Marine batteries are sold by many different measurements. One of those measurements is the physical case size. Size of the external case the battery is sold in is represented by the group number.
If looking to find the group number measurement of a marine battery, it can be found on the top of the battery, printed on a specifications label that will have other measurements on it as well. Do not be confused by the gibberish of characters written on the label because the group number will clearly be written with BCI or group next to it. BCI stands for Battery Council International that is a company which regulates the distribution of lead-acid batteries, so it determines group number categories. The factory recommended group number of a vehicle is also in the vehicle’s parts specifications booklet, so if you have the original paperwork, it might be handy to know the initial battery group number your vehicle contained. Otherwise, ask an expert who is knowledgeable about vehicle batteries to identify it.
We know group number is the physical size of the battery, but how do we use the group number once we have it? Smaller group indicates a generally larger battery, whereas larger group indicates a generally smaller battery. As the group numbers change, the shape of the battery will change too, so a smaller battery might be longer while also being thinner than the group below it, but it gains in thinness what it gave up in length.
It typically will be okay to move up, or down the group number scale a few numbers in either direction because the general size of the battery will stay the same across small sections of the scale. However, the shape of the batteries will change, so ensure there is enough room in any direction to hold the marine battery you are trying to install, otherwise, your hood may not close, or the battery may not fit at all, so your money could be wasted. Also, the battery could interfere with other parts, so be careful when improvising on the group number of a battery.