Engine Repower 2020: Part 3

Battery selection for outboard engines.

March 13, 2020
Optima Blue Top battery
The Optima Blue Top D27M batteries are rated at 800 CCA/1000 MCA and 66 AH 140 minutes reserve. John Raguso

Repowering is a great time to re-assess your boat’s complement of batteries. The battery bank, or banks, must offer enough cranking amps and reserve capacity to meet the new outboard engine’s requirements.

For example, the 250-hp Mercury Four Stroke re-power we are documenting, requires 975 cold cranking amps and a 65 minute reserve ( or 800 marine cranking amps with a 135 minute RC25 rating). This is a V-8 engine replacing a V-6.

Mercury 250 outboard engine
Rear angle view of Mercury Marine 250-hp four stroke outboard. Courtesy Mercury Marine

That’s not all. The battery must be an absorbed glass mat (AGM) type. Wet cell (a.k.a, flooded electrolyte or, “regular”) batteries and Lithium Ion ( LiOn) batteries are specifically not to be used. Finally, each engine must have its own dedicated starting battery.


Note that Mercury 250 Four Stroke boasts an 85-amp alternator, so there is plenty of output to charge a larger capacity battery bank, plus run a host of systems like livewells, lights and electronics. It also boasts a feature called, Idle Charge. Idle Charge provides up to 48% more battery charging at idle speed to support the greater power demands the systems and accessories found aboard many of today’s boats.

We have had great personal experience with AGM batteries having used them for many years.

Read Next: Why I Choose AGM Batteries For My Boat


As result of this confidence, we again selected AGM batteries by Optima for this project. Shown are a pair of Optima Blue Top D27M batteries, rated at 800 CCA/ 1000 MCA and 66 AH 140 minutes reserve. The Optima Bluetop D27M batteries measure 6.81 inches wide x 12.19 inches long x 8.75 inches high and weigh 53.8 pounds each. They retail for $298.99. Visit for more information.

Optima Batteries Spiral Cell
Among other advantages, Spiral Cell Technology enables Optima AGM batteries to better withstand vibration and shock, conditions to which batteries are subjected aboard offshore boats. 12 volt OPTIMA batteries are made of six SPIRALCELL cells each having a fully charged open circuit voltage of 2.2 volts for deep-cycle batteries and just over 2.1 volts for starting batteries. Courtesy Optima Batteries

Let’s dispel a widely-held myth: AGM batteries do not require a special charger, or at least these Optima’s don’t. I can attest to this. You can learn the technical reasons why, in this video:

Battery Terms


CCA = Cold Cranking Amps = measured at 0°F. The number of amps a battery can deliver at 0°F for 30 seconds while maintaining a voltage of at least 7.2 volts.

MCA =Marine Cranking Amps = measured at 32°F. The number of amps a battery can deliver at 32°F for 30 seconds while maintaining a voltage of at least 7.2 volts.

AH =Amp Hour Rating = The number of minutes a fully charged battery at 80°F will discharge 25 amps until it drops below 10.5 volts.


RC25 = Number of minutes for sustaining 25A draw at 80°F.

Check out all of our Engine Repower 2020 content, here.


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