Charger with desulfation: circuit diagram, principle of operation and DIY assembly

Plate sulfation is the main cause of premature failure of lead-acid batteries. Even with proper operation, after 3-4 years a dense layer of lead sulfate forms on the plates, which reduces the battery capacity by 30-50%. Standard chargers are not able to remove these deposits, but charger with desulfation solves the problem due to the pulse mode of operation.

In this article we will look at 3 working diagrams chargers with desulfation (from simple on a thyristor to advanced on a microcontroller), we will explain the principle of their operation and give step-by-step assembly instructions. You'll learn which components to choose, how to avoid soldering mistakes, and how to properly recondition a battery without risking damage.

If your battery discharges quickly, does not hold a charge, or requires frequent recharging, these are sure signs of sulfation. Buying a new battery will cost 5-15 thousand rubles, while a homemade charger with desulfation can be assembled for 1-3 thousand rubles. In this case, the battery recovery efficiency reaches 70-80% with the right approach.

It is important to understand: desulfation is not a magic wand. It only works when partial sulfation (up to 60% loss of capacity). If the plates are crumbled or closed, no device will help. Before starting assembly, check the battery with a multimeter and a load fork.

What is desulfation and why does a battery need it?

Sulfation is a chemical process in which lead sulfate crystals form on the battery plates (PbSO₄). Under normal conditions, these crystals dissolve during charging, but over time they become denser and stop interacting with the electrolyte. This leads to:

• 🔋 Reducing battery capacity (for example, from 60 Ah to 30 Ah).
• ⚡ An increase in internal resistance, which causes the battery to transmit current poorly.
• 🔥 Overheating when charging and risk of short circuit.
• 🚗 Problems with starting the engine, especially in the cold season.

Desulfation is a process destruction of the dense sulfate layer using pulse currents or alternating voltage. Unlike conventional charging, where current is supplied continuously, desulfating devices use:

• 🔄 Asymmetrical pulses (short discharge pulse after charging).
• 🌊 AC voltage low frequency (1-100 Hz).
• 🔌 Charge-discharge mode with pauses for the crystals to dissolve.

The effectiveness of the method depends on the degree of sulfation. If the crystals have not yet had time to turn into large insoluble formations (usually this happens after 1-2 years of improper use), the chances of recovery are high.

⚠️ Attention: Desulfation will not help if the plates are physically destroyed or shorted. Before the procedure, check the density of the electrolyte in each jar - a spread of more than 0.03 g/cm³ indicates a malfunction.

Operating principle of a desulfation charger

The main difference between such a device and a conventional memory is dynamic charging mode. Instead of direct current or voltage, it supplies the battery pulses of different polarity and duration, which “rock” sulfate deposits.

Let's see how this works using an example asymmetric pulse mode (most common method):

1. Charging pulse (0.1-1 sec): current passes through the battery, partially reducing lead sulfate.
2. Pause (0.1-0.5 sec): Allows time for the electrolyte to diffuse and dissolve the crystals.
3. Discharge pulse (short, 0.01-0.1 sec): the reverse current “knocks down” the dense layer of sulfate.

The cycle is repeated hundreds of times, gradually destroying the deposits. In advanced schemes they add:

• 📊 Voltage control (shutdown when reaching 14.4 V for a 12V battery).
• 🔄 Automatic mode change (for example, after 10 cycles of desulfation it goes into a buffer charge).
• 🌡️ Overheat protection (shutdown at temperatures above 50°C).

Operating mode	Voltage, V	Current, % of capacity	Pulse duration
Charging impulse	14,4–15,5	5–10%	0.5–1 s
Pause	0	0	0.2–0.5 s
Discharge pulse	–0,5––1,5	1–3%	0.05–0.1 s
Buffer charge	13,2–13,8	1–2%	Constantly

A critical caveat: desulfation requires precise current control. Exceeding 10% of the battery capacity (for example, 6 A for a 60 Ah battery) leads to overheating and destruction of the plates.

3 working circuits for chargers with desulfation

We have selected three schemes of varying complexity, which have been tested in practice. Choose based on your skills and available components.

1. A simple circuit based on the KU202 thyristor (for beginners)

Suitable for reconditioning batteries with a capacity of up to 100 Ah. Does not require programming, assembled using available radio components.

Benefits:

• ✅ Minimum number of components.
• ✅ Low cost (up to 500 rubles).
• ✅ Reliability (thyristor can withstand high currents).

Disadvantages:

• ❌ No automatic shutdown.
• ❌ Requires manual voltage control.

