How do you hook up a solar panel to a boat battery

How to connect solar panel to boat battery

how do you hook up a solar panel to a boat battery

Solar panels for boat/yachts

The cost of electric energy on a boat or boat/yacht is very high. Especially if during parking, the owner charges the batteries with an engine that does not have an external voltage regulator or a DC-DC charger installed.

In this case, any equipment that generates electricity cheaper than the internal combustion engine becomes cost-effective and quickly pays off.

Types of solar panels

Solar panels convert the free light of the sun into electricity, and given the fact that the price of the semiconductors from which they are made decreases every year on a boat/yacht or boat, the panels pay off within a few months-a year.


They are cost-effective to install on the boat as much as possible.

However, the result will be disappointing if you do not choose the right battery power or mount them in the wrong places. Boats and boat/yachts use three types of solar panels:

In single-crystal panels, each cell is cut from a single silicon crystal. Although some semi-flexible models also use single-crystal cells, as a rule, panels of this type are rigid and do not tolerate bending.

The coefficient of conversion of light into electrical energy in them reaches 22%, but most often it is 16-18%.

Most single-crystal panels have a solid, rigid back wall. Recently, two-sided models have appeared, allowing you to collect light from both sides.

This is convenient when there is a reflective surface under the panel, such as the white top of the cab.

Solar panel for boat/yacht

  1. Cell efficiency, % 22.2-22.4
  2. Power at the operating point (Pmpp), Wp 310
  3. No-load voltage (Uoc), B 23.1
  4. Operating point voltage (Umpp), B 18.8
  5. Operating point current (Impp), A 16.46
  6. Short-circuit current, (Isc), A 17.54
  7. Type Monocrystalline.
  8. Surface material ETFE or PET

In polycrystalline solar cells, each cell consists of several small crystals. Such panels are less efficient than single-crystal ones, especially at low light levels, but they are lighter and cheaper.

During the production of amorphous wafers, the vaporized silicon is deposited on the substrate. Amorphous panels are the cheapest and very flexible, but their efficiency is the least.

Each silicon cell, regardless of its size, generates a voltage of about 0.6 volts when exposed to direct sunlight.

The voltage of the entire battery can be approximately determined by multiplying 0.6 by the number of cells. For example, the voltage of a solar panel consisting of 30 cells is 18.0 volts.

The output current of the cell depends on its type, quality, and surface area.

Therefore, to get the same power output using amorphous and monocrystalline panels, amorphous will have to occupy twice the area.

In addition, the capacity of amorphous batteries is about 10% less than the nominal one to two years after production.

In the future, it will stabilize.

Characteristics of solar panels

In the specification for the solar battery, the manufacturer specifies the following characteristics:

  • Voc — open circuit voltage. This is the voltage of the solar battery disconnected from the battery
  • Isc — short-circuit current. The maximum current that the panel outputs if its terminals are closed between them. The output voltage of the battery in this case is zero
  • Imp — maximum load current
  • Vmp-voltage at maximum power

Pmax — the maximum power of the solar battery. This is the product of the two previous parameters. Sometimes only the maximum power and the corresponding voltage on the load are given.

In this case, the load current can be found by dividing the power by the voltage.

None of the above characteristics describes the actual performance of the solar battery-the output current at the battery charging voltage

Current-voltage characteristic of the solar panel

The panel voltage at maximum power depends on the number of cells and their temperature. It is always higher than the recommended charging voltage, but decreases when connected to the battery.

Because of this, even under standard test conditions, the output power at the battery charging voltage is always 20-25% less than the nominal one.

Solar panels are tested under standard conditions. From the point of view of the owner of the boat or boat/yacht, the most important of them is the assumption that the sun’s rays fall on the battery at an angle of 90 degrees, and its temperature is 25 ° C.

The test results are shown in the form of a current-voltage characteristic. Sometimes manufacturers provide data for several different temperatures. The maximum power of the solar battery corresponds to the bending of the current-voltage characteristic at 25 ° C.

Connecting solar panels

Two ways to connect solar panels to the electrical system of a boat or boat/yacht. On the left-the junction box provides a safe and reliable electrical connection and is guaranteed to withstand atmospheric influences.

It is installed on the back side of the panel. If surface mounting is intended, the junction box can be mounted on the front side of the panel.

