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Marine Battery Tutorial

Understanding Battery Ratings

In order to determine which battery is right for a specific application, you should understand the following ratings:

  • Ampere Hour Rating (Reference Rating): This is the number of amps which a battery can deliver for a 20-hour period. This test is also referred to as the 20-hour rate. The larger the ampere hour rating, the more power the battery can deliver over time.
  • Marine Cranking Amps (MCA): This is the number of amps a battery can deliver at 32 degrees Fahrenheit for 30Mseconds, and maintain at least a voltage of 1.2 volts per cell. This differs from cold cranking amps which are measured at 0 degrees Fahrenheit.
  • Reserve Capacity (RC): This is the time, in minutes, for which a battery will deliver 25 amperes at 80 degrees Fahrenheit. This represents the time which the battery will continue to operate essential accessories in the event of alternator or generator failure or while the key is off.

Determining the Ampere Hour Draw Of Your Boat

In order to determine the ampere hour draw of your vessel, you need to know what electrical equipment you have and what the 12-volt amp draw is. The following is a listing of typical 12-volt equipment aboard most boats and the average amp draw per hour.
Average Amp Draw per Hour for Equipment Aboard Most Boats
Bilge Pump (500 gph) 2.0
Bilge Pump (1000 gph) 2.9
Bilge Pump (1500 gph) 4.9
Bilge Pump (2000 gph) 8.4
Navigation Lights (3 mile) 1.5
Live Well Pump 7.0
Fresh Water Pump 4.0
Refrigerator (12 volt) 6.0
Macerator 9.0
Anchor Windlass (900 lb) 75.0
12 Volt House Lighting 0.15 per 10 watts
Spot Lights (100K cp) 8.0
Spreader Lights (3K cp) 3.0
RADAR (24 mile) 5.0
GPS 0.8
VHF Radio - transmit 6.0
VHF Radio - receive 0.5
Fish Finder (LCD) 1.0
Depth Finder (Color) 3.0
SSB - transmit 30
SSB - receive 2.5
Auto-pilot 5.0
Stereo (50 watt) 0.5
TROLLING MOTORS (12 volt) 24 lb thrust 27
TROLLING MOTORS (12 volt) 30 lb thrust 30
TROLLING MOTORS (12 volt) 36 lb thrust 36
TROLLING MOTORS (12 volt) 42 lb thrust 40
TROLLING MOTORS (12 volt) 55 lb thrust 50

Calculate The AMP Hour

Capacity Battery You Need In order to determine the proper amp hour rating capacity you need for your boat, simply add up the 12-volt accessories you have, multiply by 20; that should give you a very good approximation of your boat’s amp hour battery requirement. It is usually advised to buy a battery at least 20% over this requirement, as 12-volt capacity varies with usage and as batteries age.

Selecting the Proper Battery

A complete line of batteries for all marine applications are manufactured. From personal watercraft to mega yachts
  • Marine Starting Batteries: Marine starting batteries have been designed to deliver high bursts of power for short periods of time, to start marine engines. The power level you need depends on the cranking requirements of your engine. Marine starting batteries are not designed to provide trolling or deep cycle power, which requires plates built to different specifications.
  • Deep Cycle Batteries: Deep cycle batteries incorporate thicker grids, denser active material on the plates and alloys specifically designed to provide many cycles. These batteries are designed to provide continuous operating time to run trolling motors, live wells, inverters, 12-volt lighting, depth finders, etc. They can be charged and recharged many times without damaging the internal components of the battery. These batteries have a lower MCA rating and higher reserve capacity/ amp hour rating than dual purpose or starting types.
  • Dual Purpose Batteries: Dual purpose batteries combine deep cycle capability with starting power. Some deliver enough power to start a 350-hp engine and provide at least 7 hours of continuous 10 amp 12-volt draw.

Series Versus Parallel Installations

Batteries can be arranged differently to achieve increased capacity or increased voltage to match your specific requirements. It is extremely important not to mix battery types (Flooded, AGM).
  • Parallel Installation; Two batteries connected + to + and - to - in a parallel system that increases capacity and maintains a specific voltage. This configuration doubles the power or amp hour rating of the battery while maintaining the voltage. Thus, two 25-amp hour, 12-volt batteries in parallel will give you a 50-amp hour 12-volt system.
  • Series Installation; A series system increases the voltage and keeps the battery capacity the same. The same two batteries in a series arrangement will increase the voltage to 24 volts and maintain a battery capacity of 25 amp hours. To install batteries in series, one battery’s positive post is connected to the second battery’s negative post.


  • Batteries should always be installed in a ventilated area.
  • Batteries release explosive gasses during the charging phase and should not be exposed to spark or flame.
  • When installing a battery in your boat, it is important to use either a box or a tie-down system to keep the battery stationary once underway. This will reduce unnecessary vibration.
  • Make sure all connections to the battery terminals are tight.
  • It is important to coat the terminals and connections with a corrosion inhibitor. The corrosion inhibitor should be reapplied every several months. Failure to do this will result in poor connections and wire corrosion, especially in salt water environments.
  • Corrosion increases the resistance in the wires, requiring more amps to be drawn to run electrical equipment.
  • When installing a new battery, be sure to remove any plastic battery terminal protectors before attaching wires.


  • Most marine batteries, except AGM types, have removable vent caps so that electrolyte levels can be checked regularly.
  • You should check the electrolyte level every month.
  • When storing a battery for the winter, check and fill with distilled water as needed, recharge the battery fully, and store in a cool place.
  • When preparing the battery after winter storage, recharge the battery to its full charge state.

When Is it Time to Change the Battery?

  • You had to jump start your battery
  • The battery can barely turn the starter over
  • Lighting and electronics dim or go out when starting
  • The battery will not hold a charge
  • The battery was submerged
  • The battery discharges frequently between use
  • You buy a used boat

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