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Group 24 Marine Starting Battery Guide: 935 MCA AGM Explained
Short answer: A Group 24 marine starting battery like the Pro-Guide 24MX935 is a 12V AGM cranking battery rated at 935 marine cranking amps (MCA), 750 cold cranking amps (CCA) and 140 minutes of reserve capacity, packed into a compact Group 24 case (10 1/4 x 6 13/16 x 8 7/8 in) that weighs 42 lb. It gives single-engine boats strong, sealed, maintenance-free starting power in the most common marine battery size, backed by an 18-month free replacement warranty.
Most boats leave the factory with a Group 24 tray, box and hold-down, and the starting battery in that slot does the one job you can't afford to have fail: turning the engine over at the ramp and again at the end of the day. This guide explains what MCA, CCA and reserve capacity really mean, how to size a starting battery for your outboard, how to charge an AGM correctly, and how AGM compares with flooded and lithium starting batteries.
Key Takeaways
- Strong cranking in a compact case: 935 MCA and 750 CCA in a standard Group 24 footprint that fits most factory battery trays and boxes.
- Reserve for the unexpected: 140 minutes of reserve capacity at 25A keeps bilge pumps and nav lights running if the charging system quits.
- Sealed AGM construction: no water to add and no free-flowing acid to slosh in rough water.
- Lightest in the line: 42 lb (19 kg), compared with 50 lb for the Group 27 and 60 lb for the Group 31 Pro-Guide AGM starting batteries.
- Backed by Pro-Guide: 18-month free replacement warranty.
What Is a Group 24 Marine Starting Battery?
"Group 24" is a case-size standard from the Battery Council International (BCI). It defines the outside dimensions of the battery, not its chemistry or power. Group 24 is the most common size in small and mid-size boats, which is why so many center consoles, bass boats, pontoons and walleye rigs ship with Group 24 trays and hold-downs.
A marine starting battery (also called a cranking battery) is built to deliver a large burst of current for a few seconds to spin the starter motor, then get recharged by the engine's alternator. That is a different job from a deep cycle battery, which delivers a smaller current for hours to run a trolling motor or electronics. Starting batteries use more, thinner plates to maximize surface area and burst current, so they are rated in cranking amps rather than amp-hours.
The Pro-Guide 24MX935 combines the two: a dedicated AGM (absorbent glass mat) starting battery in the Group 24 case. Pro-Guide built its AGM starting line to deliver reliable engine starts in demanding marine environments, and the 24MX935 is the most compact model in that line.
Pro-Guide 24MX935 Specifications
| Model / SKU | Pro-Guide 24MX935 (PGM24MX935) |
| Type | AGM marine starting battery |
| Group size | BCI Group 24 |
| Voltage | 12V |
| Marine cranking amps (MCA) | 935 |
| Cold cranking amps (CCA) | 750 |
| Reserve capacity | 140 minutes at 25A |
| Charge current (standard / max) | 10A / 15A |
| Charging temperature | 0°F to 122°F |
| Discharge temperature | -4°F to 140°F |
| Terminals | 1 SAE post, 1 stud terminal |
| Dimensions (L x W x H) | 10 1/4 x 6 13/16 x 8 7/8 in (260 x 173 x 225 mm) |
| Weight | 42 lb (19 kg) |
| Warranty | 18-month free replacement |
Want the full numbers? Download the 24MX935 data sheet (PDF).
The 24MX935 prints its key ratings right on the label: 935 MCA, 750 CCA and 140 minutes of reserve capacity.
MCA vs. CCA vs. Reserve Capacity Explained
Starting batteries are compared on three numbers. Here is what each one tells you, using the 24MX935 as the example.
| Rating | What it measures | 24MX935 |
| Marine cranking amps (MCA) | Amps the battery can deliver for 30 seconds at 32°F while holding at least 7.2V | 935 MCA |
| Cold cranking amps (CCA) | The same test at 0°F, where batteries are weaker | 750 CCA |
| Reserve capacity (RC) | Minutes the battery can supply 25A before it is fully discharged | 140 minutes |
MCA is the boating number, because most boats are launched in water well above freezing. CCA is the tougher, lower number and matters if you fish early spring, late fall or in cold climates. MCA is always higher than CCA for the same battery because a battery delivers more current when it is warmer.
