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. 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.
Two batteries or more can be hooked together to get more power. If two are used together they have to be the same. If the amp rating is different between the batteries one will be damaged. How you hook the batteries together is important. If both are 12 volt batteries and you hook both + (positive) terminals together and hook both - (negative) terminals together, the output between the + side and the - side of the batteries will still be 12 volts, but the amp output doubles. This is called wired in parallel.
If you wire the two together so that the + on one goes to the - of the other and then you hook up the things you power to the free + terminal on one battery and the - terminal of the other battery, the output will be 24 volts not 12 and the amps stays the same. This is like two batteries stacked in a flash light. These are said to be wired in series.
A 6 volt battery usually has a higher amp rating then a 12 volt battery of the same size. By wiring two 6 volt batteries in series you can get a 12 volt power supply that has greater amperage then a single 12 volt battery can give. The two do not have to work as hard to do the same job as the one 12 volt battery does. This is why most golf carts use 6 volt batteries. It is nice to have options like that.
There are different types of battery charging systems available. An automatic multistage charging system is best. Because marine batteries are more expensive the car batteries, this type will charge the battery without overcharging it. Many sense the type of battery you have and will pick the correct charging method. This will help battery last longer.
A car battery can be used on a boat, but a battery made for the marine industry can give you better service. They are made to match the load requirements of a boat, they are usually sealed and leak-proof, and are often a dual purpose high load deep cycle type. The AGM batteries are the boater's choice. They provide high power with very low internal resistance compared to conventional batteries and they have two times the life of conventional batteries.
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