Design reference for lithium-ion batteries and battery chargers

Portable electronic products all use batteries as power sources. With the rapid development of portable products, the use of various batteries has increased significantly, and many new types of batteries have been developed. In addition to the more familiar alkaline batteries, rechargeable nickel-cadmium batteries, and nickel-metal hydride batteries, there are also lithium-ion batteries that have become mainstream in recent years. Here we will introduce the relevant knowledge about lithium-ion batteries, including its characteristics, main parameters, and application range, and finally provide a design reference for lithium-ion battery charging circuits.

Development and application of lithium-ion batteries

Lithium-ion batteries are the most widely used rechargeable batteries. They can be made into flat rectangular, cylindrical, rectangular and button-shaped batteries according to the requirements of different electronic products. Single cells can be used for low-power applications, or multiple cells can be combined in series and parallel to obtain higher voltage and capacity for power tools and notebook computers. The electrolyte in lithium-ion batteries can be gel, polymer (lithium-ion/lithium polymer batteries), or a mixture of gel and polymer. Because no polymer has been found that can effectively transport lithium ions at room temperature, most lithium-ion/lithium polymer batteries are actually hybrid batteries that combine gel and polymer.

Lithium-ion batteries are different from ordinary chemical batteries. Their charging and discharging process is achieved through the embedding and de-embedding of lithium ions in the positive and negative electrodes of the battery. When the battery is charged, lithium ions are generated on the positive electrode of the battery, and the generated lithium ions move to the negative electrode through the electrolyte. The carbon used as the negative electrode has a layered structure and has many micropores. The lithium ions that reach the negative electrode are embedded in the micropores of the carbon layer. The more lithium ions are embedded, the higher the charging capacity. Similarly, when the battery is discharged, the lithium ions embedded in the carbon layer of the negative electrode are released and move back to the positive electrode. The more lithium ions that return to the positive electrode, the higher the discharge capacity. What we usually call battery capacity refers to the discharge capacity. During the charging and discharging process, lithium ions are in a cyclic motion state from the positive electrode to the negative electrode to the positive electrode. Since lithium-ion batteries use lithium in an ionic state rather than metallic lithium, they are less dangerous and more safe.

Battery Characteristics

The performance parameters of batteries mainly include electromotive force, capacity, specific energy and resistance. Electromotive force is equal to the work done by the non-electrostatic force (chemical force) of the battery when a unit positive charge moves from the negative electrode through the inside of the battery to the positive electrode. The electromotive force depends on the chemical properties of the electrode material and has nothing to do with the size of the battery. The total amount of charge that a battery can output is the capacity of the battery, usually measured in ampere hours.

In a battery reaction, the amount of electrical energy produced per kilogram of reactant is called the theoretical specific energy of the battery. The so-called specific energy refers to the energy stored in a unit weight or unit volume, expressed in Wh/kg or Wh/L. Wh is the unit of energy, W is watt, h is hour; kg is kilogram (weight unit), L is liter (volume unit). The actual specific energy of a battery is smaller than the theoretical specific energy. This is because not all reactants in the battery react according to the battery reaction, and the internal resistance of the battery will also cause a voltage drop. On the other hand, the larger the cross-sectional area of ​​the battery through which the current flows, the smaller its internal resistance. The biggest feature of lithium-ion batteries is their high specific energy. Today's lithium-ion battery technology can achieve a specific energy of 80~120 wh/kg, while the specific energy of traditional lead-acid batteries is only 30~40 wh/kg. Therefore, lithium-ion batteries can store higher energy in a smaller volume, which is helpful for mobile electronic devices that emphasize thinness and shortness and electric vehicles that require high endurance.

The following are some of the characteristics of lithium-ion batteries: High capacity: The weight of a lithium-ion battery is half that of a nickel-cadmium or nickel-metal hydride battery of the same capacity, and its volume is 20-30% of that of nickel-cadmium and 35-50% of that of nickel-metal hydride.

●High voltage: The operating voltage of a lithium-ion battery cell is 3.7V (nominal value), which is equivalent to three nickel-cadmium or nickel-metal hydride batteries connected in series.

