Pulse chargers for car batteries: circuits, operating principle and assembly

A car battery is not just a power source, but a critical element on which the engine starts in any season. Traditional transformer charging is gradually giving way to more efficient and compact ones. pulse devices, capable of restoring battery capacity 2–3 times faster. However, not all circuits are equally useful: some are suitable for deeply discharged batteries, others are optimized for AGM batteries, and others can simply burn the plates if configured incorrectly.

In this article we will look at 5 proven pulse charger circuits - from protozoa to IR2153 to advanced with microcontroller control - and also we will open the key mistake of 90% of DIYers, due to which even a properly soldered circuit fails in the first hours of operation. You will learn how to calculate the inductor parameters, why a Schottky diode cannot be replaced with a conventional rectifier, and what “chips” manufacturers of factory chargers use (for example, CTEK MXS 5.0 or Optimate TM221).

What is a pulse charger and why is it better than a transformer charger?

The pulse charger (IZU) converts the mains voltage 220V/50Hz into high frequency pulses (usually 20–100 kHz), which are then straightened and fed to the battery. The main difference from transformer analogues is lack of massive iron core, which reduces the weight and dimensions of the device by 5–10 times.

Advantages of pulse circuits:

• 🔋 Efficiency up to 95% (versus 50–70% for transformer ones) - less heating, higher charging speed.
• 📏 Compactness: A device the size of a cigarette pack can deliver up to 10A.
• 🔄 Flexibility of customization: It is easy to implement multi-stage algorithms (for example, IUoU for desulfation).
• 💰 Cheap components: modern MOSFET transistors and PWM controllers cost pennies.

But there are also pitfalls. Pulse circuits are sensitive to soldering quality (especially in the high-voltage part), require precise selection of the inductor and can cause interference in the network. For example, cheap Chinese chargers often make noise in a car radio - this is a consequence of the lack of EMC filters.

⚠️ Attention: If you have never soldered SMD components, start with the transformer circuit. In pulse chargers, an error in the installation of the high-voltage part (for example, an incorrect gap in the transformer) can lead to breakdown and electric shock 300V+.

Top 5 pulse charger circuits: from simple to complex

We selected the schemes according to the following criteria: reliability, repeatability, component availability and the possibility of modification for different types of batteries (WET, AGM, GEL). All options are tested in practice and have feedback from the amateur radio community.

1. The simplest circuit on IR2153 (for beginners)

Ideal for first experience. Controller IR2153 contains a half-bridge driver and generator in one housing, which simplifies assembly. Power: up to 150W (charge current 5–7A).

Features:

• ⚡ Suitable for lead-acid batteries 12V (not AGM!).
• 🔧 Minimum number of parts: transformer, 2 MOSFETs, Schottky diode, several resistors.
• ⚠️ There is no protection against short circuit and polarity reversal - requires improvement.

Typical mistake: using conventional diodes instead Schottky (for example, 1N5822). This leads to overheating and a drop in efficiency by 15–20%.

2. Feedback circuit based on TL494 (universal)

A more advanced option with current and voltage stabilization. Microcircuit TL494 allows you to implement 3-stage charging algorithm (boost → absorption → float). Power: up to 300W.

Benefits:

• 📊 Precise voltage regulation (e.g. 14.4V for absorption and 13.6V for support).
• 🛡️ Built-in overload protection.
• 🔄 Possibility of modification for lithium batteries (for example, LiFePO4).

3. Circuit with microcontroller (STM32/Arduino)

For those who want full control over the charging process. Microcontroller (eg STM32F103) controls PWM, monitors battery temperature, and keeps a charging log. Power is limited only by the power section.

Example implementation:

• 📱 Bluetooth control (app on smartphone).
• 🌡️ Temperature sensor DS18B20 for voltage correction.
• 🔋Support desulfation pulse currents.

⚠️ Attention: When using Arduino Uno in the power section, be sure to isolate the control circuits from high voltage using optocouplers (for example, PC817). Otherwise, you risk burning the computer’s USB port when flashing the firmware.

4. Circuit with synchronous rectification (for experienced ones)

Increases efficiency up to 97% by replacing Schottky diodes with MOSFET transistors. Difficult to set up, but worth it for powerful memory devices (500W+).

Key points:

• 🔄 Requires precise selection dead zone time (dead time).
• 💎 Drivers with galvanic isolation are used (for example, IR2110).
• ⚡ Suitable for charging AGM and GEL batteries.

