Solar-powered Desulfator for Off-grid Lead-acid Batteries

Lead-acid batteries are widely used in solar systems, backup power supplies, vehicles and many off-grid applications. However, when they remain partially charged or unused for long periods of time, they can suffer from a phenomenon known as sulfation.

Sulfation occurs when lead sulfate crystals build up on the battery plates. As this accumulation increases, the battery may lose part of its ability to accept and deliver energy, its internal resistance may increase, and a battery that appears to be completely dead may still have some potential for recovery.

This problem becomes particularly interesting in solar-powered systems. Solar energy provides a renewable and freely available power source, but a small solar panel may not provide enough or stable enough voltage to directly power a pulse desulfation circuit.

This led to the following question:

How can a small solar power source be used to operate a pulse desulfator for lead-acid batteries while creating a system that is autonomous and independent from the electrical grid?

This is the idea behind Solar Pulse.

What does the device do?

For this project, I designed an off-grid solar-powered lead-acid battery pulse desulfator.

The system combines two main electronic circuits that work together.

The first circuit is a boost converter. Its purpose is to increase the voltage available from the solar panel so that the desulfation circuit can receive the electrical conditions required for operation.

The second circuit is the pulse desulfator. Instead of applying only a continuous DC voltage, it generates electrical pulses that are applied to the lead-acid battery. This pulsed operation is intended to help address the effects associated with sulfation and potentially improve the condition of a sulfated battery.

Electronic Components :

One 12 V / 1 A solar panel

Two NE555 timer ICs

One TIP31C NPN transistor

One 10N60 MOSFET

Two 1.8 kΩ resistors

One 10 kΩ trimmer potentiometer

One 10 kΩ potentiometer

Two 430 Ω resistors

One 22 Ω resistor

Two 220 Ω resistors for the red and green LEDS

One red LED and one green LED

One 56 pF non-polarized capacitor

Two 100 nF non-polarized capacitors

One 1 nF non-polarized capacitor

Two inductors: one 100 µH and one 1 mH

Two 1N4148 diodes

One 1N4007 diode

Two 1N5819 Schottky diodes

One 330 µF / 35 V polarized capacitor

One 1000 µF / 25 V polarized capacitor

Two heat sinks

One 12 V cooling fan

Four terminal blocks

Two screws for mounting the two transistors

Connecting wires

One perfboard

One switch

One XL4005E1 buck converter

One 5 V display module

Three 3.7 V / 9000 mAh lithium battery cells

Soldering tin

One DT-9205A digital multimeter

One HANTEK DSO4254C oscilloscope

One damaged lead-acid battery

Alligator clips

Type-C port

One connection terminal block

Materials and Tools :

One soldering iron

One pair of scissors

One universal pliers

One nail clipper, used as a wire cutter

Four screws for securing the caps

Caps, used as the base/support

Adhesive tape

Two spray paint cans: black and white

One plastic board/sheet

One hacksaw

One 30 cm ruler

Emery paper / abrasive paper, for polishing the cut edges

One hot glue gun

Super glue

The first stage was the construction of the DC–DC boost converter, whose purpose is to increase the voltage supplied by the solar/battery system to a higher, adjustable level required by the rest of the circuit. The converter is based on a NE555 timer, which generates a periodic PWM switching signal. This signal drives the TIP31C NPN transistor, which rapidly switches the current through the inductor. When the transistor is ON, energy is stored in the magnetic field of the inductor; when it turns OFF, the inductor releases this energy toward the output through the diode, producing a voltage higher than the input voltage. The output capacitor acts as a reservoir, storing the transferred energy and smoothing the boosted voltage. Finally, the 10 kΩ potentiometer is used to adjust the PWM duty cycle generated by the NE555, allowing the boosted output voltage to be calibrated to the desired value.

The assembled boost converter was then tested by connecting the solar panel to its input and exposing the panel directly to sunlight. The output voltage was measured using a digital multimeter set to DC voltage. Under the available sunlight conditions, the boost converter successfully increased the input voltage and reached approximately 25 V DC at its output, confirming that the boosting stage was functioning correctly.