●High stability: Since it does not contain metallic lithium, it is less dangerous and is not subject to the restrictions of aircraft transportation regulations prohibiting carrying it on passenger aircraft.

●Long life: When a rechargeable lithium-ion battery is fully charged, the voltage is about 4.2 volts. When discharged, the voltage will drop, but it should not be lower than about 2.5 volts. The storage voltage or factory voltage is about 3.6 to 3.7 volts. The service life is mainly determined by the number of charging times. A good rechargeable lithium-ion battery has a life of more than 500 times (one charge from 2.5 volts to 4.2 volts), and lithium-ion batteries do not have a memory effect.

●Fast charging: Using a constant voltage/constant current charger with a rated voltage of 4.2V, the lithium-ion battery can be fully charged in 1~2.5 hours. However, please note that if the charging voltage exceeds 4.3 volts, there is a risk of explosion. If the battery voltage is lower than 2.0 volts, the lithium-ion battery is damaged and can no longer be used or charged.

Battery Charging Method

From the above, we can know that although lithium-ion batteries have the advantages of high capacity and long life, special attention should be paid to charging and discharging. Therefore, all rechargeable lithium-ion batteries need to be equipped with their "charge and discharge management IC" to limit the charging and discharging voltage to ensure that the battery explosion is not caused by exceeding the safe voltage. When the battery voltage is lower than 2.5V, the output is cut off to avoid shortening the battery life. Except for a few standard products, most lithium-ion batteries have different sizes and appearances, mainly based on practical applications, and the capacity specifications are also different. Therefore, the charging current is designed by each manufacturer. There is a so-called fast charging or slow charging mode based on the current size; however, the high current charging mode usually damages the service life. Although the battery pack already contains a charging management IC, this is only a minimum protection measure to prevent battery explosion or combustion, rather than a normal use method. In order to fully achieve the battery life and efficiency, the charger design still needs to be far away from this upper and lower limits.

In addition to over-discharge, lithium-ion batteries are not suitable for high-current discharge, which will reduce the discharge time (higher temperature will be generated internally and energy will be lost). Therefore, the battery manufacturer specifies the maximum discharge current of the product, which should be less than the maximum discharge current during use. Lithium-ion batteries have high requirements for charging quality and require a precise charging circuit to ensure charging safety. In particular, the termination charging voltage accuracy is required to be within 1% of the rated value (for example, the tolerance for charging a 4.2V lithium-ion battery is ±0.042V). Overvoltage charging may cause permanent damage to lithium-ion batteries, and in severe cases, the battery may explode; the charging current of lithium-ion batteries should be selected according to the specifications of the battery manufacturer. Although the charging current of some batteries can be rated up to 2C (C is the capacity of the battery, such as 1000mAh, and the charging rate of 1C is 1A), high charging current will reduce the battery life, so the commonly used charging rate is 0.25C~1C. Because the electrochemical reaction during the charging process generates heat, there is a certain amount of energy loss; in addition, lithium-ion batteries are not all charged at a constant current, but also at a constant voltage mode, so the actual charging time is about 2.5 hours; the charging temperature of lithium-ion batteries is in the range of 0℃~60℃. If the charging current is too large, the temperature will be too high, which will not only damage the battery but may also cause an explosion. Therefore, when charging with a large current, the battery temperature needs to be detected, and charging can be stopped when the set charging temperature is exceeded to ensure safety. In addition, there is a set current limiting resistor in the charger circuit to ensure that the charging current does not exceed the set limit current.

At present, lithium-ion battery chargers often use a three-stage charging method, namely, pre-charging mode (Pre-Charging Mode), constant current charging (Fast Charging Mode), and constant voltage charging mode (Constant Voltage Mode). The termination discharge voltage of lithium-ion batteries is 2.5V. A well-designed charger can salvage and repair over-discharged batteries, that is, pre-treatment before formal charging. Before charging, first check the battery voltage: if the battery voltage is greater than 3V, charge in the normal way; if the battery voltage is lower than 3V, charge with a small current (about 10% of the constant current mode charging current) is called pre-charging mode, so that the passivation film dissolved in the deep discharge state can be restored. In addition, when the battery is over-discharged, some copper metal may be released at the anode to cause a short circuit. At this time, if forced charging with a high current will cause the battery to overheat, and the pre-charging stage can avoid this phenomenon. After charging to 3V, charge in the normal constant current mode.

