Lithium battery charger is a charging equipment specially designed for lithium batteries, the core of its working principle is to control the current and voltage, according to the chemical characteristics of lithium batteries for safe and efficient charging. The following is a detailed description of the basic principle, charging stages, key components and safety protection:
1.Basic working principle
The charging process of lithium batteries (such as common lithium-ion batteries) is the process of converting electrical energy into chemical energy: the charger converts the utility power (220V AC, 12V DC in some scenarios) into the low-voltage DC power required by lithium batteries, and by precisely controlling the voltage and current, lithium ions migrate from the positive electrode (usually lithium cobalt, ternary material, etc.) to the negative electrode (graphite) and are embedded to complete the charging.
Core requirements: Li-ion batteries are sensitive to voltage and current. Excessive voltage will lead to electrolyte decomposition, battery bulging and even explosion; excessive current will trigger overheating and affect life. Therefore, the charger must strictly follow the charging logic of “constant current first, constant voltage later”.
2.Charging stage (using the common 18650 lithium battery as an example)
Li-ion battery charging is usually divided into 3 stages, and the charger will automatically switch stages to match the battery status:
1.Pre-charge phase (low voltage trickle charge)
Scenario: Start when the battery voltage is too low (e.g. below 3.0V, threshold value varies slightly for different batteries).
Principle: The charger outputs a small current (about 10% of the standard charging current, e.g. 0.1C, C is the capacity of the battery, unit Ah), and slowly raises the battery voltage to 3.0~3.3V, avoiding damage to the battery by high current shock (especially for over-discharged batteries).
2.Constant current charging phase (main charging phase)
Principle: When the battery voltage reaches the pre-charging threshold, the charger enters the constant current mode, charging the battery with a set constant current (usually 0.5C~1C, fast charging can reach more than 2C), at this time, the battery voltage will gradually rise with the increase of power.
Characteristics: The charging speed in this stage is fast and can charge 70%~80% of the total capacity of the battery. For example, a 1000mAh battery charged with 1C (1000mA) current can be charged to 80% in about 1 hour.
3. Constant voltage charging phase (top-up + protection)
Principle: When the battery voltage rises to the full charge voltage of the lithium battery (the nominal voltage of a single lithium battery is 3.7V, and the full charge voltage is usually 4.2V; the part of Li-ion ternary battery is 4.35V, and Li-Fe-Phosphate battery is 3.65V, etc.), the charger switches to the constant voltage mode to keep the voltage unchanged, and then the charging current will be gradually reduced.
Characteristics: As the battery power is close to full charge, the current continues to drop until it falls to the set cut-off current (usually 0.05C~0.1C), and the charger stops charging to avoid overcharging. This stage can charge the battery to 100%, but it takes longer (about 30% of the total time).
3.Core components of a charger and what they do
01.Rectifier Filter Circuit
Convert the alternating current (AC 220V) of the utility power to direct current (DC) and remove the stray waves through capacitor filtering to output a stable DC high voltage (e.g. about 310V).
02.Switching Power Supply Circuit
Through the high-frequency switching tube (such as MOS tube) will be converted to high-frequency alternating current (AC), and then through the transformer step-down, the output of low-voltage AC suitable for lithium batteries, and finally through the secondary rectifier filtering to get low-voltage DC (such as 5V, 9V, 12V, etc.).
03. Control chip (MCU/PMIC)
The “brain” of the charger is responsible for detecting the battery voltage and current, controlling the switching of charging stages (pre-charging→constant current→constant voltage), and realizing the protection functions of over-charging, over-current and overheating. Common chips such as TI's BQ series, Huawei's self-developed charging chip.
04.Feedback circuit
Real-time monitoring of battery status through voltage sampling (e.g. resistor voltage divider) and current sampling (e.g. shunt resistor), and feedback data to the control chip to ensure accurate charging parameters.
05.Protection Circuit
Over-voltage protection (OVP): When the output voltage exceeds the set value (e.g. 4.3V or more), cut off the output to prevent the battery from overcharging.
Over-current protection (OCP): When the charging current is too large (e.g. short-circuit, battery abnormality), stop the power supply.
Overheat protection (OTP): When the charger or battery temperature is too high (e.g. over 60℃), charging will be suspended and resumed after cooling down.
Reverse connection protection: cut off the circuit when the positive and negative terminals of the battery are reversed to avoid short circuit damage.
4.Differences between different types of lithium battery chargers
01.Single section charger:
For 3.7V single-cell Li-ion battery (e.g. 18650), the output voltage is 4.2V, and the current depends on the design (e.g. 1A, 2A).
02.Multi-section series charger:
Such as lithium batteries for electric vehicles (multiple cells in series, voltage may be 12V, 24V, 48V, etc.), need to match the total voltage of the battery pack (e.g., 48V battery pack full voltage is about 54.6V), the charging logic is the same, but the voltage and current parameters are higher.
03.Fast charger:
Shorten the charging time by increasing the current in the constant-current phase (e.g. 5A, 10A) or by using high-voltage fast-charging technology (e.g. 20V for PD protocol), which requires both the battery and the charger to support fast-charging protocols (e.g. QC, PD, SCP, etc.).
Post time: Aug-06-2025