The core working principle of a lithium battery charger is based on the chemical properties of lithium batteries. By precisely controlling voltage, current and charging time, it converts external alternating current (or direct current) into direct current that meets the charging requirements of the battery, achieving safe and efficient energy injection. At the same time, it avoids the damage to battery life and safety caused by overcharging, overcurrent and overheating.
To understand its working principle, two prerequisites must be clarified first: The first is the charging "taboos" of lithium batteries (such as mainstream lithium-ion batteries) - they are extremely sensitive to voltage and current. Overcharging can cause the electrolyte to decompose, the battery cells to bulge, and even catch fire, while overcurrent can lead to internal heating and loss of control. The second is the core objective of the charger - to strike a balance between "fast charging" and "battery protection", and thus the "phased charging" strategy is generally adopted.
part.01 Key Process: Phased Charging (Core Principle)
The charging process of lithium batteries is not "charging at a constant high current until fully charged", but follows a three-stage (or two-stage) strategy of "constant current charging → constant voltage charging → trickle charging (or cut-off charging)", which is determined by the chemical reaction characteristics of lithium batteries:
Phase One: Constant Current Charging (CC stage, rapid energy replenishment)
Trigger conditions: When the battery voltage is relatively low (such as below 3.0V, with slight differences among different cells) or when it is just connected to the charger, the internal resistance of the battery is small and it can withstand a larger current.
Working logic: The charger outputs a fixed maximum charging current through the control module (such as 1A, 2A, 6A, commonly known as "1C, 2C, 6C charging", where C is the battery capacity, for example, 1C=2000mA for a 2000mAh battery). At this time, the battery voltage will rise rapidly with the charging time.
Core objective: To quickly replenish 70% to 80% of the battery‘s capacity within the safe range (the highest efficiency stage).
Phase Two: Constant Voltage Charging (CV stage, fine energy replenishment)
Trigger condition: When the battery voltage rises to the "cut-off voltage" (the nominal voltage for a single lithium-ion battery is usually 3.7V, and the charging cut-off voltage is 4.2V; the cut-off voltage for lithium iron phosphate batteries is 3.65V, and a dedicated charger is required), it enters the constant voltage stage.
Working logic: The charger maintains the output voltage stable at the "cut-off voltage". At this point, the charging current gradually decreases - because as the battery power increases, the electromotive force inside the cell rises, hindering the injection of external current, and the current slowly drops from the maximum value to the set threshold (such as 0.1C).
Core objective: While avoiding overcharging, charge the battery to over 95% (at this stage, the charging speed slows down, which is the key to protecting the battery).
Phase 3: Trickle Charging/Cut-off Charging (Trickle stage, safe closure)
Trigger condition: This stage is entered when the charging current drops to an extremely low value (such as 0.05C-0.1C).
Work logic
Some chargers will output a tiny "trickle current" to replenish the battery‘s self-discharge power and maintain a fully charged state (but long-term trickle may slightly affect the lifespan, so most smart chargers will skip this step).
Mainstream chargers will directly cut off the charging circuit (or enter a "float charging" state with a voltage slightly lower than the cut-off voltage), preventing the battery from remaining in a fully charged high-voltage state for a long time and extending the cycle life.
Core objective: Ensure safety, prevent overcharging, and balance "full charge" and "battery life".
part.02 Security Protection: The "Critical Defense Line" to Avoid Risks
The safety design of lithium battery chargers is an important part of their working principle. The core protection functions include:
Overcharge Protection (OCP
If the voltage during the constant voltage stage is out of control and exceeds the cut-off voltage (such as above 4.3V), the control chip will immediately cut off the charging current to prevent the electrolyte from decomposing due to excessive voltage in the battery cell.
2. Overcurrent Protection (OCP
If the charging current exceeds the set maximum value (such as a sudden increase in current due to battery short circuit or poor interface contact), the protection module will trigger current limiting or power off to prevent the battery and charger from burning out due to overheating.
3. Overheat Protection (OTP
When the temperature inside the charger (such as a transformer or MOSFET) or the battery exceeds the threshold (typically 60℃-85℃), the temperature sensor will send back a signal, and the control module will reduce the current or stop charging to prevent thermal runaway.
4. Short-circuit Protection (SCP
If the output terminal of the charger is short-circuited (such as when the positive and negative poles of the data cable come into contact), the short-circuit protection will be triggered instantly, cutting off the output to prevent fire or component damage.
5. Reverse connection protection
If the positive and negative terminals of the battery are reversed (commonly seen in detachable battery chargers), the protection circuit will block the current to prevent the battery from discharging in reverse, which could lead to leakage or explosion.
part.03 Differences among Different Types of Chargers
For lithium batteries in different scenarios (such as mobile phone batteries, electric vehicle batteries, and drone batteries), the charger principle is the same, but the parameters and designs are different:
Small device chargers (for mobile phones and headphones) : They are mostly fixed voltage ranges such as 5V/2A and 9V/2A, and are equipped with fast charging protocols like PD/QC (achieving higher power by adjusting the voltage range, for example, 18W=9V×2A). The charging management chip is integrated inside the device (the charger only provides adjustable voltage, and the device end controls the current).
Large equipment chargers (electric vehicles, energy storage batteries) : They are mostly "balanced chargers". As multiple batteries are connected in series (for example, 48V for an electric vehicle =13 3.7V cells), the voltage of each battery needs to be tested separately to prevent overcharging of any one of them. The principle is more complex, and the power can reach several hundred watts.
In summary, the working principle of lithium battery chargers can be summarized as a phased strategy of "constant current fast charging first, then constant voltage precise charging, and finally safe finishing", combined with real-time monitoring and protection of voltage, current and temperature, to achieve a balance between efficiency and safety. When choosing a charger, it is necessary to ensure that its output parameters (voltage, current, cut-off voltage) match those of the lithium battery to avoid battery damage or safety risks caused by mixing them.