On the right — two cables with silicone insulation and a plastic cable entry, located at the back of the panel. The electrical polarity is clearly indicated by the insulation color. Alternative to the junction box.

The panel voltage at maximum power depends on the number of cells and their temperature.

It is always higher than the recommended charging voltage, but decreases when connected to the battery.

Because of this, even under standard test conditions, the output power at the battery charging voltage is always 20-25% less than the nominal one.

You can find out exactly how much power falls if you measure the current given off by the solar battery during battery charging.

For example, a 50-watt solar panel with a rated voltage of 17 volts provides a current of 2.94 amps (W / volt = ampere). According to the current-voltage characteristic at a temperature of 25 degrees, we find that at a voltage of 13.0 volts, the output current of the solar battery is 3.0 A (A voltage of 13 volts is suitable for charging a discharged battery and a battery with a connected load).

Although the output current changed slightly compared to the value at the rated voltage, the output power decreased to 13.0 volts x 3.0 amps = 39 Watts. This is 22% less than the rated power.

There are other losses that need to be considered before installing solar panels on a boat/yacht or boat. On land, the panels are mounted on supports located at an angle to the horizon. In this case, the maximum amount of sun rays falls on the surface.

But if you install a 80 watt panels on a boat or boat/yacht in this way, they will lose the sun after each turn. To avoid this, the panels on boats are almost always installed in a fixed place horizontally.

However, even in the tropics, a sunny afternoon (the time when the sun is directly overhead) lasts only a few hours a day. The rest of the time, the sun’s rays fall on the panel at smaller angles and the amount of energy transmitted by them is noticeably reduced.

Solar panel power

Dependence of the output voltage of the solar panel on its surface temperature

The relationship between temperature and power for three solar panels. The curves represent the maximum power output in bright sunlight, not the realistic output under normal operating conditions.

At a surface temperature of 50 ° C, the output of a 36-cell panel is reduced by 15 volts, and on a 30-cell panel by 11 volts. This is too little for efficient battery charging in hot climates.

The actual power of the panel is reduced even more if the cloud obscures the sun or the shadow of the rigging, sails, or mast falls on the surface of the battery.

Even partial shading of a single cell in a circuit connected in series significantly reduces the output current.

Sharp shadows affect the output power more than shadows with fuzzy edges. If no shunt diodes are installed on the cells, then a sharp shadow on one cell will reduce the output current of the entire panel in proportion to the shaded area (for example, 50% shading just one cell will reduce the output of the entire panel by 50%).

A cell that is in the shadow consumes current from its neighbors and overheats.

Shunt diodes reduce problems from shading. They isolate the cell that has fallen into the shadow and stop the development of “hot spots”.

However, each cell removed from the general circuit reduces the voltage of the entire panel.

Since the output voltage of the panel is reduced due to heating, there may be a situation when it is below the level suitable for charging the battery. In this case, the benefit of shunt diodes disappears.

Sharp shadows falling on the surface of the solar panel on a boat/yacht or boat should be avoided

Even in a sunny climate, the energy actually generated by the panel during the day rarely exceeds the level of 4-5 hours of operation at maximum power.

Often this value is even smaller. Calculations are better based on the assumption that the daily electricity generation corresponds to 3-4 hours of battery life at rated power.

This method of comparing the actual energy produced by the solar battery with the maximum is called Peak Solar Hours (PSH).

There are websites that calculate PSH for different parts of the world and for different periods of the year.

However, almost all of them assume that the solar panels are installed at an angle to the horizon and no shadow falls on them.

In this case, the PSH is significantly inflated. Since the realistic estimate of PSH is 3, the number obtained from the online calculator must be reduced by at least 30%.

A 6-watt solar panel running 3 hours a day in a 12-volt electrical system will produce 18 Wh = 1.5 ampere-hours of electrical energy per day.

30-watt — 90 watt-hour or 7.5 amp-hours per day (number of amp-hours per day at 12.0 volts = rated power / 4).

If the daily consumption of electrical energy is known, for example, 60 ampere-hours at a voltage of 12 volts, then the power of the solar panel is determined by multiplying the ampere-hours by 4 (60 Ah x 4 = 240 W)

Solar battery voltage

The output voltage and current of the solar battery relative to the “solar half-day”. The voltage drops as the temperature rises in the sunny afternoon and early afternoon.

