Group 27 AGM Starting Battery: 935 MCA With More Reserve

Short answer: A Group 27 AGM starting battery like the Pro-Guide 27MX935 is a sealed 12V marine cranking battery rated at 935 marine cranking amps (MCA), 750 cold cranking amps (CCA) and 175 minutes of reserve capacity in a Group 27 case (12 1/16 x 6 13/16 x 8 7/8 in, 50 lb). The larger case gives you the same cranking power as a Group 24 with about 25% more reserve, so boats that run electronics, pumps and stereos with the engine off still have the power to start.

Group 27 is the step-up size for boats that carry more electrical gear than a basic fishing rig. This guide covers what a Group 27 AGM starting battery is, how to read cranking and reserve ratings, how to match a battery to your outboard or sterndrive, how much accessory time reserve capacity really buys you, and how to charge and install an AGM so it lasts.

Key Takeaways

• Cranking power: 935 MCA and 750 CCA for dependable starts.
• More reserve: 175 minutes at 25A, compared with 140 minutes for the Group 24 Pro-Guide 24MX935.
• Drop-in Group 27 fit: 12 1/16 x 6 13/16 x 8 7/8 in, sized for standard Group 27 trays and boxes.
• Sealed AGM: no watering and no free liquid acid, well suited to boats that pound through chop.
• Warranty: 18-month free replacement.

What Is a Group 27 AGM Starting Battery?

Battery group sizes are case dimensions set by the Battery Council International (BCI). A Group 27 battery is longer than a Group 24, which leaves room for more plate material and typically more reserve capacity. Many pontoons, center consoles, ski boats and cuddy cabins use Group 27 trays for exactly that reason.

AGM stands for absorbent glass mat. Instead of plates sitting in free liquid acid, the electrolyte is held in fiberglass mats pressed between the plates. That makes the battery sealed and spill-resistant, and the tightly packed construction generally handles vibration better than a typical flooded battery.

A starting battery is built to dump a large current for a few seconds to turn the starter, then be recharged by the engine. The Pro-Guide 27MX935 is a dedicated AGM starting battery in the Group 27 case, designed for reliable engine starts in demanding marine environments.

Pro-Guide 27MX935 Specifications

Model / SKU	Pro-Guide 27MX935 (PGM27MX935)
Type	AGM marine starting battery
Group size	BCI Group 27
Voltage	12V
Marine cranking amps (MCA)	935
Cold cranking amps (CCA)	750
Reserve capacity	175 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)	12 1/16 x 6 13/16 x 8 7/8 in (306 x 173 x 225 mm)
Weight	50 lb (22.7 kg)
Warranty	18-month free replacement

Want the full numbers? Download the 27MX935 data sheet (PDF).

The 27MX935 label lists its three key ratings: 935 MCA, 750 CCA and 175 minutes of reserve capacity.

Cranking Amps Explained: MCA, CCA and RC

Three ratings describe a marine starting battery. Knowing what each one measures makes it much easier to compare batteries and match one to your engine.

• Marine cranking amps (MCA): the current a battery can deliver for 30 seconds at 32°F while staying at or above 7.2V. This is the most relevant number for typical boating weather. The 27MX935 is rated 935 MCA.
• Cold cranking amps (CCA): the same 30-second test at 0°F. Cold slows the chemical reaction inside a battery, so CCA is always lower than MCA. The 27MX935 is rated 750 CCA. If you launch in early spring or late fall, give this number extra weight.
• Reserve capacity (RC): the number of minutes a fully charged battery can supply a steady 25A. The 27MX935 is rated 175 minutes. This is the number that separates it from the Group 24 version.

You may also see "CA" (cranking amps) on some data sheets. For marine batteries, CA and MCA describe the same 32°F test. Starting batteries usually do not carry an amp-hour rating, because they are not designed for long, deep discharges. The 27MX935 is rated by cranking amps and reserve capacity instead.

How Much Accessory Time Does 175 Minutes of Reserve Give You?