After the initial test, the desulfator circuit was analyzed using an oscilloscope to observe the quality and nature of the generated signals. At the output of the NE555, a periodic rectangular signal with a reduced duty cycle was observed. This reduced duty cycle is intentional: it provides short high-frequency (HF) pulses with sufficient intervals between them, allowing energy to be transferred to the battery in the form of rapid pulses rather than a continuous voltage. These pulses are the fundamental mechanism used by the circuit for the desulfation process.

When the TIP31C switches, the current through the inductor is rapidly interrupted, producing a high-amplitude voltage pulse. This pulse is then transmitted toward the battery through the diode and capacitor network.

An important observation was made when the reservoir capacitor was connected: the waveform measured at the output changed significantly. This modification occurs because the capacitor stores electrical energy and releases it during the pulse sequence, interacting with the inductor and the rest of the circuit. It therefore changes the amplitude, duration and shape of the pulses delivered to the battery.

This change in the waveform is important because the objective is not simply to obtain a high DC voltage, but to produce short, controlled electrical pulses capable of delivering energy to the battery plates. The resulting pulsed waveform is therefore more representative of the intended desulfation/regeneration operation than a simple continuous DC signal.

The regenerator/desulfator circuit was assembled on a perforated prototyping board using a NE555 timer, a TIP31C NPN transistor, an inductor, diodes, capacitors, resistors and a potentiometer. Its main purpose is to generate controlled electrical pulses that can be applied to the lead-acid battery during the regeneration process.

The NE555 timer is the control element of the circuit. It is configured as an astable oscillator, meaning that it continuously generates a periodic rectangular signal. The oscillation frequency and duty cycle are determined by the timing resistors and capacitor connected to the 555. This signal is used to control the switching transistor.

The output of the NE555 is connected to the base of the TIP31C NPN transistor. When the 555 output goes HIGH, the TIP31C conducts and allows current to flow through the inductor. During this conduction interval, electrical energy is stored in the magnetic field of the inductor. When the 555 output returns LOW, the TIP31C switches OFF rapidly. The current through the inductor cannot stop instantaneously, so the magnetic field collapses and produces a short voltage pulse.

This pulse is then directed toward the output through the diode and capacitor network. The capacitor acts as a reservoir, storing electrical energy and smoothing the pulses sufficiently to provide the required output voltage. The diodes also control the direction of current and help prevent unwanted reverse current through the circuit.

The potentiometer is used to adjust the operating conditions of the oscillator and therefore allows the output voltage/pulse characteristics to be calibrated. This adjustment is important because the desulfator must operate within an appropriate voltage and pulse range rather than simply producing the highest possible voltage.

The principle can therefore be summarized as:

NE555 → switching signal → TIP31C → inductor → voltage pulse → diode/capacitor network → battery.

The objective is to produce controlled high-voltage pulses rather than a simple continuous DC voltage. These pulses are intended to assist in reducing sulfate deposits on the plates of a lead-acid battery, with the aim of improving its ability to accept and deliver charge.

3.2 First test without calibration

After assembling the circuit, a first functional test was carried out without calibrating the potentiometer. The circuit was powered and its operation was observed using the oscilloscope and measuring instruments. This initial test was intended mainly to verify that the NE555 was oscillating, the TIP31C was switching, the inductor was producing the expected pulses, and the complete circuit was functioning.

At this stage, no attempt was made to optimize the output voltage or pulse parameters. The calibration was deliberately left for the next step, after confirming that the assembled circuit was operational.

After completing the soldering and verifying the electrical connections, the circuit board was given its final finish. The solder joints and conductive tracks were visually inspected to ensure that there were no unwanted short circuits or poorly soldered connections. Excess solder and wire ends were trimmed, and the components were properly secured to give the assembly a cleaner and more reliable appearance. This final stage prepares the PCB for its integration into the complete system and for the subsequent practical tests.

After preparing and measuring the plastic panels, the next step was to cut the enclosure according to the dimensions of the electronic components. The openings were made for the display, potentiometer, ventilation, connectors, switch, and other necessary connections. A ruler was used to ensure accurate dimensions, while a hacksaw and cutting tools were used for the main cuts. The edges were then smoothed with abrasive paper to remove irregularities and obtain a cleaner finish. This step prepared the enclosure to accommodate and protect the complete electronic system.