Reserve capacity is your safety margin. If the alternator or regulator fails, RC tells you roughly how long the battery can carry essential loads on its own. At 25A, the 24MX935 lasts about 140 minutes. Lighter loads last longer, but treat a starting battery's reserve as emergency capacity, not something to drain every trip. As a rough, conservative estimate, 140 minutes at 25A is about 58 amp-hours delivered; using only about half of that to protect the battery leaves around 29Ah, or about 6 hours of a 5A load such as a fish finder, nav lights and a bilge pump cycling.
How to Size a Starting Battery for Your Outboard
Every outboard and sterndrive manufacturer publishes a minimum battery rating in the owner's manual, usually as a minimum MCA, CCA or both. Use that as your floor, then add margin:
- Meet or beat the manual. Your battery's MCA and CCA should be equal to or higher than the engine maker's minimum. If your manual calls for up to 935 MCA or 750 CCA, the 24MX935 covers it.
- Add margin for cold starts. If you launch in cold weather, compare the CCA figure, not just MCA. Cold engine oil and a cold battery are the hardest combination for any starter.
- Account for what runs before the engine starts. Graphs, stereos and livewell pumps used while the engine is off all pull from the battery you need to crank with. If your starting battery also runs house loads, step up to more reserve capacity, or isolate electronics on their own battery (see Should You Run a Separate Battery for Your Boat Electronics?).
- Check the physical fit. Measure length, width and height, including clearance above the terminals. The 24MX935 measures 10 1/4 x 6 13/16 x 8 7/8 in.
- Match your terminals. The 24MX935 has an SAE post and a stud terminal, so you can use clamp-style or ring-terminal cable ends.
In general, compact Group 24 starting batteries suit single small to mid-size outboards where the manufacturer's minimum falls within the battery's rating. Larger engines, twin-engine setups and boats with heavy electrical loads often call for a bigger case with more cranking amps and reserve, such as the Pro-Guide 31MX1150 (1150 MCA, 925 CCA).
AGM vs. Flooded vs. Lithium Starting Batteries
All three chemistries can start an outboard. Here is how they typically compare for marine starting duty.
| Factor | AGM (Pro-Guide 24MX935) | Typical flooded lead-acid | Typical lithium starting |
| Maintenance | Sealed; no watering | Check and top up water regularly | None |
| Spill risk | Electrolyte held in glass mats | Free liquid acid can spill if tipped | No liquid electrolyte |
| Vibration resistance | Generally good; tightly packed plates | Generally lower | Generally good |
| Cold-weather cranking | Rated 750 CCA at 0°F | Varies by model | Can be limited by the BMS in freezing temperatures |
| Charging | Standard alternators and AGM-mode chargers | Standard alternators and chargers | Often needs a lithium-compatible charger |
| Upfront cost | Mid | Lowest | Highest |
For most boaters replacing a factory starting battery, AGM is the practical sweet spot. It drops into the same tray, works with the engine's existing charging system, does not need watering, and holds up better to pounding and spray than a typical flooded battery. Pro-Guide describes its AGM starting line as offering better performance and longer life than traditional flooded options. Lithium starting batteries save significant weight, but they cost more and you need to confirm compatibility with your engine's charging system. If weight is your priority, browse starting lithium batteries.
What Is the 24MX935 Best For?
The 24MX935 is a dedicated starting battery. It is not intended to power a trolling motor all day. Here is where it fits best:
- Single-outboard fishing boats: bass boats, walleye rigs, jon boats and bay boats with a Group 24 tray and an engine whose manual calls for no more than 935 MCA.
- Factory replacements: swapping out a tired flooded or AGM Group 24 cranking battery without changing trays, boxes or hold-downs.
- Weight-conscious boats: at 42 lb, it is the lightest Pro-Guide AGM starting battery, which helps smaller hulls and makes the install easier.
- Boats with a separate house or trolling bank: let the 24MX935 crank the engine while dedicated deep cycle batteries run the trolling motor and electronics. See One Pack or Three? Choosing the Right Trolling Motor Battery Setup.
One customer reported using the 24MX935 for full days of fishing and said it fired up their Suzuki outboard without issue at the end of the day.
Top view of the 24MX935: built-in carry handle, covered positive and negative terminals and the rating label.
How to Charge an AGM Starting Battery
On the water, your outboard's charging system recharges the 24MX935 after each start. Between trips and over the winter, an onboard or portable charger keeps it topped off. Follow these rules:
- Use the AGM setting. If your charger has selectable profiles, choose AGM. AGM batteries are sensitive to overcharging, and a flooded or lithium profile is not the right match.
