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The engine turns over slow at the dock at dawn, the fish finder flickers, and the battery in the box is the difference between a day on the water and a tow back in, so marine battery types are worth sorting out before you buy. A marine battery is built for a boat: it starts the engine, runs the electronics, and takes the pounding of waves and trailer rides. This guide walks through the types and the figures on the label.
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Marine battery types: the quick answer
A marine battery is a battery built for a boat, and it comes in starting, deep cycle, and dual-purpose use types. A starting battery turns the engine over, a deep cycle battery runs electronics and gear over a long day, and a dual-purpose battery does each job from one box. The reference listings here are dual-purpose AGM batteries: the Mighty Max MM-G31M lists 110 Ah, 825 CCA, and 1000 MCA, and the UPLUS Group 27M lists 92 Ah and 800 CCA. Lithium is a separate chemistry, and it needs a charger that lists lithium support.
Step 1: Starting, deep cycle, and dual purpose
A starting battery is built to give a short, strong burst that turns an engine over, then lets the engine charging system take over. A deep cycle battery is built to give steady power over a long stretch and to be drawn down and recharged again and again. A dual-purpose battery is built to do each job: it starts the engine and runs the electronics from one box. The Mighty Max MM-G31M and the UPLUS Group 27M are dual-purpose batteries, and each lists starting and deep cycle use on the label. A boat that runs navigation gear, lights, and an engine all day wants deep cycle endurance. Pick the use type from the loads on the boat, not from the size of the box.
| Battery | Type | Capacity | Cranking | Reserve |
|---|---|---|---|---|
| Mighty Max MM-G31M | Dual-purpose AGM | 110 Ah | 825 CCA, 1000 MCA | 240 RC |
| UPLUS Group 27M | Dual-purpose AGM | 92 Ah | 800 CCA | 175 RC |
These are the figures the listings state. The Mighty Max listing states more amp-hours and more cranking amps than the UPLUS listing, and a reserve capacity figure of 240 RC against 175 RC on the UPLUS. Read the figures as a set, since no one figure says how the battery behaves on its own. Chemistry is separate from use type. AGM is a sealed lead-acid build, and the UPLUS listing notes that AGM batteries resist vibration better than flooded lead-acid batteries. Lithium is a separate chemistry that needs a charger that lists lithium support, and the listings here state no lithium figures.
Step 2: Read the capacity figure
Capacity is the amp-hour figure, written as Ah, and it says how much charge the battery holds. The Mighty Max MM-G31M lists 110 Ah, and the UPLUS Group 27M lists 92 Ah. A larger amp-hour figure means the battery holds more charge, so it runs the same loads for longer. Group size is a separate figure: the Mighty Max is Group Size 31M and the UPLUS is BCI Group 27M, and the group size has to match the battery box and the tray on the boat. The Mighty Max listing also states up to 700 cycles at 50% depth of discharge.
Step 3: Read the cranking and reserve figures
Cranking figures say how much starting power the battery gives. CCA is cold cranking amps, and MCA is marine cranking amps: the Mighty Max lists 825 CCA and 1000 MCA, and the UPLUS lists 800 CCA. The UPLUS listing states it starts engines from 70 to 350 HP, so check the engine size against the cranking figures on the label. Reserve capacity, written as RC, says how long the battery keeps a load going once the charging stops: the Mighty Max lists 240 RC and the UPLUS lists 175 RC. Read the cranking and reserve figures as a set with the amp-hour figure.
Step 4: Check size, weight, build, and warranty
Size and weight decide whether the battery fits the box and whether the boat carries it. The Mighty Max measures 13.00 x 6.81 x 8.35 inches, with 9.37 inches of total height, and weighs 70.70 lbs. The UPLUS measures 6.61 x 12.05 x 9.11 inches and weighs 49.0 pounds. Build matters on the water: the listings here are AGM, sealed and maintenance-free, and the UPLUS listing notes vibration resistance for wave pounding and trailer transport. The Mighty Max listing notes a rugged ABS shell and a two-year warranty in its bullets, while the UPLUS listing states a 2 Year Warranty.
The Mighty Max listing gives a weight and a warranty term in the bullets that differ from the spec table: the bullets say 70.70 lbs and a two-year warranty, while the spec table says 72.5 pounds and 1 Year Manufacturers Warranty. Trust the bullets here, and measure the box before you order.
Ways to make the battery last longer
- Recharge the battery after a day on the water, and do not leave it flat between trips.
- Keep the terminals clean and tight, and check the cables for wear where they pass through the hull or the box.
- Store the battery where it stays dry and out of the sun, and check the charge before the season starts.
- Match the charger to the battery type: a charger that lists AGM support suits an AGM battery, and one that lists lithium support suits a lithium battery.
FAQ: power away from the grid
What type of battery is a marine battery?
A marine battery is a battery built for a boat, in starting, deep cycle, or dual-purpose form. The reference listings here are dual-purpose AGM batteries, so they start the engine and run the electronics from one box.
Is a marine battery an AGM battery?
Not always. AGM is one build type: the Mighty Max and UPLUS listings here are AGM, while other marine batteries use flooded lead-acid or lithium builds. Check the label for the build type before you buy.
What does CCA mean on a marine battery?
CCA is cold cranking amps: the starting power the battery gives in cold weather. The Mighty Max lists 825 CCA and the UPLUS lists 800 CCA.
What is the difference between a starting battery and a deep cycle battery?
A starting battery gives a short, strong burst to turn the engine over, while a deep cycle battery gives steady power over a long stretch. A dual-purpose battery does each job from one box, which is what the Mighty Max and UPLUS listings state.
Can a marine battery start an engine and run electronics?
A dual-purpose battery can: the Mighty Max lists 110 Ah with 825 CCA and 1000 MCA, and the UPLUS lists 92 Ah with 800 CCA and starts engines from 70 to 350 HP. Pick the use type from the loads on the boat.
Related solar guides
See the marine battery maintenance, the lead acid battery maintenance, the life of lithium battery and every battery guide.