When the battery voltage is greater than 3V, the charging characteristics of normal charging are shown in Figure 1 (taking a 4.2V lithium-ion battery as an example). Start charging in the set constant current mode. At this time, the battery voltage rises at a faster slope. As the battery power storage increases, the battery voltage rise slope will gradually decrease. When it rises to close to 4.2V, the constant current charging stage ends. The charger changes to 4.2V constant voltage charging. When charging in the constant voltage stage, the voltage is almost unchanged, but the charging current continues to decrease. When the charging current drops to a certain value, the timer is activated. After a period of counting and timing, the charging ends and the charging procedure is completed.

Figure 1: Typical Li-ion battery charging curve

The output voltage regulation accuracy of constant voltage charging is important for maximizing battery capacity and extending battery life. When the battery voltage is lower than 4.2V, the battery may be undercharged, which will not affect the life, but will reduce the battery storage capacity. For example, as long as the undercharge reaches 1% of the total voltage, the battery storage capacity will be reduced by 8%. On the other hand, if the battery voltage is too high, the battery will be overcharged, shortening the service life and even causing danger to the user. In order to ensure the safety of lithium-ion battery charging, the ambient temperature at the beginning of charging must be between 0℃ and 45℃. Charging at a lower temperature will form more metallic lithium, which will increase the battery impedance and battery degradation. Charging in a high temperature environment will increase the reaction between lithium ions and electrolytes and accelerate battery degradation.

Generally speaking, it is recommended that the battery should be charged to 70-80% before storage when not in use for a long period of time. This is also to prevent the lithium-ion battery voltage from being lower than 2.0 volts after a long period of natural discharge, causing the lithium-ion battery to fail and become unusable. Frequent use of lithium-ion batteries to exhaust their power will shorten their life by at least half compared to frequent charging and discharging.

Lithium-ion charger design example

To meet the demand for more accurate and safer charger applications for low-power portable products, many IC manufacturers have developed low-cost linear chargers. Figure 2 shows a design example of a stand-alone linear charger circuit using the LD6275 charging IC from Tongjia Technology, which requires only a few external parts and has a maximum charging current of 1.5A.

LD6275 is a highly integrated lithium-ion battery linear charger IC with active power path management. When the load current is loaded/unloaded, it adjusts the battery charging current in real time, effectively monitors and manages the input current (i.e. the output current of the USB port), and complies with the requirements of the surge current limit and soft activation function specified by USB-IF. In addition, the IC integrates a temperature detection function. If the IC temperature exceeds the set value, it will automatically reduce the charging current to protect the chip from damage.

LD6275 steps down the 5V DC power supply from the power adapter/USB port to charge the lithium-ion battery. To prevent the power adapter from overcurrent overload, the external resistor RCISET can be used to set the maximum charging current limit. It also supports the computer USB port charging mode and is set according to the external pins EN1 and EN2. Please see Table 1 for each mode. By setting the USB 500mA and USB 100mA operation modes, the PC USB port can be protected from overload.

Figure 2: LD6275 application circuit diagram

Table 1: Charging mode settings

LD6275 has an adaptive power path management (APPM) function, which mainly supplies power to the system end and charges the battery as a supplement, as shown in Figure 3; when the system power consumption exceeds the supply limit of the input power supply, the battery can also actively start the discharge function and supply the required power to the system end, as shown in Figure 4.

Figure 3. APPM

Figure 4. APPM

LD6275 opens two levels of battery setting voltage and charging current adjustment, which can be adjusted dynamically according to the needs. For example, in order to comply with the Japanese JEITA specification, the charger setting can be adjusted according to the battery temperature, as shown in Figure 5 below.