Scheme:

220V ~
|
[Fuse 5A]
|
+----|----+
|         |
[Transformer] [Diode Bridge]
| 24V 10A (eg KD213)
|         |
+----|----+
|
[Thyristor KU202]
|
+----|----+
|         |
[Ammeter] [Battery]

To desulfate, add R-C chain (100 Ohm resistor + 1 uF capacitor) in parallel with the thyristor. This will create short discharge pulses.

⚠️ Attention: Thyristor KU202 must be installed on a radiator! Without cooling, it will overheat after 5-10 minutes of operation.

2. Circuit on TL494 chip (semi-automatic)

A more advanced version with overcharge protection. Suitable for batteries with a capacity of 40-200 Ah.

Features:

• 🔧 Automatic shutdown when reaching 14.7 V.
• 📉 Adjustable charge current (from 1 to 10 A).
• ⚡ Built-in short circuit protection.

Scheme:

220V ~ → Transformer 18V 15A → Diode bridge
|
+-----------------+-------|-------+
|                 |               |
[TL494] [MOSFET IRF3205]
|                 |               |
[Resistors R1-R5] [Schottky Diode] |
|                 |               |
[Capacitors] [Choke] |
|                 |               |
+-----------------+               |
[battery]

To desulfate, add timer on NE555, which will periodically turn off the charge and turn on the discharge through a resistance of 1-2 Ohms.

3. Arduino circuit (full control)

The most flexible option for those who know how to program. Allows you to configure desulfation parameters via a computer.

What you will need:

• 📌 Arduino Uno/Nano.
• 📌 Module L298N (to control current).
• 📌 Transformer 12-24V 10A.
• 📌 12V relay for switching modes.

Example code (simplified):

void setup() {
pinMode(chargePin, OUTPUT);
pinMode(dischargePin, OUTPUT);
}

void loop() {
// Charging pulse
digitalWrite(chargePin, HIGH);
delay(1000);
digitalWrite(chargePin, LOW);

// Pause
delay(200);

// Discharge pulse
digitalWrite(dischargePin, HIGH);
delay(50);
digitalWrite(dischargePin, LOW);

// Voltage control
float voltage = readVoltage();
if (voltage > 14.7) {
// Shutdown when recharging
digitalWrite(chargePin, LOW);
while(1); // Stop the program
}
}

A complete diagram with a detailed description can be found in official Arduino repositories (search for projects by keywords "battery desulfator").

Step-by-step instructions for assembling the charger

Let's look at the assembly using an example circuits on the thyristor KU202 - it is simple and reliable.

Step 1: Preparing Tools and Components

You will need:

• 🔧 Soldering iron (40-60 W) and solder.
• 🔪 Wire cutters and pliers.
• 📏 Multimeter for checking connections.
• 🛠️ Heat shrink tube or electrical tape.
• 🔌 Case (you can use a plastic box from an old power supply).

Before soldering check all components:

• 🔹 Thyristor KU202 should open when voltage is applied to the control electrode.
• 🔹 The diode bridge should not pass current in the opposite direction.
• 🔹 The transformer should produce 24V without load (check with a multimeter).

Step 2: Assembling the circuit

Follow this order:

1. Solder the diode bridge to the secondary winding of the transformer. Observe polarity!
2. Connect the KU202 thyristor between the diode bridge and the positive terminal of the battery. Install it on the radiator.
3. Add an R-C network (100 ohm resistor + 1 µF capacitor) in parallel with the thyristor to create discharge pulses.
4. Connect the ammeter in series to the charging circuit (between the thyristor and the battery).
5. Install a 5A fuse at the 220V input.

Important: All connections must be firmly insulated. Use heat shrink tubing or several layers of electrical tape.

Step 3: Check and Setup

Before connecting to the battery:

1. Check the circuit with a multimeter in "continuity" mode for a short circuit.
2. Connect a 12V 21W incandescent lamp to the circuit instead of the battery and make sure that it lights up.
3. Measure the output voltage - it should be within 14-15V.

If everything works, connect the battery and observe the charging current. He should pulsate (this is noticeable by the fluctuations of the ammeter needle).

⚠️ Attention: Do not leave the device unattended during the first 30 minutes of operation! Monitor the temperature of the thyristor and battery case. The norm is up to 40°C, if higher, turn it off and check the circuit.

What to do if the circuit does not work?

