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Landfall-Learning > Science > Electricity


Two Kinds of Electricity

Landfall operates on two separate electrical systems. When we are tied up to a dock, we can plug into shore power, which is a 110-AC volt system similar to that in your house or classroom. When we are underway or at anchor, we use a 12-volt DC electrical system, very similar to the system in an automobile. One of our biggest needs for electricity is to power our electronics.

The 110-volt AC system allows us to plug our television or computer into a wall socket just as you would on land. Most important to us, using the 110-volt shore power system means we do not drain our battery bank very much when we are plugged into the shore power. There is also an automatic battery charger wired between the 110-volt AC system and the 12-volt DC system to maintain the charge in the battery bank.

When we are off of the dock, we depend solely on the 12-volt DC battery bank for electricity. We have an electrical inverter on board that allows us to operate our 110-volt appliances. The inverter takes the 12-volts DC coming in from the battery bank and changes it into the required 110 AC volts.

The biggest difference between an automobile's battery system and Landfall's battery system is that the car's engine is almost always running when the batteries are being used. This means that the car's alternator is constantly recharging the batteries. Since Landfall is a sailboat, we try to run the engine as little as possible. When the engine isn't running, there is nothing charging the batteries.

Every time we turn on a light or turn on the water faucet, we drain power from the battery bank. Therefore, we constantly have to monitor how much electricity we use and the charge status of the battery bank. When it starts to get low, we have to replenish the charge.

Charging the Battery Bank

There are four ways that we can recharge the batteries. The easiest way is to plug into the dock and use the automatic battery charger. However, since we plan to spend most of our time away from docks and marinas, this method does not meet our long-term needs.

The second method is to run the engine and let the alternator charge the batteries the same way an automobile's alternator would. Unfortunately, running the engine consumes fuel, and we need to conserve fuel to ensure that we can run the engine when there is no wind. Running the engine is also smelly and noisy, so we prefer alternative methods.

The third way to charge the battery bank, and a simple, abundant, clean and quiet way to recharge it, is to use the sun. Solar panels collect sunlight and convert the rays into electricity. This electricity can then be fed back into the battery bank. But while this method is clean, quiet and free, it does not work at night or on overcast days. Therefore, we must supplement this method with the fourth way to recharge the battery bank, by using a wind generator.

A wind generator is a propeller connected to an alternator (again, like in an automobile). When the wind turns the propeller, it drives the alternator to generate electrical current, and the current is fed into the battery bank.

By combining wind generation and solar generation, we can create enough electricity to recharge the battery bank without wasting our fuel or polluting our environment with engine exhaust and noise.

What is a Battery Bank?

We use the term "battery bank" because rather than a single battery, we have several batteries that are wired together to meet our electrical needs. In fact, there are actually three separate battery banks on Landfall.

The first battery bank is used solely for starting the engine. This ensures that if we drain the other batteries too much without recharging, we still have enough power to start the engine and then recharge the other banks.

The second bank is used to run the windlass, which raises and lowers the heavy anchor and rode. This is a separate bank because it requires a lot of energy to lift the weight of the anchor and the heavy chain rode from the bottom.

The largest battery bank is called the "house bank". The house bank supplies the electricity to the cabin lights, appliances and electronics that are constantly in use on the boat. The house bank consists of six, 6-volt "golf cart" batteries. The golf cart batteries are wired in pairs. By wiring two 6-volt batteries together, you can create the equivalent of a single 12-volt battery. The three pairs are wired in parallel. So, this system has the equivalent of three 12-volt batteries.

Amp Hours, Series and Parallels

The amount of power in a battery is measured in "amp-hours". A 12-volt battery that can store 10 amp-hours of electricity will provide enough electricity to run a single 12-volt light that draws one amp of power for 10 hours.

When batteries are wired in series, as each of our golf cart battery pairs is, the voltage is equal to the total voltage of both batteries added together, but the amperage available is limited to the amperage of one of the batteries.

When batteries are wired in parallel, the voltage remains the same but the total amperage of the bank is the amperage of the batteries added together.

Therefore, if two 12-volt batteries rated at 50 amps each are wired in parallel, you have created the equivalent of a single 12-volt battery with a total of 100 amps.

Needless to say, we have had to do a lot of math to determine how much power each of our appliances draws and how much power we have available in our battery banks.



  1. Landfall has six, 6-volt batteries, wired in series into three pairs. What is this bank equivalent to in volts?
    (Answer: Three 12-volt batteries).
  2. Both batteries in each of the pairs are rated at 50 amps. How many total amps would each of the pairs be rated at?
    (Answer: 50 amps).
  3. Each of the three pairs is wired in parallel. How many amps will the total house bank produce?
    (Answer: 150 amps).
  4. The house bank is used to run several navigational instruments and household appliances. If the radar draws 10 amps, the two-way radio draws 5 amps, and the cabin lights draw 5 amps each hour, what is the total power demand on the house bank for that hour?
    (Answer: 20 amps per hour).
  5. If all of the devices in question 4 are left on continuously, how many hours can we go before we are required to recharge the house bank?
    (Answer: 10 hours).
  6. If the wind generator adds an average of 8 amps per hour back to the batteries and the solar panels add an average of 8 amps per hour, how many hours can we go without starting the engine to supplement the charging process?
    (Answer: 50 hours, there is a 4-amp hour short fall with the wind generator and solar panels steadily depleting the battery bank).
  7. How could we avoid running the engine all together and maintain the batteries?
    (Answer: By turning off any of the devices listed above for all or part of the time, we could reduce the draw on the batteries. By turning the lights or radar off for 12 hours each day, we could reduce the draw of those items by 50%)
  8. Discussion Questions:
    What happens if we are in the middle of the ocean and we forget to recharge the battery banks, and we keep drawing on them until they are empty?
    (Possible answers:
    The cabin lights won't work.
    The running lights won't work.
    The electrical navigational equipment, including the GPS, RADAR, and two-way radio won't work.
    The fresh water pump won't work because the solenoid that controls the gas flow to the stove won't work.
    The engine won't start.)
  9. What would you do if this happened?
    (Possible answers: Wait for the solar and wind generators to recharge the batteries.
    Use the sextant to navigate.
    Use flashlights to see in the dark.
    Use the handheld, battery-powered GPS, we keep on board for just such an emergency.
    Use the small handheld, battery-powered two-way radio we keep on board for just such an emergency. )