To really understand and know the difference between a marine and car battery, it will be helpful to first know the construction of lead acid batteries. Lead acid batteries are made of a variety of different forms of lead plates, which are immersed in dilute sulfuric acid. The reaction of lead plates with the electrolyte generates an electric charge, that is stored within the battery. The thickness of the battery plates serves an indicator for the application that the battery will be ideally suited.
Plate Size
There are two categories of lead acid batteries. The starter batteries are the first type, and they are used in cars. The second are deep cycle batteries, and they are normally used to charge solar powered systems. Car batteries have more lead plates than deep cycle batteries, but they have thinner plates. Deep cycle plates have less plates, but these plates are much thicker and heavier. Marine batteries on the other hand are best classified as hybrids, and they somehow fall in between the categories. They are much similar to car batteries, but have thicker plates.
Voltage
Just like car batteries, most marine batteries produce power supplies of 12 volts DC. A car battery is rated in CA (cranking amps), while the marine battery has a marine cranking amp rating (MCA). A deep cycle marine battery has reserve capacity rating which indicates how long the battery will run for before it begins to lose power. A marine battery is also capable of a deeper and more frequent discharge than a car battery.
Car Batteries
A car battery has a bigger amount of thinner lead plates than a marine battery, and for this reason it discharges a high initial current needed to spin the starter motor as well as crank the engine. When the engine begins to run, the alternator then recharges the battery to turn the lights on, radio, heater, a/c, etc. When a car battery is properly maintained, it can last for several years and can be charged repeatedly. Plate damage can occur when the battery is discharged at more than 20% of full capacity for extended periods, or if it sits for a while without use.
Marine Batteries
Marine batteries tend to be much sturdier than car batteries in order to prevent damage occurring from vibration and shaking when the boat cruises of the water. While sturdier, the majority of marine batteries are not meant to discharge more than 50% capacity, when compared to deep cycle batteries which can discharge 80% continually, without damage to the plate. A high quality deep cycle batteries are widely recommended for boats that are equipped with radios, rolling motors, navigation lights, bilge pumps, GPS/fish finders as well as other electrical devices.
Safety
When you work with marine or car batteries, practice extreme caution when recharging or handling. Lead acid batteries have the potential to cause shock, explosive hazards, burns, which explains why professionals use facial protection and gloves. A common accident normally occurs when battery terminals are short-circuited with jewelry or metal wrenches.