The solar battery operates at rated power for a short period of time. The output power of the panel can be increased if you adjust its position during the day

To charge the battery, the voltage of the solar battery, like any other charger, must be higher than the battery voltage. And the difference should exist even when the battery voltage rises to 14.0 volts.

A 12-volt solar panel consisting of 30-44 cells, with an open circuit, provides a rated voltage of 18.0 to 26.0 volts. At first glance, this is enough to charge the battery. In fact, this is not always the case.

On a “sunny afternoon”, the black silicon in the solar cell heats up. If the temperature of the panel exceeds 25 ° C, its output voltage will decrease compared to the nominal — 1.0 volts for every 12 ° — 15 ° C increase in temperature.

At a surface temperature of 50 ° C, the output voltage of the 30-cell panel will drop to 13.3 volts. The 33-cell panel has up to 14.8 volts, and the 36-cell panel has up to 16.3 volts.

Flexible solar panels are installed on the roof of the boat. The modules are made to order, so they fit exactly into the place chosen by the customer

The charge rate of batteries connected to a solar panel with 30 cells will constantly decrease, as the voltage on the batteries will increase, and such a panel will not fully charge the battery.

Solar panels laid horizontally are heated more strongly — there is no air gap between their back side and the base on which they are installed.

To compensate for the increased voltage drop, they increase the number of cells. In some models, up to 42 pieces.

During installation, a blocking diode is sometimes added to the panel circuit in addition to the shunt diodes described earlier. The blocking diode additionally drops about 0.6 volts.

Because of this, a 30-cell panel with a blocking diode, especially in hot climates, does not charge the batteries well. The efficiency of the 33-cell panel also decreases as the battery voltage increases.

In a southern climate, the panel must have at least 30 cells to charge the batteries. A 33-cell solar battery will provide enough voltage to charge, but a margin for losses (voltage drop on diodes, in cables, connections, and poor sunlight) she will have a small one.

The 36-cell panel can handle battery charging in almost any situation. In temperate climates, the 33-cell panel provides a suitable voltage for charging batteries at all times, except on the hottest days.

For efficient battery charging in hot climates, the minimum panel voltage (under standard test conditions), after subtracting the voltage drop across the diodes, should be 16.0-17.0 V. In temperate climates — 15.0 to 16.0 volts.

Solar Cell Voltage Regulators

As the battery is charged, a self-regulating solar panel consisting of 30 cells reduces the output current.

If you take into account the heating of the panel in a hot climate, the voltage drop in the blocking diode and other parts of the circuit, a self-regulating solar panel will not charge the batteries well, regardless of its rated power. More cells are needed for efficient charging.

Pricing table with an Table ID of “classic-blue_11” is not defined.

But a panel that maintains a voltage suitable for charging the batteries will slowly recharge them while the boat or boat/yacht is not in use.

The critical point occurs if the rated power of the panel at a voltage of 14.0 volts exceeds 0.5% of the battery capacity (for example, a panel with an output current of 1 A, connected to a battery with a capacity of 200 Ah).

If the power of the panel is higher, it is necessary to install a voltage regulator or turn off the panel when the boat is parked.

Due to the extreme sensitivity of lithium-ion batteries to overcharging, any solar panel used with any lithium-ion battery must always have a voltage regulator.

A cheap regulator consists of a simple voltage measuring circuit and a relay.

When the voltage reaches the set value, the relay is triggered and disconnects the solar battery from the batteries.

Other regulators switch the output of the solar panels to a resistor (shunt regulator) or to a load, such as a water heater (redirect regulator).

More sophisticated solar cell voltage regulators have multi-stage battery charging programs and track maximum power (MPPT).

Some models disconnect the battery as soon as a negative current appears in the circuit and replace the blocking diode in this way.

For the alignment of liquid-acid or AGM batteries, a conditioning mode is provided. One of the ways to activate it is to turn off the regulator and charge the battery at full voltage of the solar panel.

MPPT Solar Controllers

The Maximum Power point tracking controller is an extended version of the pulse-width modulated shunt controller.

