Marine Battery Types: Starting, Deep Cycle, Dual Purpose
Starting, deep-cycle and dual-purpose marine batteries explained, plus flooded vs AGM vs gel vs lithium, BCI group sizes, CCA, MCA and reserve capacity.
The word “marine battery” covers batteries built for three different jobs and four different chemistries, sold in a handful of standard case sizes. Mixing up the job is the most common mistake: a starting battery on a trolling motor dies young, and a deep-cycle battery may not meet an outboard maker’s cranking spec. This page sorts the types out and translates the numbers on the label.
What are the main types of marine batteries?
There are three marine battery types by job: starting (cranking), deep cycle and dual purpose. Each can be built with flooded lead-acid, AGM, gel or lithium (LiFePO4) chemistry, so “AGM deep cycle” and “flooded starting” are both real products.
- A starting battery is a battery designed to deliver a large current for a few seconds to turn an engine over, then be recharged by the engine’s alternator.
- A deep-cycle battery is a battery designed to deliver a moderate current for hours and be drained deeply and recharged many times, which is what a trolling motor or fish finder needs.
- A dual-purpose battery is a battery built as a compromise: enough cranking current for a small outboard and enough reserve to run accessories.
The difference is inside the case. Starting batteries use plates with a large surface area for instant current, while marine and deep-cycle types use thicker plates with more room below them for shed material to collect (Wikipedia, automotive battery).
How do starting, deep-cycle and dual-purpose batteries compare on paper?
Compare one maker’s line and the pattern is plain: starting batteries list cranking amps and no amp-hours, deep-cycle batteries list amp-hours, and dual-purpose batteries list both. Interstate Batteries’ published specs, checked October 2026:
| Interstate model | Type | Group | CCA | Reserve capacity | Ah (20 h) | Warranty | List price |
|---|---|---|---|---|---|---|---|
| 24M-XHD | Starting, flooded | 24M | 800 | 135 min | Not listed | 12 months | $169.95 |
| SRM-24 | Deep cycle, flooded | 24M | 550 | 140 min | 81 | 12 months | $159.95 |
| SRM-27 | Deep cycle, flooded | 27M | 600 | 160 min | 88 | 12 months | $169.95 |
| SRM-31 | Deep cycle, flooded | 31M | 675 | 210 min | 98 | 12 months | $194.95 |
| 31M-AGM | Dual purpose, AGM | 31M | 925 | 200 min | 100 | 36 months | $459.95 |
| LI27M12V100 | Deep cycle, LiFePO4 | 27M | Not listed | 240 min | 100 | 10 years limited | $804.95 |
Prices are Interstate’s suggested retail in USD on 2026-10-06; street prices are often lower. The lithium warranty is 5 years free replacement and years 6 to 10 performance.
Note that even Interstate’s deep-cycle flooded batteries carry a CCA figure. A CCA number on the label does not make a battery a starting battery; the amp-hour and reserve-capacity numbers tell you whether it can run a trolling motor.
Which marine battery should run a trolling motor?
A deep-cycle battery, or a dual-purpose battery with enough amp-hours. Minn Kota’s 2026 battery guide says any 12 V lead-acid deep-cycle marine battery will run its motors, recommends at least 110 Ah (usually group 27 or larger), and calls dual-purpose batteries a good choice when their amp-hour rating meets that guidance.
A pure starting battery is the wrong tool. It is built for shallow discharges, and Wikipedia’s summary of lead-acid behaviour is blunt: starting batteries are not designed for deep discharge and their life is reduced when they get it.
How many amp-hours you need for a given motor and day is worked out in trolling motor battery sizing.
What is the difference between flooded, AGM, gel and lithium?
Flooded is cheapest and needs topping up with water; AGM is sealed and maintenance-free at up to twice the price; gel is sealed but charges slower and dislikes heat; lithium (LiFePO4) costs the most and weighs the least. The comparison below follows Minn Kota’s own battery guides:
| Chemistry | Maintenance | Weight | Cost | Watch out for |
|---|---|---|---|---|
| Flooded lead-acid | Top up with distilled water | Heavy | Lowest, about $100 or less for budget models | Lasts a minimum of 2 to 3 years; vent the compartment |
| AGM | None (sealed) | Similar to flooded or slightly heavier | Up to twice flooded for the same group size | Can be more permanently damaged by a full drain |
| Gel | None (sealed) | Lead-acid class | Varies by maker | Less heat-tolerant, less capacity than AGM of the same size, charges slower |
| LiFePO4 lithium | None | Roughly a third to half of lead-acid | Highest | Needs a lithium charge profile; cold-charging limits |
Minn Kota Precision chargers made in 2021 or later have a dedicated lithium profile; earlier Precision models can charge lithium on the AGM setting, but Minn Kota says to confirm with the battery maker. The full cost and runtime case is in lithium marine batteries vs AGM.
What does the BCI group size mean?
