This articles looks at how to plan the loads a boat’s electrical system will endure. First, you need to know the total amount of power needed. This will then tell you how much power your batteries must be capable of producing. Once you know that you can then look at charging methods.

You can figure loads by doing an electrical audit. This involves taking each item that is DC powered (from cabin lights to navigation instruments) what their draw is in amps and the amount of time they will run in a 24 hour period.

For instance an instrument system that draws 4 amps and is run 12 hours means it requires 48 amp hours of battery to rum. On a dawn to dusk summer delivery an auto pilot drawing 10 amps running for 18 hours will need 180 amp hours of battery. Even a cabin light can draw an amp. Spend some time in your bunk reading with the lights on for 3 hours and that’s 3 amp hours.

Once you have gone through and gathered the numbers and done the math for all your DC devices you can then come up with a total amp hours required.

500 amp hours of load is not satisfied by 500 amp hours of battery capacity. In fact, since batteries should be drawn down to approximately 50% of their capacity, 500 amp hours of load actually would call for 1000 amp hours of battery.

Many typical sized batteries are 100 amp hours or under. A typical sailboat with cabin lights, electric bilge pumps, sailing instruments, and an outplot will have a load of around 150 amp hours. Using the 50% rule this goes up to 300, or at least 3 normal (such as Group 27) sized batteries. Addition of a radar that might need to be run for 18 hours can add 100’s of amp hours, and multiple
batteries.

Other items such as laptop computers used for navigation or AC inverters used to run children’s video games can further add to your boats load.

With all this information in hand you can evaluate your boat’s alternator.

The alternator’s capacity in amps should be 25% of that of the batteries. For instance in the example above a 150 amp hour load, which needs 300 amp hours of batteries would require a 75 amp alternator.

Obviously each boat’s load is different, therefore so are battery requirements and alternator needs are too. Keep in mind that these are house battery requirements. For our purposes the starting battery is separate.

Next week our final article on batteries will consider installation, testing and items like straps and terminals.

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