5. Ready-made modules from China (for the lazy)

If you don’t want to solder, you can buy a ready-made module on AliExpress (for example, DPS5005 or Riden RD6018) and modify it for the battery. Cost: 1500–3000₽.

Pros:

• 🔌 Ready-made solution with protection and display.
• 📦 Compact (palm size).

Cons:

• ⚠️ Often absent temperature compensation.
• 🔧 Requires modification to work with car batteries (for example, adding a relay to turn off when reaching 14.7V).

Scheme	Difficulty	Max. power	Suitable for battery	Component cost
IR2153	⭐⭐	150W	WET, Ca/Ca	500–800₽
TL494	⭐⭐⭐	300W	WET, AGM	1000–1500₽
STM32 + MOSFET	⭐⭐⭐⭐	500W+	All types	2000–4000₽
Synchronous rectification	⭐⭐⭐⭐⭐	1000W+	AGM, GEL	3000–6000₽
Ready module (DPS5005)	⭐	300W	WET, AGM	1500–3000₽

How to calculate a transformer and inductor: step-by-step instructions

Errors in the calculation of magnetic elements are the main reason for failure of the IZU. Here you cannot “estimate by eye”: even a slight deviation in the number of turns or the core gap will lead to overheating or breakdown.

Step 1. Determining the power of the transformer

Formula:

P_tr = P_out / (η * cosφ)
where:
P_out — load power (W),
η - efficiency (0.85–0.95 for pulse circuits),
cosφ - power factor (0.9–0.95).

Example: for a memory for 10A/14.4V (P_out = 144W) at η = 0.9:

P_tr = 144 / (0.9 * 0.95) ≈ 168W.

Step 2. Core selection

For frequencies 50–100 kHz suitable ferrite brands N87, N97 (EPCOS) or 3C90 (Ferroxcube). The core size is determined by the formula:

Ae  Aw ≥ (P_tr  10^4) / (2  f  B_max  j  k)
where:
f—frequency (kHz),
B_max — max induction (T, for N87 ≈ 0.35 T),
j - current density (3–5 A/mm²),
k is the fill factor (0.3–0.4).

I bought a core with a gap of 0.1–0.3 mm|Checked the saturation induction (for N87 ≥ 0.35T)|Calculated the number of turns of the primary winding|Used stranded wire (licendrate)|Checked the insulation between the windings (≥ 1kV)-->

Step 3. Calculation of the output filter choke

The inductor inductance (L) determines the current ripple:

L = (V_out  (1 - D)) / (2  f * ΔI)
where:
D - fill factor (0.3–0.6),
ΔI - permissible current ripple (10–20% of I_out).

Example: for V_out = 14.4V, f = 50kHz, ΔI = 1A, D = 0.5:

L = (14.4  0.5) / (2  50000 * 1) ≈ 72 µH.

⚠️ Attention: If the inductance of the inductor is less than the calculated one, the ripple current will exceed the permissible values, which will lead to battery overheating and reducing its service life. Especially critical for AGM batteries!

Typical assembly mistakes and how to avoid them

Even experienced radio amateurs make mistakes that ruin all their efforts. Here TOP-5 fatal mistakes and ways to prevent them:

1. Incorrect gap in transformer
   

   Too large a gap → low inductance → breakdown of transistors. Too small → core saturation. Solution: for ferrite EI33 at frequency 50kHz optimal clearance - 0.2–0.3 mm.

2. Ignoring snubber circuits
   

   Without RC circuits, voltage surges occur at the MOSFET drains, which kill the transistors. Solution: install resistor 10–22 Ohm and capacitor 1nF parallel to each transistor.

3. Savings on Schottky diodes
   

   Replacing with conventional rectifier diodes (for example, 1N4007) leads to a drop in efficiency by 15–20% and overheating. Solution: use SB540 or 1N5822.

4. No galvanic isolation
   

   If the control circuits are not isolated from the power part, one breakdown can kill both the MK and the power source. Solution: use optocouplers (PC817) or pulse transformers.

5. Incorrect feedback setting
   

   If the feedback loop is too “slow”, the memory will “sway” and produce pulses with an amplitude of up to 20V+, which will kill the battery. Solution: configure R_C and C_C in the compensation circuit according to the datasheet on the PWM controller.