After cutting, sanding, and preparing the different faces of the enclosure, the next step was to paint them using black and white spray paint. The surfaces were first cleaned to remove dust and particles that could affect the adhesion and appearance of the paint. The panels were then painted with several light and uniform coats, allowing sufficient drying time between coats. This operation gives the enclosure a cleaner, more professional appearance while also providing a protective surface finish. The combination of black and white was chosen to improve the visual contrast between the different faces and make the controls and display easier to identify.

After painting and allowing the enclosure panels to dry completely, the next step was to install the base supports made from bottle caps. The caps were positioned at the bottom of the enclosure and fixed using screws, ensuring that they were properly aligned and firmly attached. These supports raise the enclosure slightly above the working surface, providing mechanical stability, better ventilation, and protection for the components and wiring located inside. This step also gives the final assembly a more stable and finished appearance.

The ninth step consists of the final assembly of the different parts of the system and their electrical interconnection. After completing the construction and finishing of the enclosure, the different electronic modules and energy sources are installed inside the housing and connected together to obtain a complete and functional device.

9.1. Assembly of the different parts

First, the different faces of the enclosure are assembled to form the final structure of the device. The main components, including the regenerator/desulfator circuit, the lithium battery pack, the Boost converter, the display, the solar panel connection, and the control elements, are positioned in their respective locations.

The components are arranged in a way that provides sufficient mechanical support while also making the electrical connections accessible for testing, maintenance, and adjustment.

9.2. Connection of the Boost converter module

The Boost converter module is then connected to the system. Its main function is to increase the input voltage to a higher and suitable voltage level required by the electronic circuit.

The input and output terminals of the Boost converter are connected while carefully respecting the positive (+) and negative (−) polarities. The output voltage can then be adjusted according to the voltage required by the circuit.

This module therefore serves as an important interface between the available battery voltage and the voltage required for proper operation of the electronic system.

9.3. Connection of the display

The digital display is connected to allow the user to monitor the voltage of the system directly. Its integration makes it possible to observe the electrical operating conditions without continuously using an external measuring instrument.

This display is particularly useful during testing because it allows the evolution of the supply voltage to be monitored while the different parts of the system are operating.

9.4. Connection of the solar panel

The solar panel is connected to the corresponding input of the system. Its purpose is to convert solar energy into electrical energy and provide energy for the system while also contributing to the charging of the batteries.

During this connection, the polarity of the solar panel is checked carefully to ensure that the positive and negative terminals are correctly connected.

The solar panel therefore constitutes the renewable energy source of the system.

9.5. Connection of the battery

The battery is then connected to the power section of the device. It acts as an energy storage element and supplies the electronic circuit when the energy supplied by the solar panel is insufficient or unavailable.

The battery connection is made while respecting the correct polarity. Once connected, the stored electrical energy can be distributed to the different sections of the device through the appropriate power circuits.

9.6. Connection of the Type-C charging port

A USB Type-C port is also integrated into the enclosure and connected to the internal lithium battery charging circuit. Its purpose is to provide an additional and convenient method for charging the internal lithium power batteries.

The Type-C port receives an external electrical supply, which is then handled by the appropriate charging circuit before being delivered to the lithium batteries. This arrangement allows the internal batteries to be recharged independently of the solar panel when necessary.

The integration of the Type-C charging port therefore provides greater flexibility in powering the device, since the internal batteries can be charged either through the solar-energy system or through an external Type-C power source, depending on the implemented charging architecture.

9.7. Final interconnection

After connecting each individual section, the different modules are interconnected to form a complete electrical system. The solar panel, lithium batteries, Boost converter, display, Type-C charging interface, and regenerator/desulfator circuit are therefore integrated into the same enclosure.

Before applying power, all connections are checked, particularly the polarity of the batteries, the input and output terminals of the Boost converter, and the charging connections. This preliminary verification helps prevent short circuits, incorrect connections, or possible damage to the electronic components.

Before the test, the faulty battery had a voltage of 0.44 V. After 3 hours of operation of the regenerator/desulfator circuit, the battery voltage increased to 2.16 V.

This increase indicates an improvement in the battery’s electrical condition following the treatment, although the battery remained significantly degraded.

The final step was to perform the complete testing and demonstration of the device under real operating conditions. The system was tested with a lead-acid battery while being powered by the solar energy system. The voltage evolution of the battery was monitored during the desulfation process to evaluate its recovery.

