18650 battery cell charging technology for portable systems

　　Today, portable applications place many unique requirements on batteries: batteries must have high energy density to provide a steady stream of energy (burst or continuous mode) for portable applications; batteries must be lightweight and have a small footprint; batteries must be safe protected from possible abuse and have an indefinite idle life;

　　Finally, batteries should be extremely low cost. Because lithium-ion or lithium-polymer batteries meet most of the above requirements, they have become the battery of choice for current portable applications. General Characteristics of Rechargeable Li-ion Batteries Rechargeable lithium-ion batteries have several advantages over other existing batteries, making them a more suitable power source for portable applications. They can provide higher energy density (up to 200Wh/kg or 300-400Wh/L) and higher battery voltage (4.1V for carbon anode cells and 4.2V for graphite anode cells). Lithium-ion batteries can be quadrilateral in shape and have longer charge retention or idle life and higher charge cycles.

　　The higher chemical energy density and higher cell voltages of lithium-ion batteries allow us to create smaller and lighter batteries for portable applications, for which lighter and smaller power sources are often critical. However, to fully utilize battery capacity or extend battery life, charging parameters must be extremely tightly controlled. The key to extending battery life is to properly select charging parameters such as current, voltage and temperature. During the charging process, the accuracy of applied voltage plays a very important role in improving battery efficiency and extending battery life. Exceeding the charge termination voltage will cause overcharging, which will increase the battery's power supply in the short term, but will cause battery failure and create safety issues in the long term.

　　For every 1% increase in charge termination voltage, the battery's initial capacity will increase by approximately 5%. This apparent short-term gain effect can have serious consequences on the battery's charge/discharge cycles. Overcharging leads to a reduction in the number of charges. On the other hand, undercharging, although it does not pose a safety problem, will significantly reduce the battery's capacity. Generally speaking, the charging principle of lithium-ion batteries is conceptually very simple. The equivalent circuit of a battery is usually considered to be a parallel circuit consisting of a large capacitance capacitor and an internal leakage resistor RLeakage.

　　The resistance and inductance between the battery wires and the cell itself are represented by effective series resistance (ESR) and effective series inductance (ESL) respectively. These parameters are a function of the battery's mechanical structure and specific chemical composition. The ESR of a battery is between 50 and 200m(, and the ESL is an inductance on the order of nanohenries. As we will see below, ESR poses special challenges for accurately sensing the battery voltage during charging. Lithium-ion batteries There are many charging methods. The simplest 18650 battery cell charger usually refers to a constant voltage (CV) charger (see Figure 1). It consists of a current-limited constant voltage source connected to both ends of the battery. It The current is limited below the battery capacity, and the output voltage is adjusted to the battery termination voltage (4.1V for carbon anode batteries and 4.2V for graphite anode batteries).

　　A depleted battery will absorb as much current as possible from the charging source. When charging a battery, the voltage across the battery will rise and the charging current will gradually become smaller. When the charging current drops below 0.1C, the battery can be considered to be fully charged. Because trickle charging is not recommended, the charger must be completely turned off or disconnected when charging is completed. To prevent defective batteries from being charged with an uncertain current, a backup timer should be used to terminate the charging process. While constant voltage charging is a relatively low-cost method, it does require a long charging time. Since the power supply voltage remains constant, as the battery continues to be charged, the charging current will drop rapidly, causing the charging speed to also drop rapidly. The current rate at which the battery is charged will then be much lower than the current rate it can handle.

　　A faster charging method is constant current/constant voltage (CC/CV) charging, as shown in Figure 2. When charging begins, the CC/CV charger first applies a constant current equivalent to the battery capacity C. To prevent overcharging during a constant current charge cycle, the voltage across the battery package needs to be monitored. When the voltage rises to a given termination voltage, the circuit switches to constant voltage source operating mode. Even if the voltage across the battery package reaches the termination voltage, the actual battery voltage will be lower than the termination voltage because there is a voltage drop across the ESR.

　　During constant current charging, the battery can be charged at a high current rate close to its end voltage without any risk of high voltage being applied and overcharging. After constant current charging, the battery's capacity will reach approximately 85% of its rated value.

　　After the constant current period ends, the charger switches to the constant voltage period. During the constant voltage cycle, the charger determines whether to end charging by monitoring the charging current. Like a constant voltage charger, the charging cycle ends when the charging current decreases below 0.1C of the battery. Figure 2 shows a complete CC/CV charging process.

　　Although implementing the CC/CV charging method requires more complex circuits, various CC/CV charging methods dominate 18650 battery cell charging applications because it can significantly reduce charging time. So far we have assumed that the object being charged is a good quality battery. But this is not always the case.

　　The battery being charged may be defective and unable to accept a charge. Additionally, attempting to fast charge a defective battery may create safety hazards. An ideal charger must be able to detect all possible battery failure modes and charge accordingly. To simplify matters, we intentionally ignored another factor in the previous discussion, battery temperature.

　　If the temperature of a 18650 battery cell exceeds the specified temperature range, it will be unsafe to charge it. Currently, all chargers must track changes in voltage, and CV/CC chargers even need to track current and voltage.

