exide xmc 31 megacycle agmMarine batteries come in various sizes, having different capacities, voltages and performances in general.
Depending on the size, batteries are categorized in various size groups. Of course, there are always 'special' size batteries, so pay attention when buying new batteries.

Battery Dimensions By Group Size

Following table shows industry standard group sizes according to their external dimensions. Battery compartments are usually made for a certain battery standard, so knowing both values is important - larger battery can't fit into smaller compartment, while smaller can, but must be additionally fixed to avoid any damage to the boat or battery.

Group Size LxWxH (inches) LxWxH (cm)
34 10.25x6.8125x7.875 26x17.3x20
24 10.25x6.8125x8.875 26x17.3x22.5
24H 10.25x6.8125x9.375 26x17.3x23.8
27  12.0625x6.8125x8.875 30.6x17.3x22.5
29H 13.125x6.725x9.125 33.3x17.1x23.2
30H 13.5x6.8125x9.25 34.3x17.3x23.5
31 13x6.8125x9.4375 33x17.3x24

Of course, there are other standards and even 'special sized' batteries, but these are the most common.

Marine Batteries - Abbreviations and Glossary

Here are several, most common abbreviations used to describe marine ant other batteries and their characteristics:

Ah (Ampere/Amp hours) - this value describe how many amps of current can new, fully charged, 12V battery, at 80°F (~25°C), deliver for 20 hours, without voltage falling below 10.5V.

RC (Reserve Capacity) - this value is number of minutes that a new, fully charged, 12V battery, at 80°F (~25°C) can deliver 25A current and maintain at least 10.5V of voltage.

CA (Cranking Amps) or MCA (Marine Cranking Amps) - maximum current that a new, fully charged, 12V battery (at 32°F - 0°C) can deliver for 30 seconds, with the voltage NOT dropping below 7.2V.

CCA (Cold Cranking Amps) - maximum current that a new, fully charged, 12V battery (at 0°F; -18°C) can deliver for 30 seconds, with the voltage NOT dropping below 7.2V.

HCA (Hot Cranking Amps) - maximum current that a new, fully charged, 12V battery (at 80°F; 27°C) can deliver for 30 seconds, with the voltage NOT dropping below 7.2V.

If you are looking for a marine battery that has only CCA rating, and you want to know it's MCA or HCA, it is good to know that CCA, CA/MCA and HCA are related (approximately):

CCA = HCA X 0.60 = MCA x 0.80

HCA = CCA x 1.66 = MCA x 1.33

MCA = CCA x 1.25 = HCA x 0.75

This is logical, since CCA describe performance of batteries at lower temperature and lead acid batteries are more efficient and perform better on higher temperatures (up to the point, of course).

Flooded (wet) lead acid batteries use liquid electrolyte, which can be spilled out. Depending on the use and temperature, distilled water must be periodically added to keep the batteries function properly and to avoid the damage.

Valve-regulated lead–acid (VRLA) batteries have safety valve inside battery which opens at certain pressure and let oxygen flow from positive plate to negative plate to recombine with hydrogen and create water. Such VRLA Sealed Lead Acid (SLA) batteries are maintenance free batteries and are used more and more on various boats and yachts.

Gel Cell batteries use electrolyte in the form of gel - diluted sulfuric acid is mixed with fumed silica to create gel which is placed between battery plates.

Absorbed Glass Mat (AGM) batteries use electrolyte that is held in the glass mats made out of woven, very thin glass fibers.

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