1. Check the fuse - it could have burned out the first time you turned it on.

2. Make sure that the thyristor opens (apply voltage to the control electrode).

3. Ring the diode bridge - one of the diodes often fails.

4. Measure the voltage on the secondary winding of the transformer (should be 24V).

5. If the thyristor heats up without load, it is broken, replace it.

How to properly restore a battery by desulfation

Even the most advanced device will not help if the recovery technology is violated. Follow this algorithm:

Battery preparation

Before desulfation:

• 🔋 Check electrolyte level — add distilled water if the plates are bare.
• 🔧 Clean the terminals from oxides (use soda solution).
• 🌡️ Measure the density of the electrolyte hydrometer. The norm is 1.27 g/cm³ at 25°C.

If the density in the banks differs by more than 0.03 g/cm³, the battery most likely cannot be restored.

Desulfation modes

Use this cycle (for a 60 Ah battery):

1. Charging current 1-2 A up to a voltage of 14.4 V (2-3 hours).
2. Pulse desulfation (10-20 charge-discharge cycles).
3. Pause 1-2 hours to equalize the electrolyte density.
4. Check digit current 0.5 A to 10.5 V (capacity check).

Repeat the cycle 3-5 times. After each cycle, check the electrolyte density.

Process control

Keep an eye on these parameters:

Parameter	Norm	Deviation	Action
Battery voltage	12.6–14.4 V	>15 V	Disconnect charge, check circuit
Case temperature	<40°C	>50°C	Interrupt the process, cool the battery
Charge current	1–10% of capacity	>10%	Reduce the current with a resistor in the circuit
Electrolyte density	1.25–1.29 g/cm³	<1.15 g/cm³	The battery cannot be restored

Critical point: if after 3 desulfation cycles the battery capacity has not increased by at least 20%, further restoration is pointless. In this case, sulfation has entered an irreversible stage.

Common mistakes and how to avoid them

Even experienced radio amateurs make mistakes when assembling and operating desulfating chargers. Here are the most common:

• 🔌 Using a transformer that is too powerful (for example, 30A for a 60 Ah battery). This leads to overheating and destruction of the plates.

  Solution: The charging current should not exceed 10% of the battery capacity (for example, 6A for 60 Ah).

• 🌡️ Ignoring battery temperature. When heated above 50°C, the electrolyte begins to boil and the plates begin to warp.

  Solution: Use a thermocouple or infrared thermometer for monitoring. Interrupt the process every 30 minutes to cool.

• ⚡ No reverse polarity protection. If you accidentally confuse “+” and “–”, the diode bridge will fail.

  Solution: Add a diode (for example, 1N4007) to the circuit in the opposite direction in parallel with the battery.

• 🕒 Too long desulfation (more than 20 hours in a row). This leads to overcharging and loss of electrolyte.

  Solution: The optimal time for one cycle is 6-8 hours with breaks.

• 🔋 Desulfation of a completely discharged battery (voltage below 10.5 V). In this state, the plates may crumble.

  Solution: First, charge the battery with a regular charger up to 12 V, then proceed with desulfation.

⚠️ Attention: Never desulfate gel (GEL) or AGM batteries according to the schemes for lead-acid! These batteries require special algorithms with reduced voltage (maximum 14.1 V). Exceeding will result in irreversible damage.

FAQ: Answers to frequently asked questions

Is it possible to restore a battery with closed banks?

No. If one of the cans is short-circuited (voltage 0 V, boils when charging), desulfation will not help. In such cases, it is necessary to replace the battery or repair the bank (which is often impractical).

How long does complete desulfation take?

From 1 to 3 days, depending on the degree of sulfation. Recommended schedule:

• Day 1: 3 cycles of 6 hours.
• Day 2: 2 cycles of 4 hours.
• Day 3: test charge and capacity check.

If after this the capacity has not been restored by 50% or more, further attempts are useless.

What voltage should be at the output of the device?

For 12V battery:

• Maximum charge voltage: 14.4–14.7 V.
• Desulfation voltage: 15.0–15.5 V (short pulses).
• Buffer voltage: 13.2–13.8 V.

Exceeding 15.5 V leads to intense gas evolution and destruction of the plates.

Is it possible to use a charger with desulfation for Li-Ion batteries?

Absolutely not! Lithium-ion batteries require completely different charging algorithms (eg CC/CV). Pulse currents or alternating voltage will destroy them in a few minutes. Li-Ion requires specialized chargers with balancing.

How to check that desulfation was successful?

There are 3 reliable ways:

1. Capacity check: Fully charge the battery, then discharge with a current of 1-2 A to 10.5 V, noting the time. Capacitance = current × time. If it returns to 80% of the nominal value - success.
2. Density check: After desulfation, the electrolyte density should be 1.27–1.29 g/cm³ in all banks.
3. Load test: Connect the load plug. The voltage should not drop below 10 V for 10 seconds.