protect battery life |
| Voltage under load | Stays flat until nearly empty | Sags as it discharges | Sags as it discharges |
| Maintenance | None | None (sealed) | Check water levels |
| Cycle life | 3,000+ cycles at 100% depth of discharge | Typically a few hundred cycles | Typically a few hundred cycles |
| Warranty (Pro-Guide) | Up to 10 years pro-rata | 18 to 24 months free replacement | 12 months free replacement, 30 months pro-rata |
| Up-front cost | Highest | Middle | Lowest |
Lithium costs the most up front but lasts thousands of cycles, so it usually costs less per cycle over its life. AGM is the simplest upgrade from flooded because it uses the same charging approach (with an AGM profile) and needs no maintenance. For the full chemistry comparison, read What Is a LiFePO4 Battery?
How to Choose a Group 31 Marine Battery
- Decide on the job. A starting battery delivers a short, high burst for the engine. A deep cycle battery delivers steady power for hours. A dual purpose battery does both.
- Meet your engine's cranking requirement. Your engine manual lists a minimum CCA or MCA. Choose a battery at or above it.
- Size for your accessories. For deep cycle use, compare reserve capacity (lead-acid) or amp-hours and watt-hours (lithium).
- Check fit. Group 31 batteries are about 13 in long and 6 13/16 in wide. Height ranges from 8 1/2 to 9 1/8 in for standard models and is 10.83 in for Group 31 Tall. See the battery group size chart and the battery box size chart.
- Match your charger. Lithium, AGM and flooded batteries each need the right charging profile. Pro-Guide on-board chargers support all three.
Frequently Asked Questions
What is the best Group 31 AGM marine battery?
For combined starting and deep cycle use, the Pro-Guide Group 31 AGM Deep Cycle (800 CCA, 1,000 MCA, 205 minutes reserve capacity) is the best all-rounder. For cranking only, the Group 31 AGM Starting battery offers 925 CCA and 1,150 MCA.
Is a lithium Group 31 battery better than AGM?
For most boaters, yes. The Pro-Guide 31M180-DP weighs 41.9 lb compared with 72.75 lb for the Group 31 AGM Deep Cycle, can use nearly all of its 180Ah, holds its voltage under load and is rated for more than 3,000 cycles. AGM costs less up front.
Can a Group 31 battery start a boat engine and run a trolling motor?
A dual purpose battery can. The Pro-Guide 31M180-DP and 31M235-DP are rated at 1,000 CCA (lead-acid equivalent) and store 180Ah and 235Ah. The Group 31 AGM Deep Cycle also has 800 CCA alongside 205 minutes of reserve capacity.
How much does a Group 31 marine battery weigh?
Pro-Guide Group 31 batteries weigh 41.9 to 52.9 lb in lithium, 54 lb flooded and 60 to 72.75 lb AGM.
How long does a Group 31 marine battery last?
Lithium models are rated for more than 3,000 cycles at 100% depth of discharge, with up to a 10-year pro-rata warranty. Lead-acid batteries typically last a few hundred cycles, and Pro-Guide AGM models carry 18- to 24-month free replacement warranties.
Where can I buy the 31M225 flooded battery?
Pro-Guide flooded batteries are sold through dealers. Call 1-800-827-3232 and our team will point you to the nearest one.
Shop Group 31 Batteries
Pick the job, check your engine's cranking requirement and measure your compartment. Then choose lithium for the lightest, longest-lasting option, or AGM for a sealed lead-acid upgrade.
Prefer AGM? Browse AGM batteries, or call our team at 1-800-827-3232 for help choosing the right Group 31 battery.
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