posted 03-28-2006 09:21 AM ET (US)

**A short summary of battery size characteristics**There are three dimensions to battery size:

--physical size

--storage capacity

--cranking current

**PHYSICAL SIZE**

The physical size of a battery is usually stated in terms of a somewhat obscure collection of group sizes. The most common ones are

GROUP - - L x W x H (in inches)

24 = 10-1/4 x 6-13/16 x 8-7/8

27 = 12-1/16 x 6-13/16 x 8-7/8 (i.e., about 2-inches longer)

31 = 13 x 6-13/16 x 9-7/16 (even longer and now taller)

The typical OEM Boston Whaler battery box will accept a Group-24 battery.

**STORAGE CAPACITY**

The storage capacity of a battery is stated in Ampere-hours (A-h). This is a measure of the total electrical capacity the battery can store. If a current of one ampere is withdrawn from a battery for one hour, that is 1 A-h. The rated Ampere-hour figure is the amount of current that can be continuously supplied by the battery in a 20-hour period. This is better explained by an example.

If a battery has a rating of 40 A-h, this implies that it will supply a current of 2-amperes for 20-hours. This capacity is available only when the battery begins with a full charge and is in new condition. As a battery ages it will gradually lose some of its storage capacity, and it may be more realistic to de-rate the Ampere-hour figure somewhat to judge the true capacity.

The typical Group-24 battery will have an A-h rating from 40 to 80, depending on its construction and quality.

The A-h rating will also indicate how long the battery must be charged to become fully charged. To fully charge an 80 A-h battery from completely discharged, you would need to use an 8-Ampere charging current for 10-hours, or a 16-Ampere charging current for 5-hours, or a 20-Ampere charging current for four hours, and so on. (The charging current usually tapers off as the battery terminal voltage rises so these examples are not quite obtainable in practice.)

**CRANKING CURRENT**

Ratings of cranking current refer to how much current can be delivered for 30-seconds until the battery voltage drops to 7.2-volts. This is measured and rated at two temperatures. The rating at 0-degrees-F is called the Cold Cranking Amperes (CCA). This is not particularly applicable to marine installations. The rating at 32-degrees-F is called the Marine Cranking Amperes (MCA).