The duration of the desulfation process depends on the initial condition and level of degradation of the battery. In general, the treatment takes approximately 1 to 3 days, with more severely degraded batteries requiring longer treatment.

A video demonstration was also recorded to show the complete operation of the device, including the solar power source, the electronic system, the voltage display, and the connection to the lead-acid battery.

These final tests demonstrate the complete operation of Solar Pulse, from solar energy generation to the pulse desulfation process.

1. General System Overview

The system is divided into several functional blocks, each with a specific role:

1. 12 V / 1 A Solar Panel

2. Input Protection Diode

3. 12 V to 25 V BOOST Converter

4. First NE555 Timer

5. TIP32C Transistor

6. Desulfation Stage

7. Second NE555 PWM Generator

8. 10N60 MOSFET

9. Pulse Coil

10. Schottky Output Diode

11. XL4005E1 Buck Converter

12. Display Module

13. Output Terminal

14. Three 3.7 V Li-ion Cells

15. 3S BMS

16. USB Type-C Charging Port

17. Red LED

18. Green LED

19. POWER Switch

The complete system can be understood as three main functional sections:

- Energy generation and voltage conversion

- Pulse generation and desulfation

- Monitoring, control, and battery management

The solar panel provides the initial energy. The boost converter raises the available voltage to approximately 25 V. The desulfation stage then uses this energy to generate short, controlled electrical pulses through a coil. The output section directs and monitors the resulting electrical energy.

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2. 12 V / 1 A Solar Panel

The solar panel is the primary energy source of the system.

It is rated at approximately 12 V / 1 A, corresponding to a theoretical nominal power of approximately:

12 W

However, these values are nominal. A solar panel does not behave like a regulated 12 V power supply.

Its actual voltage and current depend on several factors:

- solar radiation;

- panel orientation;

- temperature;

- connected load;

- operating point;

- environmental conditions.

Therefore, the voltage supplied by the panel can vary significantly.

The purpose of the solar panel is to provide the electrical energy required by the rest of the system.

The panel output first enters the protection stage and then supplies the boost converter.

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3. Input Protection Diode

The protection diode is placed between the solar panel and the electronic system.

Its primary purpose is to prevent reverse current.

For example, if the voltage on the system side becomes higher than the voltage available from the solar panel, current could potentially flow backward toward the panel.

The diode prevents this unwanted current path.

It therefore acts as an electrical one-way valve:

Solar panel → system

while blocking the opposite direction.

A Schottky diode is particularly useful because it generally provides:

- relatively low forward voltage;

- fast switching;

- low conduction losses.

This is important in a solar-powered system because the available power is limited. Every unnecessary voltage drop represents wasted energy.

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4. 12 V to 25 V BOOST Converter

The next major block is the BOOST converter.

Its purpose is to increase the available input voltage from approximately 12 V to approximately:

25 V DC

This higher voltage is required as the energy source for the desulfation stage.

A boost converter does not create additional energy. Instead, it converts voltage and current while accounting for its own losses.

The fundamental operating principle is based on an inductor and high-speed switching.

During one part of the switching cycle, energy is stored in the magnetic field of the inductor.

During another part of the cycle, that stored energy is transferred toward the output.

By repeating this process rapidly, the converter can produce a DC output voltage higher than the input voltage.

Therefore, the general energy path is:

Solar panel → protection → BOOST converter → approximately 25 V DC

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5. First NE555 Timer

The first NE555 is used as the timing and control element of the BOOST converter.

The NE555 generates a periodic switching signal.

The important parameters of this signal are:

- switching frequency;

- duty cycle.

The NE555 does not directly generate the 25 V output.

Instead, it controls the switching action of the power stage.

It can therefore be considered the timing brain of the boost converter.

Its function is essentially to determine:

«when the power transistor should switch ON and when it should switch OFF.»

The repeated switching action allows the inductor to alternately store and transfer energy.

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6. TIP32C Transistor

The TIP32C is a PNP bipolar power transistor.

In the proposed architecture, it is part of the control/power section associated with the boost converter.

The NE555 produces the timing signal, while the transistor stage allows that control signal to interact with the higher-power switching section.

