Inverters

Why do you need an inverter on a boat?

For many small boats, a diesel generator is not an option. Its cost is high, and the lack of space on a boat and the noise generated during operation are additional obstacles to the organization of the alternating current (AC) system on board. An inverter converts direct current (DC) 12 or 24 Volts into 220 V and is an excellent alternative to a generator.

The inverter increases comfort on board. It allows you to enjoy the quiet at the anchorage, saves money, and has very few disadvantages. Most boat owners will agree to this.

Basic types of inverters.

The inverter produces AC voltage from a DC one and operates in the opposite direction from the charger. The conversion occurs in two stages: first, the DC voltage is transformed into a variable, which is then raised to the required level.

 An inverter voltage chart with a modified and pure sinusoid.

All models are divided into two major groups: inverters with pure sinusoid and devices whose voltage is in the form of a stepped square wave, sometimes called a modified sinusoid. Both types of voltage can be obtained by switching the electronic keys both at the network frequency (50 times per second) and at a frequency several orders of magnitude greater than the network key (20000 Hz and more). There are four possible combinations of inverters: a linear-square wave, a linear-sinusoidal wave, a high-frequency-square wave and a high-frequency-sinusoidal wave.

Linear and high-frequency devices differ in weight and size. A 2000 W linear inverter with both pure and modified sine weight is about 20 kg. A high-frequency model of the same power weighs half as much and takes up one-third of the linear device capacity.

High-frequency inverters with pure sinusoids are often less efficient than models with modified waves and without load consume more current. Since the battery charge is limited, the intrinsic consumption of the long-waiting inverter strongly influences the battery condition of the boat or yacht.

What batteries are suitable for an inverter?

Microwave furnace with a capacity of 1200 W consumes from a 12-volt battery – 100 A. Since the inverter’s efficiency when converting the DC voltage to a variable of less than 100%, the actual current is even higher – 110-115 A. This is twenty times greater than a 220 V electrical system.

Long and short-term discharge.

The more current the devices connected to the battery consume, the smaller the capacity thereof in ampere-hours. In high current discharges, the effective capacity of the lead-acid battery (which it can charge before the voltage falls below the threshold) is only a part of the nominal value. Therefore, under the load, the battery voltage drops sharply, and the inverter shuts down. After a while, the battery is restored and gives another short power. But sooner or later, repeated high current discharges will destroy the battery over a long period.

When using batteries of equal capacity, under the same load, gel batteries better hold voltage than liquid acid batteries, and AGM is better than gel ones. Lithium-ion accumulators maintain stable voltage up to almost complete discharge.

Under no circumstances must the long-term discharge current of batteries exceed 25% of the liquid acid tank and 40% of the AGM battery. Allowable discharge current for lithium batteries is up to 100% capacity.

Load intensity is only one side of the impact of the inverter on the battery. Its duration is the other. To avoid excessive discharges, the lead-acid battery capacity must be two and a half to four times the expected electricity consumption between chargers (lithium battery capacity – one and a half to two times).

If a yacht or motorboat does not have huge batteries, the engine generator does not have an external multi-stage voltage regulator or a DC-DC charger. There is no additional generator. The powerful inverter should be turned on with full load only in rare cases and for a short time.

How do you protect a battery?

Since inverters can quickly release batteries, they always incorporate a circuit that shuts down the device when the battery voltage becomes very low.

Suppose the inverter is connected to the battery with a relatively small capacity, then under high load. (це речення або неправильно переклали, або не дописали шось)  In that case, the battery voltage will drop rapidly, although its charge will not change so much. Therefore, to prevent premature deactivation of the inverter at high short-term loads, the disconnection point must be set between 10 and 11 V.

However, if the battery is discharged with a low current, then at 10 V it will become not only «dead» but will also have significant internal damage. In other words, the built-in low-voltage shutdown protects the inverter but not the battery. It is not necessary to rely on inverter automation to protect the battery. It is needed to use other means of control.

Which inverter should you choose for your boat?

