Selecting a Battery for your Boat

Units such as CA (cranking amps), CCA (cold cranking amps), and MCA (marine cranking amps) are the number of amps a battery can supply for 30 seconds at a specific temperature.
CA - 0° C
CCA - 0° F
MCA - 32° F also called CA (cranking amps)
CCA - 80° F is often referred to as HCA (hot cranking amps)

Why is there a marine cranking amp? Well, how often is a boat used below 32 degrees? Hence, MCA more closely represents the batteries performance in the real world. Since the majority of automotive customers are mounting the battery in the engine bay of the vehicle, a temperature of 80 degrees most closely represents the mean of temperatures when the battery will be used.

Rating Definitions and Conversion Formula
CCA - Cold Cranking Amp
[SAE] Max current the battery can deliver for 30 sec. at 0F (-18C) while V >= 7.2V
[EN] Max current the battery can deliver for 10 sec. at 0F (-18C) while V >= 7.5V
[DIN] Max current the battery can deliver for 30 sec. at 0F (-18C) while V >= 9V
[IEC] Max current the battery can deliver for 60 sec. at 0F (-18C) while V >= 8.4V

CA - Cranking Amp
Max current the battery can deliver for 30 seconds at 32F (0C) while V >=7.2

MCA - Marine Cranking Amp (equal to CA)
To convert MCA or CA to CCA multiply CA by 80%

HCA - Hot Cranking Amp
Max current the battery can deliver for 30 seconds at 80F (26.7C)) V >=7.2
To convert HCA to CCA, multiply HCA by 69%

You cant directly convert between CA, CCA, and MCA, because its at a different temperature.
So to avoid confusion always included the temperature it was measured at.


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Two standard ratings are used to measure a battery's storage capacity.

Amp Hours
The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20.

Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.)
A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

Reserve Minutes
Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.)
Reserve Capacity (RC)
The Reserve Capacity rating is the second most important consideration while buying a battery. This is because of the effects of an increased parasitic (key off) load and in emergencies. Reserve Capacity is the number of minutes a fully charged battery at 80 degrees F (26.7 degrees C) can be discharged at 25 amps until the voltage falls below 10.5 volts. More RC is better in every case! In a hot climate, for example, if your car has a 360 OEM cranking amp requirement, then a 400 CCA rated battery with 120 minute RC with more electrolyte would be more desirable than one with 1000 CCA with 90 minutes of RC. If more RC is required, two six-volt batteries can be connected in series or two (or more) 12 volt batteries can be connected in parallel. Within a BCI group size, generally the battery with larger RC will weigh more because it contains more lead.

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Size
Batteries are generally sold by model, so the group numbers will vary for the same price. This means that for the SAME price you can potentially buy a physically larger battery with more RC than the battery you are replacing, e.g. a 34/78 group might replace a smaller 26/70 group and give you an additional 30 minutes of RC.


Freshness
Determining the "freshness" of a battery is sometimes difficult. A battery that is more than six months old should never be bought because it begins to sulfate. Sulfation occurs when lead sulfate can not be converted back to charged material and is created when discharged batteries stand for a long time or from excessive water loss.