The different components therefore have different responsibilities:

- NE555 → timing and control

- TIP32C → transistor power/control stage

- Inductor → magnetic energy storage

- Diode → controlled energy transfer

- Capacitor → output filtering and voltage stabilization

The TIP32C by itself does not convert 12 V into 25 V.

The voltage conversion is performed by the complete BOOST converter.

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7. Why Raise the Voltage to Approximately 25 V?

The purpose of increasing the voltage is to create an appropriate energy rail for the desulfation stage.

The boost converter provides the desulfator with a higher-voltage DC source.

The desulfator then uses this energy in a controlled pulsed manner.

The important concept is that increasing voltage does not create additional power.

For example, if the solar panel provides approximately 12 W, the output power of the boost converter cannot exceed that input power, and in reality it will be lower because of conversion losses.

Therefore, at 25 V, the available current must be significantly lower than 1 A.

This distinction between voltage, current, and power is essential when designing the system.

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8. Desulfation Stage

The desulfation stage is the central functional part of the system.

The boost converter produces the approximately 25 V DC energy source.

The desulfator converts this relatively continuous DC energy into short electrical pulses.

The main elements of this stage are:

- second NE555;

- timing components;

- PWM control section;

- 10N60 MOSFET;

- pulse coil;

- Schottky diode;

- filtering and protection components.

The purpose is not simply to apply a continuous 25 V voltage to the output.

Instead, the energy is transferred in controlled pulses.

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9. Second NE555 — PWM Pulse Generator

The second NE555 is used as the PWM generator for the desulfation stage.

The target switching frequency is approximately:

10 kHz

At 10 kHz, one complete switching period is approximately:

100 microseconds

If the duty cycle is approximately 2%, the theoretical ON time is approximately:

2 microseconds per cycle

This means that the MOSFET is commanded ON only for a very short portion of every switching cycle.

The NE555 therefore determines:

- the pulse frequency;

- the pulse duration;

- the switching timing;

- the duty cycle.

In simple terms:

The NE555 decides when the pulse should occur.

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10. Why Use a Very Low Duty Cycle?

The purpose of the low duty cycle is to operate the power stage in a strongly pulsed mode rather than continuously.

When the PWM signal commands the MOSFET ON:

- the MOSFET conducts;

- current flows through the coil;

- magnetic energy is stored in the coil.

When the MOSFET switches OFF:

- the current path through the MOSFET is interrupted;

- the coil releases the energy stored in its magnetic field;

- the circuit redirects that energy through the appropriate path;

- an electrical pulse is produced at the output.

The low duty cycle therefore produces short energy-transfer events.

A value around 2% can be used as a design target, but it should not automatically be considered the final value.

The actual duty cycle must be determined according to:

- coil inductance;

- peak current;

- MOSFET characteristics;

- switching losses;

- pulse energy;

- thermal limitations;

- output behavior.

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11. 10N60 MOSFET

The 10N60 is the main power-switching device of the desulfation stage.

The NE555 generates a relatively low-power control signal.

That signal controls the gate of the MOSFET.

The MOSFET then performs the actual high-current switching.

The functional relationship can therefore be summarized as:

NE555 = controller

10N60 = power switch

The MOSFET is responsible for rapidly connecting and disconnecting the power path associated with the coil.

This is what allows the relatively high-voltage DC source from the boost converter to be converted into a pulsed energy transfer.

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12. Pulse Coil

The coil is a fundamental part of the desulfation mechanism.

An inductor has the characteristic of resisting rapid changes in current.

When the 10N60 is switched ON, current begins to flow through the coil.

The coil stores energy in its magnetic field.

When the MOSFET is suddenly switched OFF, the current cannot instantly become zero.

The magnetic field therefore collapses and releases its stored energy.

This causes the electrical conditions around the coil to change rapidly, allowing the stored energy to be transferred through the output path.

The coil is therefore the component that enables the system to transform controlled switching into an energy pulse.

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13. Relationship Between the 25 V Rail and the Pulse Stage

The approximately 25 V generated by the BOOST converter provides the energy source for the desulfator.

The second NE555 does not provide this energy.

It only controls the timing of the energy transfer.

The complete relationship is:

BOOST → provides approximately 25 V

NE555 → generates the PWM control signal

10N60 → switches the power

Coil → stores and releases magnetic energy

This separation of functions is fundamental to understanding the architecture.

