The following covers battery group sizes, battery ratings, battery charging and chargers, for conventional lead acid batteries and Lithium batteries, used on boats.
On boating forums, I often hear people say they are going to increase the size of their batteries by replacing their Group 24 batteries with Group 27s. Unfortunately they are under a misconception that group size and battery capacity are related.
Group Number: Standards for automotive and marine batteries are set by the Battery Council International (BCI). They have designated a Group Number for 12V batteries of various sizes and configurations. It is only a grouping based on the physical outside dimensions of the battery, and whether the lugs are on the top or sides of the case. The group number has little relationship to the capacity of the battery.
The BCI definition of Group Number is:
"BCI GROUP NUMBER- The BCI Group Number defines a battery by describing the following characteristics:
A.) Its maximum dimensions (L x W x H)
B.) Voltage (6 volt or 12 volt)
C.) Post configuration
D.) The type of terminal used (top, side, flag, etc.)
It should be noted that the BCI (Battery Council International) Group Number does not designate a battery's capacity, it merely defines the above listed physical characteristics."
A link to a chart showing Battery Group Sizes and dimensions.
https://www.thebatterygenie.com/latest-bci-battery-group-size-chart/
It should also be noted that batteries in the same Group come in different capacities. So, a larger battery can accommodate more plates and increase the amp hour rating of the battery. But this does not necessarily increase the Cold Cranking Amps, Marine Cranking Amps or Reserve capacity.
Battery capacity is determined by the type of battery construction: lead acid wet cell, Sealed Valve Regulated, AGM, Gel or Lithium. The number of plates and surface area of the plates, and the thickness of the plates in lead acid batteries determines whether it is a starting battery, or deep cycle and how long it can last under a known load. It is also determined by the composition of the plates. Some are more like a lead foam or mesh, up to thick fully lead plates.
Capacity; that is, how many amp hours the battery can release energy is measured by Cold Cranking Amps (CCA) which is a measure of how long the battery can continue to put out power. Below is the definition of CCA from the American Boat and Yacht Council (ABYC) http://www.abycinc.org which is the same as the BCI definition.
ABYC E-10.4 “Cold Cranking Amperage (CCA) - The discharged load, in amperes, that a new, fully charged battery at 0 degrees F (-17.8 deg. C) can deliver for 30 seconds, and maintain a voltage of 1.2volts per cell or higher.”
A better rating for marine batteries is Marine Cranking Capacity (MCA).
ABYC E-10.4 “Marine Cranking Amperage - The discharged load, in amperes, that a new, fully charged battery at 32 degrees F (0 deg. C) can deliver for 30 seconds, and maintain a voltage of 1.2volts per cell or higher.
If you are mainly concerned with how long your battery will last then you should look at the Battery Reserve Capacity.
ABYC E-10.4 “Battery Reserve Capacity – The number of minutes a new, fully charged battery at 80 deg. F (26.7 C) can be continuously discharged at 25 amperes, and maintain a voltage of 1.75 volts per cell or higher (10.5 volts for a 12 volt battery or 5.25 volts for a 6 volt battery).
Batteries, particularly large industrial batteries, can also have what is called a C rating. This is a measure of the charge rate and discharge rate of a battery. That is: the C rating is a measure of time, how long does it take the battery to charge, or discharge.
Batteries sizes (physical dimensions) are classed by Group. Most common are Group 24 and 27. It is a common misconception that this has something to do with the capacity of the battery. This is a myth. The Group number is solely based on the outside physical dimensions of the battery, and the type of battery terminals it has, that is, top mounted or side mounted. For instance, a Group 24 battery is 10 ¼ inches by 6 13/16 inches by 8 7/8 inches. A Group 27 battery is 12 1/6 inches by 6 13/16 by 8 7/8 inches. This battery is a Group 24.
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