If you plan to install 2-3 low-power AC devices on board, which will be available from time to time, the inverter can be easily adapted to the existing DC system.

This mode of inverter operation is most common when a yacht or a boat is used only on weekends, and then for a week the vessel stands at the pier and is connected to the shore power grid. For the uninterrupted operation of the inverter on the «Output Day Vessel», it is important to select correctly the capacity of the service accumulator battery, which must withstand the load for 2-3 days.

The type of inverter depends on the size and weight of the device and whether voltage-sensitive equipment will be on board. Another important characteristic is the current consumed by the inverter in standby mode.

Pure sinusoidal inverters are necessary for TV, multimedia devices and AC engines. Microwave ovens are sensitive to both the shape and output voltage, so it is better to connect them to an inverter with a pure sinusoid.

Another important detail in the inverter selection.

The neutral conductor of the AC circuit must be grounded only on the voltage source. Thus, before starting work, the inverter on board a boat or a yacht must connect the neutral to its grounding wire and, after the vessel is connected to the shore network (or other AC voltage source), disconnect them. All marine inverters do it automatically, but coastal inverters do not. So before you buy an inverter for a boat or a yacht, make sure you buy a model specifically designed for maritime use.

Choose an inverter depending on the load.

Suppose AC voltage devices are switched on board a yacht or boat regularly. In that case, the service battery capacity will have to be increased. The battery capacity will have to be charged with a running engine generator or an additional diesel (gasoline) one.

Typical daily AC consumption graphs on a boat or a yacht. On the left this is an uncontrolled variant. The generator is started every time the AC voltage device is switched on. On the right these are all powerful AC devices, switched on only when the generator runs. The inverter works the rest of the time.

When the inverter and the generator work together, the energy-intensive AC equipment comprises 1-2 operating hours of the engine (additional generator). In between, only less powerful devices are operated during a period of silence, which are powered by a battery via an inverter.

The generator and the inverter complement each other – their strengths and weaknesses are opposite. Without the inverter, a yacht or a boat owner is forced to start the generator only to turn on the TV or heat the kettle. The result is useless fuel consumption, incredibly high cost of power generation and heavy load on the generator.

The inverters are effective at both low and high loads. For example, if you have an inverter on board, you don’t have to run an engine for three minutes to heat the products in a microwave oven. The inverters are indispensable for lighting. Finally, during the running of the propulsion engine, the inverter can be used to supply AC circuits. In this case, the batteries will not be discharged as the power source for the inverter will be the engine generator.

Recommended characteristics of the inverters.

  • High peak load. Some high-frequency models have a maximum load of only 150% of continuous one. Linear devices peak load is up to 300% nominal
  • High efficiency of conversion of DC voltage to a variable. Typically, manufacturers specify the maximum efficiency that characterizes the performance of the inverter at low loads. To select a device correctly, compare performance graphics over the entire range of capacities for multiple models
  • Parasitic power consumption on the motorboat needs to be reduced. Therefore, if the inverter is kept open for a long time without any load, the current consumed in the waiting mode must be minimal. This indicator is one of the most important features of the inverter
  • Remote Control. Often on boats and yachts, the inverter is installed in a hard-to-reach place, connected to accumulators of small capacity, and occasionally used. The remote control will disable the inverter at any time and will reduce power consumption on board
  • Automatic voltage source switch. It connects the on board electrical system to a shore network or the Every time the inverter detects an external AC voltage source, it automatically connects it and switches to battery charging (if it has a built-in charger). As soon as the external source shuts down, the inverter returns to AC generating mode
  • Powerful built-in charger with low pulsation rate and temperature compensation of charging voltage. Additional output with a limited current for charging the start-up battery
  • High level of electromagnetic interference suppression. Cheap models produce intense electromagnetic radiation and noise in the electrical grid. Devices issued for Europe must be CE-marked
  • A simple installation that depends mainly on convenient access to the AC and DC connection terminals and the ability to place the remote control panel at the selected location.