This article is a basic breakdown of what you see when staring at a marine battery. From the various acronyms and numbers on the label to the terminal post configurations.
If you want to dive much deeper into the world of batteries and be deemed a nerd on your docks (a very popular nerd once all your fellow boaters become aware of your legit battery knowledge) don’t worry. There will be many more in-depth articles soon.
Physical aspects of a marine battery:
Outer Case: Generally speaking, almost all battery cases/housings are made of polypropylene resin (a type of thermoplastic polymer resin). It’s a type of plastic widely used across multiple industries. From your batteries to your Tupperware at home.
Battery Terminal/Post Configurations: On most marine batteries, you will see two different types of terminals affixed to the top. The first one is the most common and well-known, The Automotive Post Terminal or SAE Post. This terminal post is the large cylindrical shape hunk of lead. The positive post and the negative post are of different diameters to prevent reverse polarity connections. (Negative cable to the positive post and positive cable to the negative post. Typically, a bad day when that takes place).
The second type of terminal/post found on a marine battery is the stud terminal. This is the threaded terminal. This threaded stud terminal is either 5/16th or 3/8th. Some battery manufacturers will use different size stud terminals for the positive and negative to prevent reverse polarity connections as well.
Acronyms and information you typically see on a Marine Battery label:
FLA: Flooded Lead Acid. Also known as a “wet cell” or “wet” battery. This battery chemistry has a liquid electrolyte (part water/part sulfuric acid) within the battery. When moving the battery, you can typically hear the liquid electrolyte sloshing around inside the battery case. These batteries will leak out the liquid electrolyte from the top caps if tipped or laid on their side. Will also leak if the battery case is punctured. These batteries also require maintenance in the form of checking the levels of electrolytes periodically and filling them with distilled water when necessary. FLAs are the most well-known, most used, and most readily available at your local store.
AGM: Absorbed Glass Mat. Also known as “VRLA” (Valve Regulated Lead Acid) and “SVR” (Sealed Valve Regulated). This battery chemistry is also lead acid and has a liquid electrolyte within the battery. Unlike the FLA, the liquid electrolyte is not free to slosh around and do what it wants. The liquid electrolyte in an AGM is absorbed (thanks to capillary action) into matted fiberglass. Since the liquid electrolyte is absorbed and held in place via the matted fiberglass mats, these batteries are deemed “spillproof”. They will not leak if tipped, laid on their side, or punctured. AGMs are also considered “maintenance free” (I used quotations here as I believe all batteries require some form of maintenance. Regardless of what the label and brochure say. Topic for another day). AGMs have become popular over the years and are typically available at your local store as well.
GEL: This battery chemistry is lead acid as well. The electrolyte within this battery is in a gel state. During the manufacturing process, the liquid electrolyte is mixed with silica particles and forms a gel-like substance after being injected into the battery. Like AGMs, these batteries are “spillproof” and “maintenance-free”. You will notice GEL batteries are typically not stocked at your local stores. They are not in high demand due to being very application specific.
Starting: A starting battery is designed for just that… starting an engine or motor. Starting batteries are built to provide a quick burst of tremendous energy. After this large and quick burst of energy being discharged (usually only a few percent of battery capacity) the starting battery will usually be thanked with a charge from an alternator. Due to the quick release of energy and then a quick retrieval of energy, starting batteries are built with many thin plates and a low-density active material. Starting batteries may also be referred to as “cranking batteries”.
Deep Cycle: A deep cycle battery is designed to be discharged at a much slower and longer rate than a starting battery. Deep cycle batteries also need to be designed to withstand many discharging and charging cycles. Due to this demand, deep cycle batteries are built with thicker plates and denser active material. They are just built stronger and tougher than that of a starting battery. Deep cycle batteries are usually referred to as “house batteries”.
Dual Purpose: A dual-purpose battery is designed to be able to provide adequate bursts of energy to start your engine or motor and withstand the abuse of being discharged at a long slow rate. They are designed to be both a starting battery and a deep cycle battery.
BCI: Battery Council International. Association was formed in 1924. BCI establishes technical standards that most battery manufacturers from around the world adhere to. One of the most relevant standards is their creation of the Battery Group Size. A BCI battery group size is a maximum overall physical dimension of a battery most battery manufacturers follow.
GROUP: Shortened term for BCI Group Size. The battery group size is simply the physical dimensions of the battery. Almost all battery manufacturers follow this BCI standard. It keeps everyone on the same page and makes replacing a battery easy when it comes to your battery space. There is a huge array of BCI group sizes. From the automotive world to the commercial world. For us boaters, the most used Group Sizes are 24, 27, 31, 4D, and 8D.
CCA: Cold Cranking Amps. This standard informs you of the maximum discharge (in amps) the battery can deliver to your starter motor, for 30 seconds, at 0 degrees Fahrenheit, while not dropping below 7.2 volts (for a 12v battery).
MCA: Marine Cranking Amps. This standard is the same as CCA, except for the temperature. This standard is tested at 32 degrees Fahrenheit. Due to this standard being tested at a warmer temperature, this rating will be higher than the CCA rating.
CA: Cranking Amps. This standard is the same as MCA. Some non-marine-based battery manufacturers/distributors will use CA on their labels instead of MCA.
AH: Amp Hour Capacity. This standard is the number of amps that can be pulled from a battery at 80 degrees Fahrenheit, at a slow rate, before the battery is deemed dead (10.5 volts for a 12v battery). For the U.S. this slow rate of discharge is typically set at 20 hours. This rate is known as the C20 rate. There are however various other set time discharge ratings: C5 (5 hours) and C10 (10 hours).
RC: Reserve Capacity. This is an automotive standard. In the auto world, this standard is useful as it will tell you how long the car’s battery will keep everything operational (electrically speaking) when the car’s alternator dies. Surprisingly, this standard is being utilized on many marine batteries today. This standard tells us how long, in minutes, the battery can sustain a draw/load, at 80 degrees Fahrenheit, before the battery voltage falls below 10.5 volts (on a 12v battery). The typical draw/load is set at 25amps. Other amperages may be used for this standard, however.
Part No. Some battery manufacturers will have their Part number listed on the battery label. Typically (but not always) the part number will have the Group Size listed within. For example, Interstate Battery Part No. SRM-27 is BCI Group Size 27.
That concludes my Battery 101 article. Hopefully, you enjoyed reading this and now better understand what all those words, acronyms, and numbers are on a marine battery.
And like I mentioned above, if you want to be deemed the “dock nerd”, don’t worry, there will be many more in-depth articles about marine batteries. The above is merely the beginning.