Modifications for different types of batteries

Not all batteries are the same: WET, AGM, GEL and LiFePO4 require different charging algorithms. Let's look at how to adapt the scheme for each type.

1. Charging WET (regular lead-acid) batteries

Classic 3-step algorithm:

• 🔋 Boost: current 0.1–0.2S (for example, 5A for 60Ah) to 14.4V.
• 📉 Absorption: support 14.4V until the current drops to 0.01S.
• ⚡ Float: 13.6V to compensate for self-discharge.

2. Charging AGM and GEL batteries

These batteries are sensitive to overvoltage. Optimal parameters:

• 🔋 Boost: up to 14.1–14.4V (for AGM) or 14.0–14.2V (for GEL).
• 🌡️ Temperature compensation: -30 mV/°C from rated voltage.

3. Charging LiFePO4 batteries

Requires BMS controller and precise voltage stabilization:

• 🔋 Charge up to 3.65V per element (for example, 14.6V for 4S).
• ⚠️ Charging is prohibited at T < 0°C!

Why can't AGM be charged like WET?

AGM batteries have a compressed electrolyte in glass fiber, which reduces internal resistance, but makes them sensitive to gas formation. At voltages above 14.4V, intensive hydrogen evolution begins, which leads to irreversible damage to the plates and a reduction in service life by 30–50%.

Safety: how to avoid burning your battery and getting an electric shock

Switching chargers operate with high voltages and currents, so Safety violations can cost lives. Here are the key rules:

1. Reverse polarity protection

Connecting the battery in reverse polarity is the most common cause of charger failure. Solution:

• 🔌 Use normally open relay (for example, Omron G2R).
• 🔧 Add a powerful diode to the circuit (for example, BY229) - it will burn out if there is an error, but will save the circuit.

2. Isolation of high voltage circuits

All network related items 220V, should be:

• 🛡️ Closed with a dielectric housing (for example, ABS plastic).
• 🔌 Spaced from the low-voltage part to a distance of ≥ 5 mm.

3. Short circuit protection

Use:

• 🔋 PTC thermistor (for example, MF72) in the power circuit.
• 🔧 High speed fuse (for example, 10A at the entrance).

⚠️ Attention: Never touch circuit elements while working! Even after being disconnected from the mains, capacitors can retain their charge 300V+ within a few minutes. Discharge them with a resistor 10kOhm/2W before renovation.

FAQ: Answers to frequently asked questions

Is it possible to charge an old sulfated battery with a pulse charger?

Yes, but you need to use it asymmetrical current (e.g. 10:1 charge/discharge ratio) or specialized desulfation algorithms. In the diagram on STM32 this is implemented in software, in analog circuits - using an additional timer (for example, NE555).

Important: if the density of the electrolyte in the banks differs by more than 0.03 g/cm³, it is better to replace the battery - desulfation will not help.

Why does my charger get hot and turn off after 10 minutes?

There are several reasons:

1. Insufficient ventilation (install a cooler 12V or MOSFET heatsinks).
2. The gap in the transformer is too large (check the core for cracks).
3. Poor soldering quality (resolder the power part using solder POS-61).

First measure the current consumption without load. If he > 100mA, look for a breakdown in the transformer or diodes.

What charging current is optimal for a 60Ah battery?

For standard WET batteries:

• 🔋 Optimal current: 6A (0.1C).
• ⚡ Accelerated mode: 10–12A (0.17–0.2S), but not longer than 2 hours.
• 📉 Maintenance mode: 0.5–1A.

For AGM and GEL, the current is reduced by 20–30% to avoid overheating.

Is it possible to use a pulse charger to charge 12V lithium batteries?

Only if:

1. The circuit implements voltage stabilization with accuracy ±0.1V.
2. Connected BMS controller (for example, Daly 4S).
3. The charge is carried out at a temperature 0–45°C.

Lithium does not tolerate overvoltage: excess 3.65V on the element leads to degradation and risk of fire.

What tools are needed to assemble a pulse charger?

Minimum set:

• 🔧 Soldering iron 60–80W with a thin tip (for SMD).
• 🔍 Magnifying glass or microscope (to control soldering).
• 📊 Oscilloscope (at least Chinese DSO138).
• 🔌 LATR or laboratory power supply for testing.
• 🛡️ Multimeter with transistor testing mode.

Useful for winding a transformer machine (can be made from a drill) and a thread counter.