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Solar fan with dynamic battery backup for constant speed of operation

The easiest way of making a solar fan is to connect the solar PV panel to a DC fan of a matching voltage and power rating. This simple arrangement does work, but has few serious drawbacks:

  1. The system does not operate at maximum power point, resulting in a poor utilization of the PV panel.
  2. When the sunlight intensity varies, the fan speed fluctuates very widely. This is not acceptable for the user.
  3. The system cannot be used at night.

There are other designs available in the market. These systems use a solar panel and a battery. The solar panel charges the battery. The user can run the fan on battery power whenever desired. Even though this works fine, there are limitations. The user can run the fan for a few hours until the battery drains. Since all the power is routed through the battery the battery experiences a large number of charge/discharge cycles. This limits the life of the battery. In addition to this, there are losses in the battery during charging and discharging.

A new design is proposed here which runs the fan at constant speed. The concept behind this approach is to utilize all the power generated by the PV panel to run the fan. Then inject just enough power from the battery for the fan to run at near-constant speed. During the daytime, we are drawing only fraction of power from the battery. If it is a sunny day, almost zero power may be drawn from the battery. This increases the life of the battery. Also, charge/discharge losses in the battery are reduced. Another benefit of this approach is the PV panel operates near maximum power point, thus maximizing the PV power generated. This objective has been achieved without using complex switching circuits. Note that the power draw from battery is dynamically changing based on the instantaneous PV power.

Layout of the proposed solution

Figure 1 shows the block diagram of proposed solar fan. It consists of a solar PV panel, which is connected to a current limiter. The current limiter output is connected to the LED. The current flows through LED then to the FAN+ terminal. The current limiter ensures that the fan current does not exceed the rated value. The battery is also connected to the FAN+ terminal through resistor (R) and Diode (D). The under-voltage trip circuit is used to turn off the FAN when the battery is fully discharged.

Figure 1 Block diagram of solar fan with dynamic battery backup system.

A two-pole two-way switch (SW1-SW2) is used to turn off the fan when not needed. In the fan’s off state, the switch connects the battery to the PV panel through the solar charge controller. The solar charge controller charges the battery and prevents overcharging. Such controllers are available as off-the-shelf solutions.

Please note that the LED has been introduced for voltage matching purposes, specifically for the design presented in this article. For a solar fan, this LED is not essential. An explanation on how to eliminate this LED is provided in the design section.

As shown in the block diagram, there are two sources which are feeding power to the fan. One is the battery which supplies power at a fairly constant voltage through the diode (D). This diode stops the PV current from entering the battery terminal. The other one is the PV panel, which supplies power with a widely varying voltage.

A PV panel actually acts as a current source. Hence, if the PV current does not find a path, then the panel voltage goes on increasing until the current gets injected into the circuit. Here in this design, it has to overcome the battery voltage at the FAN+ terminal. As the panel current enters the FAN+ terminal, the battery current gets reduced by that much magnitude. Thus, all the available PV power is supplied to the FAN. Still, if the fan speed is below the design value, then some current is drawn from the battery to attain the designed speed. Resistance (R) helps in pushing back the battery current by increasing the total battery circuit resistance (battery internal resistance + R).

Design of the proposed solution

Figure 2 shows the circuit diagram of the solar fan.

Figure 2 Circuit diagram of solar fan with dynamic battery backup system.

The specifications of the main components are as follows:

  • Solar PV panel power rating = 10 Wp
  • Voltage at maximum power (Vmp) = 17.5 V
  • Open circuit voltage of the panel (Voc) = 21.2 V
  • Current at maximum power (Imp) = 0.58 A
  • Voltage rating of the BLDC fan = 12 V
  • Current rating of the fan = 0.2 A
  • Number of fans connected in parallel = 0.58/0.2 = 2.9 (Rounded off to 3)
  • Nominal Voltage of SLA battery = 12 V
  • Battery capacity = 7 Ah
  • Forward voltage of 1 W white LED = 3 V
  • Max current rating of White LED = 1W/ 3V = 0.33 A
  • Magnitude of current limit = Vbe/R1 = 0.6/1Ω = 0.6 A

As shown in the circuit diagram, the PV+ terminal is connected to the current limiter circuit through switch SW1. The current limiter consists of two PNP transistors: T1 (BC556) and T2 (TIP32). T2 is forward biased using resistor R2 (2.2 kΩ). When the drop across resistor R1 exceeds 0.6 V, T1 turns ON and limits the current in T2. Note that the orange LED4 in series with R2 is used for indicating the presence of PV voltage.

Wow the engineering world with your unique design: Design Ideas Submission Guide

The output of the current limiter is connected to the fans through three white LEDs (LED1 to LED3) connected in parallel. These LEDs are used here for voltage matching purposes. The fan and the battery both are rated for 12 V. However, the voltage at the collector of T2 is held at about 15 V under all sunlight conditions. The LEDs drop 3 volts and therefore the fan gets about 12 V from the PV panel. The rated current of each fan is 0.2 A. Hence, to match the maximum current of PV panel (0.58 A), the three fans are connected in parallel. If we have a fan with a 0.6 A current rating, then single fan is sufficient.

Please note that the proposed circuit has been designed using easily available components. If we had to eliminate the series LED, then we would have had to use a 15 V battery (e.g., Li-ion battery with 14.8 V rating is available). Also, we have to use a fan designed to operate at 15 V. Fans with 18 V ratings are available. These could be operated at 15 V, with slightly reduced air flow. Alternately, three 5 V fans could be connected in series.

The battery is also connected to FAN+ terminal through the fuse (1A), schottky diode D1 (1N5822) and resistance R3 (1Ω). The p-channel MOSFET T3 (IRF9540) is also included in series. This MOSFET (T3) along with the comparator circuit is used for disconnecting the fan when the battery is fully discharged.

