A boat’s electrical system starts with a battery that will supply the electricity for the boat. The system is usually 12 volt DC (direct current), but can be 6 volt, 12 volt or 24 volt depending on how many and what type of batteries the system is designed for. For this article, the system discussed is a 12 volt system.
A boat’s wiring is a two wire system. One wire goes from the battery to the light or instrument to be used and a second wire returns to the battery from the light or instrument to complete the circuit. In a Direct Current system the electricity flows only in one direction. The electricity flows from the battery to the light and then back to the battery. Each item used will have its own two wires, one to get power to it, and one to return the electricity. This is a very simple explanation of how a boat is wired.
The batteries that that are used on a boat are of 3 basic types. They are a wet cell battery similar to the type used in a car, a gel cell battery and an AGM or Absorbed-glass mat battery. All of these types are rechargeable. The battery capacity or how much electricity it can produce is given by the voltage and amps listed on the battery. The group size of is the physical size of the battery, the height, width and length. This lets you get the right size that will fit in space you have for the battery. The battery designation will be Deep Cycle or Cranking.
A deep cycle battery will put out a steady current over a long time. A Cranking battery can put out a high amount of current for a short time to crank a motor over to start it, but it will not last a long time under continuous use like a deep cycle can. Some batteries, like AGM batteries, are often designated as BOTH and are dual purpose batteries. Pax era life Marine Cranking Amps, Cold Cranking Amps and Reserve Capacity data is also often given. These numbers tell you they responds under a load condition and it lets you compare batteries of the same physical size with each other. The boat motor on the boat will determine what cranking amps are required to start the motor.
A wet cell battery usually has cells that you can open and add water to them. When the battery heats up under use, water evaporates from them. The cells must always have the water acid liquid in them covering the cells or the battery dies. The acid doses not evaporate out, just the water, so water needs to be added regularly. These wet cell types must also be level or the acid water mixture will run out. The acid can destroy many things if it leaks out. You can get sealed, leak proof wet cell batteries to avoid having to add water. Gel Cell and AGM batteries are sealed and are leak proof. An AGM battery can be stored in any position and the discharge rate of the AGM type, when it sits unused, is better than the wet cell and gel cell batteries.
The load the battery will have on it will determine the type of marine battery that is needed. If it is to be used to start a motor and run just a few electric items, a cranking battery will work fine. If it will be used to power a trolling motor and other electronics, creating a constant drain for an extended period of time, a deep cycle will be needed. AGM dual purpose batteries are becoming popular because the can handle both starting and loads well. Another reason they are popular is that they are sealed and may be stored and used in any position. The AGM’s are the first choice in the marine industry today.