Lithium-ion battery characteristics revealed
Lithium, the third element in the periodic table, possesses unique properties determined by the number of electrons in its outermost shell. Although a lithium atom has only one electron in its outermost shell, its relative atomic mass is only 7, meaning that for the same mass density, a lithium atom carries the most electrical energy. Therefore, lithium batteries are the preferred choice for many electronic products. However, lithium batteries operate on a voltage range of 3.0V to 4.2V, and overcharging or over-discharging can damage them, necessitating careful design of charging and discharging circuits.
Analysis of Lithium Battery Charging Circuit
To address the charging challenges of lithium batteries, engineers designed the TP4054 charging management chip. This chip is a constant voltage, constant current linear charging type, ensuring a stable charging voltage of 4.2V and a charging current of up to 800mA. Its application circuit is simple, requiring only a few resistors, capacitors, and LEDs to indicate the charging status. However, the TP4054 chip does not have an automatic power-off function; when the lithium battery is fully charged, the user must manually disconnect the power supply; otherwise, the battery will remain in a charging state.
Lithium battery protection circuit design
In addition to the charging circuit, the discharge process of lithium batteries also requires protection. The discharge voltage of a lithium battery cannot fall below 3.0V, otherwise the battery life will be significantly shortened. To achieve this protection, engineers designed a circuit combination of a DW01 chip and an 8205 MOSFET. The DW01 chip can monitor the discharge voltage and current of the lithium battery. When the voltage falls below 3.0V or the current is too high, it will control the MOSFET to cut off the discharge circuit, thereby protecting the battery. At the same time, the 8205 MOSFET, as a switching element, has the characteristics of low internal resistance and high efficiency, which can ensure the stability and safety of the discharge process.
Integration of charge and discharge management solutions
To achieve effective charge and discharge management of lithium batteries, engineers cleverly combined the TP4054 charging circuit and the DW01 protection circuit. By connecting the TP4054 chip's Pin 3 (BAT) and the DW01 chip's BATT+ and BATT- pins, a complete lithium battery charge and discharge management circuit was formed. This solution not only achieves constant voltage and constant current control during charging but also provides multiple protections against overcharge, over-discharge, and overcurrent, ensuring the safe use of lithium batteries.
In electronic product development, the design of lithium battery charging and protection circuits is crucial. By gaining a deep understanding of the characteristics and application requirements of lithium batteries, engineers can design efficient and safe charging and protection circuits, providing strong support for the stable operation of electronic products. Furthermore, with continuous technological advancements and innovation, we anticipate the emergence of more and better battery management solutions to meet the application needs of various fields.
01 Analysis of Lithium-ion Batteries and Other Battery Charging Methods
▲ Analysis of Lithium-ion Battery Charging Circuit
The charging circuit for a lithium-ion battery is shown in the figure. In the initial stage, when the battery voltage has not yet reached 8.4V, the output of IC1 remains silent, and Q2 is off. At this time, LM317 operates in constant current output mode. Once the battery voltage climbs to 8.4V, Q2 turns on, and its control signal acts on the ADJ terminal, causing the output voltage of IC2 to drop significantly, thereby maintaining the charging current at a very low level and allowing the battery to enter float charging mode. This process ensures that the lithium-ion battery has a constant current supply in the early stages of charging, and switches to float charging to stabilize the voltage when it is close to being fully charged.
In-depth analysis of lithium battery charging circuits
This charger features a streamlined circuit design, stable performance, and a wide adjustment range. Through its simple design, it avoids the use of high-power, high-current SCRs, enabling charging of batteries ranging from 6V to 24V. Furthermore, the clever configuration of R6 and C4 allows the load to exhibit near-resistive characteristics. This not only allows the charger to operate at various voltages but also ensures consistent load characteristics.
In-depth analysis of lithium battery charging circuits
▲ Lead-acid battery charging circuit
The charging circuit for a 2V lead-acid battery, as shown in the diagram above, includes key components such as resistors, capacitors, a trigger diode, and a triac. This circuit enables efficient charging of the lead-acid battery, and the combination of resistors and triacs ensures the stability and safety of the charging process.
▲ Simple solar cell charge/discharge controller
Lead-acid batteries play a crucial role in solar photovoltaic power systems. However, their lifespan is susceptible to overcharging or over-discharging. This article introduces a simple solar cell charge/discharge controller that effectively prevents overcharging or over-discharging of batteries. The controller uses an LM393 dual voltage comparator to prevent overcharging and over-discharging, features adjustable settings, and is suitable for 12V 65Ah batteries, ensuring the normal operation of the solar system.
In-depth analysis of lithium battery charging circuits
02 Diverse Charging Devices ▲ Mobile Phone Charger Circuit Analysis
This mobile phone charger circuit diagram clearly shows the connection methods and working principles of each component. Under sunlight, the silicon solar cell module generates direct current, which flows through the normally closed contact J1-1 and R1, causing LED1 to light up, indicating that it is waiting to charge the battery. When K closes, the three-terminal regulator outputs 8V, starting the circuit. At this time, the overcharge voltage detection and comparison control circuit and the over-discharge voltage detection and comparison control circuit simultaneously detect and compare the battery's terminal voltage. Based on the battery's terminal voltage, the circuit will switch states accordingly, indicating the charging progress and load operating status.
▲ MP3 Charger Circuit Analysis
The circuit diagram of an MP3 charger reveals the secrets of its charging process. When charging begins, the AC power supply is stepped down by transformer T1 to produce a 12V AC voltage. This AC voltage is then converted to DC voltage by rectifier diodes D1 and D2. Next, the DC voltage is further stabilized by the filtering effect of capacitors C1 and C2. Finally, this DC voltage is intelligently controlled by the charging management chip to achieve stable charging of the MP3 battery.