The MPPT controller is a DC-DC converter. It consists of an inverter that converts the constant voltage of the solar panel into a high-frequency variable.

A transformer that changes this voltage and a rectifier that converts the transformer’s AC voltage back to DC.

Why do you need such a complex device? The output voltage of the solar panel is determined by the type of battery being charged.

However, the solar battery operates at its maximum capacity when its voltage is significantly higher than the permissible battery charging voltage.

Reducing the optimal output voltage to a battery-safe level reduces the actual power of the solar battery by 25% compared to the rated power.

The MPPT controller makes the output voltage of the solar panel independent of the battery voltage.

In complex MPPT controllers, the microcontroller controls the battery voltage, its charge level, and the output current of the solar panel.

Based on this data, the controller sets the output voltage of the panel, so that its power is maximum under this particular set of conditions.

To achieve the desired result, the control circuit in the DC converter is used.

Installing solar panels

There are four types of marine solar panels made specifically for boats and boat/yachts:

  1. rigid glass panels with aluminum frame
  2. semi-flexible panels
  3. flexible ultra-thin solar panels;
  4. very flexible, collapsible amorphous panels

Semi-flexible solar panels are easier to install, do not require complex mounting devices, and are much lighter than rigid ones.

If the panels are made to order, then they can be made almost any size and placed where it is most convenient

Rigid monocrystalline and polycrystalline panels have the lowest cost of 1 watt of power generated, and the maximum power for a given area.

However, the installation of these panels is most expensive, as it is necessary to use a rigid mount that protects the panels from damage.

Rigid panels operate at maximum power when mounted on brackets astern. However, in this case, the solar panels become vulnerable to waves and can be damaged when moored. Another good place is the upper part of the wheelhouse.

Semi-flexible polycrystalline panels are installed on the top of the cab and other curved surfaces. Amorphous silicon panels are placed on any surface, and if necessary, they are rolled up and removed for storage. In all cases, the heat loss will be less if an air gap is organized under the solar panel.

Connecting the solar panels to the battery

Given that solar panels are highly sensitive to even small voltage drops, it is necessary to use marine-quality cable and terminals during installation.

The contacts on the panel are vulnerable to corrosion and must be completely sealed.

There should be no additional connections above the deck – one piece of cable is laid before sealing in the deck. If you cannot do without connections, they are performed inside the boat.

Connection diagram of several batteries for charging from solar panels. The Sterling Power bistable relay is used. A conventional relay consumes a current of up to 0.5 A in the closed state and can negate the operation of solar panels. The bitable relay consumes current only during switching on — 0.5 mA.

The current carrying capacity of the cable is obtained by multiplying the short-circuit current of the panels by 1.25. Then, according to the table, the cable cross-section is selected, taking into account the 3% voltage drop.

If the panel is connected directly to the battery for supporting charging, then install a fuse as close to the battery as possible. Without it, any malfunction in the wiring will lead to a short circuit of the battery and possibly a fire.

If part of the panel can get into the shadow, then instead of one large one, it is better to use a set of several smaller solar panels, designed for the same voltage, but connected in parallel. In this case, the shaded panel will reduce the output, but will not affect the rest of the circuit. Shading part of the large panel will reduce the power output of the entire battery.

If a 24-volt electrical system is installed on a boat or boat/yacht, it is not correct to connect two 12-volt solar panels in series. Shading any area on any panel will affect both.

It is better to connect them in parallel, get 12 volts at the output, and use a DC-DC converter to increase the voltage to 24 volts. In this case, one panel may be completely in the shadow, but this will not affect the second one.

Multiple batteries

Some separate charging systems use diode insulators that reduce the voltage by 0.6 volts. If the solar cell is used to charge multiple batteries in a split-charge system, it must be installed before the separation diodes.

The voltage drop on the diodes in this case must be taken into account when calculating the output power of the panel.

To service several battery groups, chargers with two or three outputs are installed on boat/yachts. Some models of solar voltage regulators also have multiple outputs, allowing you to charge two batteries without additional diodes or relays.

But such devices are not widely used and are more expensive. The isolation device installed between the batteries allows you to charge several batteries at the same time without a voltage drop.

It is better to use a bitable relay, which in the closed state does not consume current and does not reduce the charging capacity of solar cells.

Leave a Comment