To understand the difference between marine and car batteries, it is helpful to know how rechargeable lead acid batteries are constructed. Lead acid batteries have an array of two different forms of lead plates, immersed in diluted sulfuric acid. The chemical reaction of the electrolyte with the lead plates creates an electrical charge, which is stored in the battery. Battery plate thickness is an indicator of the application for which the battery is best suited.
Rechargeable lead acid batteries are classified in two categories. One type is starter batteries, used in automobiles, and the second type is deep cycle batteries, often used for recharging solar power systems. Car batteries or starter batteries have more lead plates than deep cycle types, but the plates are much thinner. Deep cycle batteries have fewer plates, but the plates are heavier and thicker. Marine batteries are hybrids and fall somewhere in between, similar to car batteries, but with slightly thicker plates.
Similar to car batteries, the majority of marine batteries supply 12 volts DC. Car batteries are rated in cranking amps (CA) or cold cranking amps (CCA) while marine starter batteries may have a marine cranking amp (MCA) rating. Quality deep cycle marine batteries will have a reserve capacity (RC) rating that indicates how long the battery will operate before losing power. Marine batteries are also capable of more frequent and deeper discharges than car batteries.
Car batteries have a greater number of thinner lead plates than the marine type, so they discharge a high initial current to spin the starter motor and crank the engine over. Once the engine is running, the alternator recharges the battery to run lights, ac/heater, radio, etc. When maintained, car batteries can last for years and be charged repeatedly. However, plate damage can occur if the battery is discharged more than 20 percent of full capacity for an extended period, or sits for weeks without being used.
Marine batteries are sturdier than car batteries to prevent damage from shaking and vibration caused when a boat rides on the water surface. Although sturdier, most marine batteries are not intended to be discharged more than 50 percent capacity, compared to deep cycle batteries, which can be discharged 80 percent continually, without plate damage. Good quality deep cycle marine batteries are recommended for boats equipped with trolling motors, radios, bilge pumps, navigation lights, GPS/fish finders and other electrical devices.
Whether working with car or marine batteries, use caution when handling or recharging. Lead acid batteries pose dangerous shock, burn and explosive hazards, which is why professionals use gloves and face protection. One common mistake is accidental shock by shorting out battery terminals with metal wrenches or jewelry. Although less common, another serious concern is skin burns from sulfuric acid due to battery explosion resulting from improper charging. Ventilation is very important. When working with lead acid batteries, always seek professional advice, if unsure, rather than risk injury.