A BCI group size is a Battery Council International code for a battery’s case dimensions and terminal layout, not its capacity. Two group 31 batteries can hold very different amp-hours. Interstate’s published case sizes show the common marine groups:
| Group | Length x width x height (Interstate SRM series) | Minn Kota amp-hour range |
|---|---|---|
| 24 (24M) | 11 x 6 7/8 x 9 in | 70-85 Ah |
| 27 (27M) | 12 3/4 x 6 7/8 x 9 in | 85-110 Ah |
| 31 (31M) | 13 x 6 3/4 x 9 5/8 in | 95-125 Ah |
Dimensions vary slightly by maker and chemistry: Interstate’s 27M-AGM measures 12 7/16 x 5 3/4 x 8 7/8 inches. Measure your battery box before ordering, and check that the terminal type (top post, stud or dual) fits your cable lugs. Box sizes by group and the tie-down rules are in marine battery boxes.
What do CCA, MCA, reserve capacity and amp-hours mean?
CCA and MCA measure cranking power for an engine; reserve capacity and amp-hours measure how long a battery can run a load.
- CCA (cold cranking amps): the current a battery can deliver at 0 F for 30 seconds while staying at or above 7.2 volts.
- MCA (marine cranking amps): the same test at 32 F, so the MCA figure is always higher than the CCA of the same battery.
- Reserve capacity (RC): the minutes a fully charged battery can supply 25 amps. Interstate labels it “RC @ 25 amps.”
- Amp-hours (Ah): the current a battery delivers over 20 hours before voltage falls below 10.5 V at 80 F. A 100 Ah battery should supply 5 amps for 20 hours.
Reserve capacity is a useful cross-check when Ah is missing. Minn Kota’s rule is at least 180 minutes RC for a trolling motor battery. By that test the SRM-27 above (160 minutes) falls short and the SRM-31 (210 minutes) passes. RC also shows how capacity shrinks at higher current: 210 minutes at 25 A is 87.5 Ah, below the SRM-31’s 98 Ah 20-hour rating.
How should a small boat with an outboard set up its batteries?
Use a starting or dual-purpose battery for the outboard and separate deep-cycle batteries for the trolling motor. Minn Kota recommends this split whenever a crank battery starts a gas outboard, so a long day on the trolling motor never leaves you unable to start the engine. How much CCA your outboard needs, by horsepower, is covered in marine starting batteries.
On a 24 V or 36 V trolling motor bank, do not use one battery of the series as the starting battery; Minn Kota warns it draws current unevenly and damages the batteries over time. Its Riptide manual adds a second rule: automatic jump-start systems and battery selector switches tie battery negatives together, so the only trolling motor battery that may connect to them is the “low side” battery, the one the motor’s negative lead goes to. Series wiring is covered in 12 vs 24 vs 36 volt trolling motors, and the switch choices for a two-battery boat in boat battery switches.
Every battery then needs a way back to full. An onboard charger with one bank per battery is the simplest; see the marine battery charger guide.
How long do marine batteries last?
A flooded deep-cycle battery should last a minimum of 2 to 3 years, according to Minn Kota, and AGM a little longer when properly maintained; lithium makers back theirs with warranties of up to 10 years. Care decides where in that range you land.
- Recharge lead-acid within 12 to 24 hours of every use. Minn Kota names failure to do so as the leading cause of premature battery failure.
- Avoid running lead-acid flat. A battery cycled to 50% depth of discharge lasts longer than one regularly pushed deeper.
- Store fully charged, on a maintainer over winter.
- Keep terminals clean and tight, and fuse every circuit; the gauge and breaker table is in the trolling motor wiring guide.
Who is this guide not for?
This guide covers 12 V batteries for small fishing boats: one cranking battery and up to three trolling motor batteries. It does not cover house banks with inverters, AC shore power, or building your own lithium pack; those belong with a qualified marine electrician working to ABYC standards.
Prices are US list prices in USD. Brands, warranties and prices differ in other countries, so check local availability.
Frequently asked questions
Quick answers to what anglers ask us most
What is the difference between a starting and a deep-cycle marine battery?
A starting battery has plates with a large surface area to deliver a big burst of current for a few seconds. A deep-cycle battery has thicker plates built to deliver moderate current for hours and to be drained and recharged many times.
Can a dual-purpose battery run a trolling motor?
Yes, as long as its amp-hour rating is high enough. Minn Kota's 2026 battery guide calls dual-purpose batteries a good choice if they meet its capacity guidance of at least 110 Ah, usually group 27 or larger.
What does the group size on a marine battery mean?
It is a Battery Council International (BCI) code for the case dimensions and terminal layout, not the capacity. A group 31 case is about 13 x 6.75 x 9.6 inches, but capacity inside it varies by maker and chemistry.
What is the difference between CCA and MCA?
Both measure the current a battery can deliver for 30 seconds while holding at least 7.2 volts. CCA is tested at 0 F and MCA at 32 F, so the MCA number on the same battery is always higher.
What reserve capacity do I need for a trolling motor?
Minn Kota says to choose a deep-cycle battery with at least 180 minutes of reserve capacity when no amp-hour rating is given. Reserve capacity is the minutes a full battery can supply 25 amps.