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14. Schottky Output Diode

The Schottky diode at the output provides controlled current direction.

Its main functions include:

- directing energy toward the output;

- preventing unwanted reverse current;

- assisting the rapid transfer of energy;

- protecting other parts of the circuit from inappropriate current paths.

Because the desulfator operates with relatively fast switching, the diode must be suitable for high-frequency operation.

It must also be selected according to:

- maximum reverse voltage;

- average current;

- peak current;

- switching behavior;

- thermal dissipation.

The diode should therefore be selected according to the real electrical conditions of the pulse stage.

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15. Purpose of the Desulfator

The purpose of the desulfator is to generate controlled electrical pulses intended for the treatment of lead-acid batteries affected by sulfation.

Sulfation is associated with the formation of lead sulfate crystals on the plates of lead-acid batteries, particularly under certain charging and discharge conditions.

The pulse system is intended to provide a controlled electrical excitation rather than a simple continuous DC charging voltage.

It is important to clearly distinguish the two batteries involved in the overall system:

Internal battery

The three 3.7 V Li-ion cells provide energy for the electronic system.

External battery

The battery connected to the output terminal is the battery intended to receive the desulfation pulses, typically a lead-acid battery.

The internal Li-ion battery is therefore not the battery being desulfated.

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16. XL4005E1 Module

The XL4005E1 is generally used as a buck converter, meaning that it is a step-down converter.

It should therefore not be confused with the BOOST converter.

The boost stage increases voltage.

The XL4005E1 can reduce voltage to a suitable regulated level for the electronics associated with the display.

Its purpose is therefore to provide an appropriate supply voltage for the monitoring electronics.

This is useful because the voltage present in the power stage may not be directly suitable for a low-voltage display module.

The XL4005E1 provides an intermediate voltage-adaptation stage.

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17. Display Module

The display module provides the user interface for monitoring the system.

Depending on the exact display module selected, it may monitor:

- output voltage;

- current;

- power;

- other electrical parameters.

The display is particularly useful during testing and experimentation.

It allows the operator to observe the electrical behavior of the system in real time.

For example, it can help monitor:

- voltage changes;

- current variations;

- output behavior when a battery is connected;

- changes caused by the solar panel;

- system response under different operating conditions.

The display therefore acts as the real-time monitoring interface.

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18. Output Terminal

The output terminal is the physical connection point between the desulfator and the external battery.

It should provide clearly identified:

- positive terminal;

- negative terminal.

The terminal should be mechanically robust and electrically rated for the expected operating conditions.

Since the desulfator operates with pulses, the connection should have low contact resistance and good mechanical reliability.

Poor connections can produce:

- voltage drops;

- unwanted heating;

- electrical noise;

- unstable operation.

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19. Three 3.7 V Li-ion Cells

The internal energy-storage system consists of three Li-ion cells rated at approximately 3.7 V nominal each.

If the three cells are connected in series, they form a:

3S battery pack

The nominal pack voltage is:

3 × 3.7 V = 11.1 V

For conventional Li-ion cells with a maximum charging voltage of approximately 4.2 V per cell, the fully charged pack reaches approximately:

12.6 V

Therefore:

11.1 V = nominal voltage

12.6 V = fully charged voltage

The battery voltage is not constant during operation.

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20. 3S BMS

The BMS, or Battery Management System, protects the three-cell Li-ion pack.

Depending on the exact BMS, its functions may include:

- overcharge protection;

- over-discharge protection;

- over-current protection;

- short-circuit protection;

- cell balancing.

Cell balancing is especially important because three cells connected in series do not necessarily remain at exactly the same state of charge.

One cell may reach its maximum voltage before another cell.

The BMS helps manage these differences.

However, an important point must be emphasized:

The BMS is not the battery charger.

A dedicated charging circuit suitable for a 3S Li-ion battery is required.

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21. USB Type-C Charging Port

The USB Type-C port provides an alternative way of charging the internal Li-ion battery.

The Type-C connector should not simply be connected directly to the three cells.

The charging path should include an appropriate 3S Li-ion charging stage.

A conventional 3S Li-ion battery requires a final charging voltage of approximately:

12.6 V

Therefore, a simple 5 V USB supply cannot directly and correctly charge a 3S Li-ion pack.