Figure 5: TVSET, TISET adjustment

Since the power consumption of LD6275 itself is very small, only 1~2mA, which can be almost ignored, the heat generation power Pd of IC itself can be calculated by the following formula:

Vin is the input power supply voltage, the operating range is 4.1V~6V. VBAT is the battery voltage, which can be 0~4.2V. ICHG is the set charging current, which is set by the external resistor RCISET. When the battery voltage is lower than 3V, it will enter the pre-charge mode, and the IC is preset to charge at 10% of the current of ICHG.

Assuming that a 5.5V power supply is used to charge a single 1200mAh lithium-ion battery, at a 0.7℃ fast charging current and a battery voltage of 3V, the maximum power consumption of the IC operation can be estimated to be 1.762W. This power consumption will cause the temperature of the 3×3mm QFN package with a thermal impedance of 60℃/W to rise by 127℃. Even at an ambient temperature of 0℃, it has exceeded the maximum allowable silicon chip operating temperature of 125℃. If the charging current is set to 0.6A (0.5C), the IC temperature rise can be reduced to 90 degrees, and it can operate in an ambient temperature of 35 degrees, so it is a better setting current.

From the above, we can know that the fast charging steady current value and the operating range of the power supply voltage are very important for linear chargers. The fundamental problem of linear chargers is that the chip temperature is high during operation, so the design must make a trade-off between the charging current and the heat dissipation mechanism. However, the application scope of linear chargers is often portable products that require thinness and lightness. Plastic shells with poor thermal conductivity are often used, and metal heat sinks are not considered. In the end, product designers can only reduce the charging current and extend the charging time in exchange for a lower operating temperature. Based on the fact that portable product users hope to complete charging in 1~2 hours, linear chargers are usually more suitable for low-capacity lithium-ion battery applications below 1500mAh. If it is used in charging applications with high input/output voltage difference or high-capacity batteries, you can consider using a synchronous switching charger at this time.

Figure 6 shows the standard charging process of a lithium-ion battery charger. First, the charging IC detects whether there is an output short circuit or an overload protection mode. If the system is normal, it then detects whether the initial battery voltage reaches 3V or above. If it is higher than 3V, it will directly charge with high current in fast charging mode. If the battery is lower than 3V, it will enter the pre-charging mode and charge at 10% of the fast charging mode to wake up the battery and avoid battery damage. In the pre-charging stage, the battery voltage is still detected at any time, and the fast charging mode can be switched immediately after reaching 3V.

In fast charge mode, the battery voltage rises at a high speed. When it rises to 4.2V, it switches to 4.2V constant voltage charging. The internal resistance of the battery is used to limit the current. At this time, the charging current is the same as the CV stage in Figure 1. As time goes by, the charging current decreases exponentially. When it reaches 10% of the set current ICHG, the charger is turned off and the charging is indicated at the same time.

However, when the battery fails, the battery may not be able to store electrical energy, the voltage will not increase, and the charged energy will be converted into heat. In addition to relying on the over-temperature protection mechanism, the IC also has a timeout timer inside. Regardless of the battery voltage status at this time, as long as the set charging time is exceeded, the charger will be turned off immediately to achieve multiple protection functions for users.

The user may also remove the battery without removing the power source after charging or charging. To avoid danger, the IC should have a battery presence detection mechanism as shown in Figures 7 and 8. The charging IC will draw battery current in a short pulse (a 2ms pulse is generated every 370ms). If the battery is present, the detected battery voltage should be greater than a preset threshold; if the battery has been disconnected, the charging IC will detect a low voltage, which can be determined as a battery disconnection state, and the battery terminal voltage will be cut off to protect the user's safety.

Figure 7. Battery presence detection mechanism

Figure 8: Battery removal detection mechanism

in conclusion

Lithium-ion batteries have been widely used in portable devices such as notebook computers, cameras, and mobile communications due to their unique performance advantages. The new generation of polymer lithium-ion batteries can be made thinner, with any area and shape, which greatly improves the flexibility of battery design. At the same time, the unit energy of polymer lithium-ion batteries is 20% higher than that of current general lithium-ion batteries, and their capacity and environmental performance are all improved compared to lithium-ion batteries. Therefore, it can be foreseen that the future development trend of lithium-ion battery chargers will also be towards faster charging speeds and stronger system protection capabilities.