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Ralph Doolin
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Join Date: Jan 2000
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Marine Battery Ratings

Can you explain how marine batteries are rated and what the numbers mean when comparing batteries?

Ralph Doolin responds:
A Marine Cranking Amp or MCA is the commonly used rating for marine starting power. It is the number of amperes a lead-acid battery at 32 degrees Farenheit can deliver for 30 seconds, maintaining at least 1.2 volts per cell (7.2 volts for a 12-volt battery). This lets you know how much power you have to start your boat's engine.

Amp Hours are simply a unit of measure for a battery's electrical storage capacity, and this rating applies to deep-cycle marine batteries. The rating is obtained by multiplying the current in amperes by the time in hours of discharge. A battery delivering five amperes for 20 hours, delivers 100 Amp Hr (five amperes X 20 hours) of capacity. In other words, this rating lets you know how many amps, for how long, you can draw from your deep-cycle battery.

The reserve capacity is the time in minutes that a new, fully charged battery will deliver 25 amperes at 80 degrees Farenheit and maintain a terminal voltage equal to, or greater than, 1.75 volts per cell. This rating represents the time the battery will continue to operate if the alternator or generator of a boat fails. Reserve capacity is a battery's ability to sustain a minimum electrical load in the event of a charging system failure.

Deep-cycle batteries are designed to provide "deep cycle" power, which makes these batteries different from ordinary car batteries. Starting a car requires a high amount of energy for a short period of time (about five seconds). Only a small amount of the battery's capacity is used. Once the engine starts running, the battery is recharged quickly by the alternator, which carries the basic electrical load. On the other hand, deep-cycle batteries supply a relatively low amount of current for a long period of time. They are specially designed to power electric trolling motors and other electrical accessories in boats and recreational vehicles. Unlike ordinary car starting batteries, these batteries can run down and be recharged (deep cycled) repeatedly with minimum loss of capacity.

Here's hoping that this information is useful to you. If you need to purchase a new heavy duty battery for your boat, check out the selection of batteries in SailNet's Store.

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