An appropriate charging circuit is required between the USB Type-C input and the battery pack.

If USB-C Power Delivery is used, the system can negotiate a higher input voltage when supported by the charger and the appropriate USB-C PD circuitry.

The dedicated 3S charging stage then manages the correct charging profile.

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22. Why Add USB Type-C?

Adding USB Type-C makes the system more versatile.

The internal battery can have an alternative charging source:

Solar energy → system/battery

or:

USB Type-C → charging circuit → 3S battery

This provides several advantages.

The system can be charged:

- when sunlight is unavailable;

- during laboratory testing;

- before field operation;

- during maintenance;

- independently of the solar panel.

The USB Type-C interface therefore makes the system more practical and flexible.

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23. Red LED — Solar Panel Indicator

The red LED is used to indicate the presence of solar input.

Its intended function is:

Red LED ON → solar input is present.

For a more meaningful indication, the LED can be controlled using a suitable voltage threshold.

This avoids interpreting a very low or unusable panel voltage as a valid power source.

The red LED therefore represents the status of the solar input.

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24. Green LED — System Operating Indicator

The green LED has a different purpose.

It indicates:

System ON / System operating

When the POWER switch is turned ON, the green LED can illuminate to indicate that the main electronic system has been activated.

In a more advanced design, the green LED could be controlled by the electronic control section so that it indicates that the system is actually operating rather than merely receiving power.

The two LEDs therefore have distinct meanings:

Red LED → solar input available

Green LED → system activated/operating

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25. POWER Switch

The POWER switch is the main user control.

When the switch is OFF:

- the main control electronics are disabled;

- the boost converter control is stopped;

- the desulfation PWM is stopped;

- the 10N60 no longer performs the intended switching;

- the green LED is OFF.

When the switch is ON:

- the control electronics receive power;

- the first NE555 can control the boost stage;

- the second NE555 can generate the desulfation PWM;

- the 10N60 can perform power switching;

- the desulfation system becomes operational.

Ideally, the charging system should be arranged so that the internal battery can be charged independently of the main POWER switch if this functionality is desired.

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26. Complete System Operating Logic

The complete operating sequence can now be summarized.

The solar panel provides the initial electrical energy.

The protection diode prevents unwanted reverse current.

The first NE555 generates the control signal for the boost converter.

The TIP32C and associated power stage participate in the switching/control process.

The BOOST converter raises the available voltage to approximately 25 V DC.

This 25 V DC energy source feeds the desulfation stage.

The second NE555 generates approximately 10 kHz PWM.

The PWM has a deliberately low duty cycle, with approximately 2% used as the initial design target.

The PWM signal controls the 10N60 MOSFET.

The 10N60 rapidly switches current through the pulse coil.

The coil stores energy in its magnetic field while the MOSFET is conducting.

When the MOSFET turns OFF, the coil releases its stored magnetic energy.

The energy is transferred through the designed output path.

The Schottky diode directs the energy toward the output and limits unwanted reverse current.

The XL4005E1 provides a suitable regulated voltage for the monitoring/display electronics.

The display module allows the user to observe the electrical behavior of the system in real time.

The output terminal provides the connection to the external battery being treated.

Meanwhile, the three 3.7 V Li-ion cells form the internal 3S energy-storage pack.

The 3S BMS protects and manages the cells according to its capabilities.

The USB Type-C port, together with a suitable 3S charging circuit, provides an alternative charging method for the internal battery.

The red LED indicates the presence of solar input.

The green LED indicates that the main system is operating.

The POWER switch provides the main user control.

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27. Overall Functional Philosophy

The important design philosophy behind the system is the separation of functions.

The solar panel does not directly generate the desulfation pulses.

Instead, it supplies energy to the boost converter.

The boost converter does not directly control the pulse timing.

Instead, it creates the approximately 25 V energy rail.

The second NE555 does not provide the pulse power.

Instead, it generates the timing signal.

The 10N60 does not determine the pulse frequency.

Instead, it acts as the high-power electronic switch.

The coil does not determine the PWM frequency.

Instead, it stores and releases energy according to the switching conditions.

The Schottky diode controls the direction of energy transfer.

The XL4005E1 adapts the voltage for the monitoring electronics.

The display provides