Reserve capacity is the most practical way to estimate how long a starting battery can carry accessories with the engine off. At 25A, the 27MX935 runs about 175 minutes. That works out to roughly 73 amp-hours delivered at that rate. To protect a lead-acid battery and keep enough left to crank, plan on using only about half of it, or around 36Ah.

Engine-off load	Typical draw	Rough estimate using about half the reserve
Fish finder or chartplotter	About 2A	About 18 hours
Nav lights, graph and bilge pump cycling	About 5A	About 7 hours
Stereo, lights and livewell pump together	About 10A	About 3.5 hours

These are conservative, rough estimates. Actual runtime depends on temperature, battery age, how charged the battery was and how the loads cycle. The takeaway: the Group 27 case buys you meaningfully more engine-off time than a Group 24 with the same cranking rating. If you regularly run electronics for hours with the engine off, the cleaner solution is a separate house battery. Read Should You Run a Separate Battery for Your Boat Electronics?

Sizing a Group 27 Starting Battery for Your Engine

Your engine's owner's manual lists the minimum battery rating the manufacturer requires, usually as an MCA, CCA or both. Use this checklist:

1. Find the minimum rating in the manual and choose a battery that meets or exceeds it. The 27MX935 covers engines that call for up to 935 MCA or 750 CCA.
2. Look at CCA if you boat in the cold. A cold engine and a cold battery are the toughest start.
3. Add reserve for your electrical load. If you run accessories before starting, or have a lot of gear on the cranking battery, more reserve capacity gives you margin. This is where the Group 27 earns its place over a Group 24.
4. Measure the space. The 27MX935 is 12 1/16 in long, 6 13/16 in wide and 8 7/8 in tall. Confirm clearance above the terminals and under any hatch.
5. Consider weight. At 50 lb, it is 8 lb heavier than the 24MX935. Most Group 27 trays are already built for that.

If your manual calls for more than 935 MCA, or you are starting a larger engine or twins, step up to the Pro-Guide 31MX1150 with 1150 MCA and 925 CCA.

AGM vs. Flooded vs. Lithium for Engine Starting

Factor	AGM (Pro-Guide 27MX935)	Typical flooded	Typical lithium starting
Watering	Never	Regular checks and top-ups	Never
Spill and fume risk	Sealed; electrolyte held in mats	Liquid acid; can spill if tipped	No liquid electrolyte
Vibration and pounding	Generally good	Generally lower	Generally good
Works with existing charging	Yes; use AGM charger mode	Yes	Check compatibility first
Cold-weather starts	750 CCA at 0°F	Varies by model	Can be limited in freezing temperatures
Weight	50 lb	Similar or heavier	Much lighter
Upfront cost	Mid	Lowest	Highest

Flooded batteries cost the least, but need regular water checks and can leak acid if they tip. Lithium starting batteries are much lighter but cost the most and may require charging changes. AGM sits in the middle: sealed, maintenance-free and compatible with the charging systems most boats already have. Pro-Guide describes its AGM starting line as offering better performance and longer life than traditional flooded options. If shaving weight matters most, compare starting lithium batteries.

What Is the 27MX935 Best For?

• Pontoons and family boats: stereos, lights and USB chargers often run while anchored. The extra reserve helps you still start the engine when it is time to head in.
• Center consoles and bay boats with Group 27 trays: a direct replacement for a factory Group 27 starting battery.
• Single-engine boats with moderate electrical loads: engines whose manual calls for up to 935 MCA, where you want more margin than a Group 24 provides.
• Dual-battery setups: use the 27MX935 as the dedicated cranking battery alongside a deep cycle house or trolling bank. See One Pack or Three? Choosing the Right Trolling Motor Battery Setup.

Customers who have bought the 27MX935 describe it as a great battery at a great price that has done everything they needed, and praise the ordering experience.

From above: the 27MX935's integrated carry handle, covered positive and negative terminals and rating label.