▲ Analysis of Motorcycle Charging Circuit
This motorcycle charging circuit fully utilizes the positive half-cycle of AC power for charging, significantly improving charging speed and extending battery life. Applying this charger to ordinary motorcycles not only offers superior performance but also saves approximately 5% on fuel, making it highly practical. Its working principle is as follows: AC voltage simultaneously applies to D1 and the SCR. After half-wave rectification by D1, the voltage is used to provide a trigger voltage to the SCR through R1, R2, Q1, and R3, thus initiating battery charging. When the battery voltage rises to 13.5V, D1 conducts, and the voltage, through R5 and D2, provides bias voltage to Q2, causing Q2 to conduct and Q1 to turn off, thereby stopping the SCR output. If the battery voltage drops below 13-13.5V, the charging process restarts.
Can lithium batteries be charged directly with a power source?
Observing the charging curve of lithium batteries reveals that it is impractical to charge lithium batteries directly with a power source without using a dedicated charging chip.
1. Unable to achieve constant current charging;
2. Pre-charging is not possible; the current is at its maximum when the battery voltage is less than 3V.
3. Unable to automatically stop charging;
4. It cannot provide temperature protection when the battery charging temperature is abnormal (some batteries may not have a built-in battery protection board).
Lithium-ion batteries, with their significant advantages such as high single-cell terminal voltage and large specific capacity, have become an essential energy source for many electronic products such as mobile phones and electric vehicles. However, due to their characteristics, lithium-ion batteries must be charged with a dedicated charger to avoid damage from overcharging. This lithium-ion battery charger, hailed as a "new innovation," not only monitors the battery's charging status in real time but also intelligently controls the maximum charging current in stages. During charging, the charging current gradually increases from 10mA to 270mA. When the battery level reaches approximately 70%, the charger automatically adjusts to a maximum charging current of 220mA, then gradually decreases to 170mA, 120mA, and 70mA, finally ending the process with a trickle charge of approximately 10mA. This carefully designed charging method maximizes the amount of energy that the lithium-ion battery can receive.
Detailed steps of the charging process △ Current step and voltage detection
As the battery gradually charges, its terminal voltage gradually increases, and the voltage is higher during high-current charging than during low-current charging. Therefore, during prolonged charging, the following situation may occur: when Q5=1 and the charging current is approximately 270mA, the battery terminal voltage momentarily exceeds the set value of the charging termination voltage, causing IC4 to output a high level and forcibly reset IC2, thereby automatically adjusting the maximum charging current to 220mA (corresponding to Q4=1). Similar situations will occur repeatedly, and as charging progresses, the maximum charging current will automatically adjust between 220mA, 170mA, and 70mA.
△ Maximum current and regulation
When the battery is almost fully charged, the 70mA charging current (corresponding to Q1=1) will cause the battery terminal voltage to exceed the set value. At this time, IC2 will remain in a high state with Q0, thus continuing to charge the battery in a trickle charging mode of 10mA.
△ Trickle charging and end indicator
At this point, when the battery is nearly fully charged, it will stop trickling charging at 10mA, and the LED will indicate the charging status.
The structural secrets of lithium-ion power batteries
A standard lithium-ion battery consists of five core components: a positive electrode, a negative electrode, an electrolyte, a separator, and a casing. This layered structure resembles a sophisticated "electrochemical sandwich."
Cathode materials: Lithium metal oxides with layered structures, such as lithium cobalt oxide (LiCoO₂) and lithium iron phosphate (LiFePO₄). These materials have a stable crystal framework, allowing lithium ions to be reversibly inserted and extracted.
Anode materials: typically composed of graphite, whose layered structure provides space for lithium ions to intercalate. Novel anode materials, such as silicon-based composites, are breaking through the theoretical capacity limits of traditional graphite.
Electrolytes: As a medium for lithium-ion transport, liquid electrolytes (such as LiPF₆ soluble in carbonate solvents) have excellent ionic conductivity, while solid electrolytes (such as sulfide glass ceramics) offer higher safety.
The diaphragm, made of microporous polypropylene or ceramic coating material, not only blocks direct contact between electrons but also allows lithium ions to pass through, playing a key role in electronic insulation and ion conduction.
This multi-layered structure design creates a "highway network" for the three-dimensional migration of lithium ions.
Charging process: The "forward migration" of lithium ions
When the charger is connected to the lithium-ion battery, a precise dance of lithium-ion migration begins:
Electric field driven stage:
An external power source establishes an electric field between the electrodes, and lithium ions in the positive electrode material (such as LiCoO₂) break free from the crystal binding under the action of the electric field force.
At the same time, electrons flow from the positive electrode to the negative electrode through the external circuit to maintain charge balance.
Lithium-ion transmembrane transport:
The released lithium ions are solvated in the electrolyte, forming a lithium ion-solvent complex.
Driven by both concentration gradient and electric field, lithium ions move towards the negative electrode through the micropores of the membrane.
Negative electrode insertion process:
Lithium ions that reach the negative electrode are embedded between the graphite layers to form LiC₆ compounds.
Electrons accumulate on the surface of the negative electrode and neutralize with the embedded lithium ions, thus completing charge compensation.
Intelligent charging control:
Modern chargers employ a four-stage charging algorithm: trickle pre-charge (to restore over-discharged batteries), constant current fast charging (0.2C-1.0C), constant voltage fine charging (4.2V regulated), and automatic termination (current threshold or timer).
During charging, the battery management system (BMS) monitors voltage, current, and temperature parameters in real time, acting like an "electrochemical steward" to ensure charging safety.