The purpose of R3 is to increase the source resistance of the battery circuit. This will allow the PV side to push back the battery power more effectively which helps in maximizing the utilization of PV power. The value of R3 should be as small as possible to reduce power losses.

The other terminals of SW1 and SW2 are used to connect a solar charge controller to the battery when fan is turned OFF. Solar charge controllers charge the battery while tracking the maximum power point. When the battery is fully charged, the charging is stopped.

The circuit diagram of the battery’s undervoltage protection circuit is shown in Figure 3. It uses the comparator IC1 (LM311). A reference voltage of 5.1 V is connected to the non-inverting input (Pin2). On the inverting input (Pin3), the potential divider circuit consisting of R12 (47 kΩ), R13 POT (50 kΩ) and R14 (47 kΩ) are connected. The POT is used to set the tripping voltage at inverting input.

At the output pin 7 of the comparator, resistors R15 (2.2 kΩ), R16 (1 kΩ) and a green LED5 are connected. The terminals of R15 are connected to source and gate terminals of T3. Resistor R17 (100 kΩ) is used for introducing hysteresis.

Figure 3 Circuit diagram of comparator circuit used for battery under voltage trip.

When the battery is charged, the output of comparator is low. The drop across R15 is greater than the gate source threshold voltage of MOSFET. Hence T3 is ON. When the battery voltage drops below the trip voltage, the comparator output goes high. The drop across R15 becomes zero and the MOSFET turns OFF. The LED5 also turns OFF to indicate battery is discharged.

Testing the circuit

First, the circuit was tested with battery OFF condition by removing the fuse. At various sunlight intensities, the PV voltage (Vpv), PV current and fan voltage are noted down. Figure 4 shows the plot of fan power versus Vpv. It is observed that the PV voltage varies from about 8 volts to 19 volts. Below 8 volts, the fans stop rotating. The fan power varies from 1 W to 6 W. This produces a very large variation in the flow of air from the fans.

Figure 4 Plot of PV power to fan without battery backup.

Figure 5 shows the plot of fan power when the battery is ON. It is observed that the Vpv variation is restricted within the range from 14.7 V to 18.7 V. This indicates that the circuit tracks the maximum power point of the PV panel quite closely. The blue trace is the power supplied by the PV panel to the fan. The fan PV power at 14.7 V is about 2.5 W. This is significant improvement over the lowest power of 1 W when the battery was OFF (see Figure 4). Thus, the presence of the battery improves the power generated by the panel.

The orange trace in Figure 5 is the sum of the PV power supplied to the fan and the power drawn from the battery. This curve is almost flat with a variation of about 1 W in the full range. Thus, the fans run at almost constant speed even when Vpv varies from 14.7 V to 18.7 V.

Figure 5 Plot of PV power and total power to the fan with dynamic battery backup.

Figure 6 shows the power supplied by the battery to the fan. When the PV power is at a minimum, the battery supplies maximum power. As the PV power to the fan increases, the power draw from the battery reduces. At full PV power output, the power drawn from the battery is very small.

Figure 6 Power draw from the battery with variation in PV voltage.


Figure 7 shows the current limiter circuit and the under-voltage trip circuit assembled on a PCB. Transistor T2 and MOSFET T3 are mounted on heat sinks. T2 operates in the active region therefore, the heat dissipation from T2 is higher. Hence, a bigger heat sink has been used. When T3 is ON, it has small amount of conduction losses. Therefore, a small heat sink is sufficient.

Figure 7 PCB with a current limit circuit and battery undervoltage trip circuit.

Figure 8 shows the fully assembled proposed solar fan system. LED1 to LED3 are mounted on a metal clad PCB. This MCPCB is mounted on a small aluminum channel. The aluminum channel acts as heat sink. The LEDs, solar charge controller, switch, fuse and the SLA battery are mounted on a Bakelite panel.

The three fans and the controller PCB are mounted on two small aluminum channels. The interconnections are made to the PCB through the terminal strip soldered on the PCB.

Figure 8 Fully integrated solar fan system along with the solar charge controller.


The proposed system can be used for off-grid applications. The benefits of off-grid systems are as follows:

  • Unaffected by grid failures
  • Lower transmission losses
  • Lower wiring costs.
  • Useful during emergencies when there are prolonged grid failures
  • Reduced burden on the grid

Lastly, the system has been designed for a fan load as an example. However, the same approach can be used for other types of loads (e.g. lamp loads, heater loads, mobile and EV chargers etc.)


From the results shown in the graphs, it is possible to inject power from two different sources to the fan. The PV panel acts as a current source and dynamically controls the power drawn from the battery. Thus, without using complex switching circuits we can run the fans at constant speed when the output from the PV panel is highly fluctuating. All the available PV power at any given instant of time is fed directly to the fans. Only balance power is drawn from the battery, thus improving the battery life. Also, the energy lost in the charging and discharging of battery is reduced to some extent.

The proposed design can be easily scaled up for higher power systems. Nowadays, BLDC ceiling fans are available. The proposed circuit can be a good match for a BLDC ceiling fan in installations where a battery backup is required.

For better performance, the power losses in the circuit have to be kept as low as feasibly possible. Especially in the resistor R3 and diode D1. If necessary, multiple diodes could be connected in parallel to reduce the current in an individual diode. This will reduce the forward voltage drop to some extent.


[1] Solar day lamp designs use passive and active current-limiting circuits –

Vijay Deshpande worked as an electronics hardware engineer for more than 30 years in various industries. After retirement mainly working on low cost, Off-Grid, solar lighting systems.

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