- Stay within the rated current. Pro-Guide rates the 24MX935 for a 10A standard and 15A maximum charge current. A 10A charger such as the 12V 10A 1-Bank Battery Charger is a good match. If you charge the starting battery along with a deep cycle battery, a 12V 10A 2-Bank Battery Charger handles both banks.
- Charge between 0°F and 122°F. That is the 24MX935's rated charging range. It can discharge (crank) from -4°F up to 140°F.
- Recharge promptly. Lead-acid batteries, AGM included, can sulfate if left partially discharged. Recharge after any trip where you ran accessories with the engine off.
- Use a maintainer for storage. Over the off-season, keep the battery on a smart charger or maintainer, or charge it fully and disconnect it.
Installation and Maintenance Tips
Secure it in a box or tray
A starting battery that shifts in rough water can loosen cables or damage terminals. Mount the 24MX935 in a NOCO Group 24 Snap-Top Battery Box or a NOCO Group 24 Battery Tray with a proper hold-down.
Keep connections clean and tight
Loose or corroded terminals are a common cause of slow cranking. Tighten connections and protect them with battery corrosion preventative spray, especially in salt water.
Mind the terminal covers
The 24MX935 ships with red and black terminal covers. Keep the positive covered to prevent accidental shorts from tools or loose gear.
Carry a backup
Even a healthy battery can be drained by a stereo left on. A 12V 1000A jump starter in the console is cheap insurance.
For a full seasonal checklist, read our marine battery maintenance guide.
24MX935 vs. 27MX935: Which Should You Choose?
Both are Pro-Guide 12V AGM starting batteries with the same cranking ratings: 935 MCA and 750 CCA. The differences are size, reserve capacity and weight.
| Spec | 24MX935 | 27MX935 |
| Group size | 24 | 27 |
| MCA / CCA | 935 / 750 | 935 / 750 |
| Reserve capacity | 140 minutes | 175 minutes |
| Length | 10 1/4 in | 12 1/16 in |
| Weight | 42 lb | 50 lb |
Choose the 24MX935 if your boat has a Group 24 tray or box, or if you want the lightest, most compact option. Choose the 27MX935 if you have a Group 27 compartment and want more reserve capacity for accessories. If your engine needs more than 935 MCA, look at the 31MX1150.
Frequently Asked Questions
What is the best Group 24 marine starting battery?
The best one meets or exceeds your engine's required MCA and CCA and fits your tray. The Pro-Guide 24MX935 delivers 935 MCA, 750 CCA and 140 minutes of reserve capacity in a sealed AGM Group 24 case.
What is the difference between MCA and CCA?
MCA (marine cranking amps) is measured at 32 degrees F and CCA (cold cranking amps) at 0 degrees F. MCA is always the higher number. The Pro-Guide 24MX935 is rated 935 MCA and 750 CCA.
How many cranking amps do I need for my outboard?
Check your engine owner's manual for the minimum MCA or CCA and choose a battery that meets or exceeds it. If the manual calls for 935 MCA or less, the Pro-Guide 24MX935 covers it.
Can I use a Group 24 starting battery for a trolling motor?
It is not recommended. A starting battery like the Pro-Guide 24MX935 is built for short, high-current bursts. Use a dedicated deep cycle battery for a trolling motor.
Do AGM starting batteries need a special charger?
Use a charger with an AGM setting. The Pro-Guide 24MX935 is rated for a 10A standard and 15A maximum charge current.
How much does a Group 24 AGM starting battery weigh?
Weights vary by brand. The Pro-Guide 24MX935 weighs 42 lb (19 kg).
What warranty comes with the 24MX935?
The Pro-Guide 24MX935 includes an 18-month free replacement warranty.
Get Dependable Group 24 Starting Power
If your boat has a Group 24 tray and an engine that calls for up to 935 MCA, the Pro-Guide 24MX935 is a simple, sealed, maintenance-free upgrade that starts your outboard at the ramp and again after a long day on the water. It is the lightest AGM starting battery in the Pro-Guide line and is backed by an 18-month free replacement warranty.
Not sure which starting battery fits your engine? Call our team at 1-800-827-3232 or browse all AGM batteries and battery chargers.
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