Charging a Group 27 AGM Battery

Your engine's alternator recharges the 27MX935 while you run. At the dock or in storage, use a charger and follow these guidelines:

1. Select the AGM profile. AGM batteries charge best on an AGM-specific setting. Flooded or lithium profiles are not the right match.
2. Respect the current rating. The 27MX935 is rated for a 10A standard and 15A maximum charge current. A 12V 10A 1-Bank Battery Charger fits a single starting battery. To charge the starting battery and a house or trolling battery from one unit, use a 12V 10A 2-Bank or 12V 10A 3-Bank Battery Charger.
3. Charge between 0°F and 122°F. That is the rated charging range. The battery can crank from -4°F to 140°F.
4. Recharge after heavy engine-off use. If you drew down the reserve running accessories, put the battery on a charger when you get home. Lead-acid batteries left partially discharged can sulfate and lose capacity.
5. Maintain it over winter. Keep it on a smart maintainer, or charge it fully, disconnect it and recheck it periodically.

Install and Care Tips

Use a Group 27 box or tray

Secure the battery so it can't slide or bounce. The NOCO Group 27 Snap-Top Battery Box and NOCO Group 27 Battery Tray are sized for Group 27 batteries.

Choose your terminal connection

The 27MX935 offers an SAE post and a stud terminal, so it works with clamp-style cable ends or ring terminals. Tighten connections firmly; loose cables cause slow cranking and heat.

Protect against corrosion

Salt air and spray attack terminals. A coat of battery corrosion preventative spray after installation helps keep connections clean.

Keep a jump starter aboard

A compact 12V 1000A jump starter can get you home if someone leaves the stereo on too long.

For seasonal care, see our marine battery maintenance guide.

27MX935 vs. 24MX935 vs. 31MX1150

The Pro-Guide AGM starting line comes in three sizes. Here is how the Group 27 fits in.

Spec	24MX935	27MX935	31MX1150
Group size	24	27	31
MCA	935	935	1150
CCA	750	750	925
Reserve capacity	140 min	175 min	200 min
Weight	42 lb	50 lb	60 lb

Choose the 24MX935 if you have a Group 24 tray and modest electrical loads. Choose the 27MX935 if you have a Group 27 tray or want more reserve with the same cranking power. Choose the 31MX1150 if your engine calls for more than 935 MCA or you want the most reserve in the line.

Frequently Asked Questions

What is a Group 27 AGM starting battery used for?

It starts marine engines and supplies short bursts of high current. The Pro-Guide 27MX935 adds 175 minutes of reserve capacity, which helps on boats that run accessories with the engine off.

Is a Group 27 starting battery better than a Group 24?

Not always, but it offers more reserve in a larger case. The Pro-Guide 27MX935 and 24MX935 share 935 MCA and 750 CCA, while the 27MX935 provides 175 minutes of reserve versus 140.

What does 175 minutes of reserve capacity mean?

It means a fully charged Pro-Guide 27MX935 can supply 25A for about 175 minutes before it is fully discharged. Use only part of that so you can still start the engine.

Can a Group 27 AGM starting battery run a trolling motor?

It is not designed for that. The Pro-Guide 27MX935 is a starting battery; use a deep cycle battery for a trolling motor.

What size charger should I use for a Group 27 AGM starting battery?

Use a charger with an AGM setting within the battery's rating. The Pro-Guide 27MX935 is rated for 10A standard and 15A maximum charge current.

How much does the 27MX935 weigh?

The Pro-Guide 27MX935 weighs 50 lb (22.7 kg) and measures 12 1/16 x 6 13/16 x 8 7/8 in.

What warranty does the 27MX935 have?

The Pro-Guide 27MX935 comes with an 18-month free replacement warranty.

Upgrade Your Group 27 Starting Battery

If your boat has a Group 27 tray and you want 935 MCA of cranking power plus extra reserve for the gear you run at anchor, the Pro-Guide 27MX935 is a sealed, maintenance-free choice that drops right in. It is backed by an 18-month free replacement warranty and a team that will help you match the right battery to your engine.

SHOP THE PRO-GUIDE 27MX935

Have questions about cranking amps or fit? Call our team at 1-800-827-3232 or browse all AGM batteries and battery charger kits.

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