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.

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. Then cross reference this to the charts below. The first chart provides amp load versus minutes, the second chart provides the 20-hour rate. 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.

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.

Installation And Maintenance

Exide 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. Additionally, 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.


All Exide 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.

It’s Time To Replace Your Battery When

• 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 becomes submerged.

• The battery discharges frequently between use.

• You buy a used boat.