　　But as pointed out earlier, we cannot ignore potential safety issues while improving charger efficiency and extending battery life, which makes us increasingly need smarter charging operations. To prevent accidental application of reverse voltage to the battery, all 18650 battery cell packs contain some complex circuitry. Generally speaking, protection functions include preventing over-discharge, over-charging, excessive charge/discharge current, and avoiding high voltages being applied to the battery.

　　During the charging or discharging of the battery, if any parameter exceeds the limit set by the specific battery, the connection between the cell and the battery terminals will be broken. Normally, the charger will reset after a period of time after the reverse voltage is removed or the battery is preset. In addition to electronic protection, the battery also contains mechanical secondary overcurrent protection devices.

　　A polymer positive temperature coefficient (ppTC) overcurrent protection device is connected in series between the battery package and the cell terminals. When overcurrent occurs, the ppTC device switches from a low impedance state to a high impedance state, thus protecting the circuit. The heat generated by the device due to the I2R heating effect causes its temperature to rise, and the above changes are the result of the rapid increase in device temperature. A good charger design must be able to determine whether a 18650 battery cell can be quickly charged safely and efficiently.

　　Below are some examples of chargers that support portable applications. Stand-alone Charger Lp3946 The Lp3946 is a stand-alone single-cell Li-ion battery charger with an integrated pass transistor and current sensing resistor. In addition to the charging function, it can also operate in low dropout (LDO) mode.

　　This feature is very useful during the manufacturing process because it eliminates the need to insert batteries when testing and performance verification of the product. The charging cycle begins with plugging in the power adapter. The charger first verifies the input voltage and if it is within the allowed range, the charger starts the battery certification process.

　　During this phase, a current source applies 50mA to the battery terminals while monitoring the voltage. If the voltage across the battery is higher than 3.0V, indicating that the battery is in good condition, then the constant current charging cycle will start. The magnitude of the current is a function of the battery capacity, which can be referred to the value recommended by the battery manufacturer. Typical charging current is 1C, but some batteries require lower charging current. The Diff-Amp output can simulate reproducible charging current. The Diff-Amp output also represents the current in LDO operating mode.

　　In a single-supply power supply system, to avoid accuracy errors when the operating voltage is close to ground potential, the output of the Diff-Amp is biased to 0.5V. For ease of use and to reduce the number of external components, the fast charging current of the Lp3946 is set at the factory according to the user's requirements.

　　The constant current source can also be factory set to any value from 500mA to 950mA in 50mA increments. Other factory-preset parameters include termination voltage (4.1 or 4.2V) and end-of-charge current (0.1C, 0.15C or 0.2C). During the constant current cycle, the battery voltage must be monitored very accurately to avoid overcharging. As described earlier, if the battery's termination voltage is exceeded, the battery life will be gradually shortened, and this effect is cumulative.

　　During the constant current charging process, due to the voltage drop on the ESR of the battery, the voltage across the battery cannot accurately represent the voltage of the battery cell. When the constant current cycle ends at the termination voltage, the voltage drop of the ESR provides a safety margin so that excessive voltage is not applied to the battery. During the constant current cycle, the battery is charged to about 80-85% of capacity. After the constant current period ends, the constant voltage period starts.

　　In this cycle, the charging current of the battery is ICharger=(VBatt-Vcell)/ESR. During the charging process, Vcell continues to increase and the charging current decreases.

　　As the charging current decreases, the error caused by ESR will also gradually decrease, so the voltage across the battery can more accurately represent the actual voltage of the cell. Charging is terminated when the current drops below the preset end-of-charge (EOC) current. Generally, the recommended EOC current levels are 0.1C, 0.15C and 0.2C. Once the EOC current is detected, the charging cycle ends.

　　At this point, the charger circuit is turned off and the maintenance cycle is initiated. During the maintenance cycle, when the battery voltage is monitored to drop below 3.9V, the charging cycle restarts (assuming the detected rectifier signal is valid). When power consumption causes the battery voltage to drop below 3.0V, fast charging is replaced by a pre-qualification cycle.

　　This is primarily for safety reasons and to avoid fast charging a potentially faulty battery. During normal operation, if the cell voltage drops to this level, the internal protection circuit will be activated to cut off the connection between the cell terminals and the battery package terminals.

　　If the battery is not permanently damaged, a low level of current is applied to gradually increase the battery voltage and reset the internal protection circuitry.

　　As a backup protection device, the timer counter records the total charging time. If the battery still does not reach the termination voltage after 5.6 hours of constant current or constant voltage charging, charging will be abandoned. As a visual indicator of the charge cycle, the CHG signal illuminates a red LED to indicate the start of the charge cycle.

　　The EOC signal lights up a green LED, indicating the end of the charging cycle. During maintenance cycles, the green LED remains on as long as the AC adapter remains plugged into the wall. If a fault condition is detected, the red and green LEDs will turn on simultaneously. The BIpB input pin is a multi-function pin.

　　Its main function is to allow the Lp3946 to operate as an LDO when no battery is inserted. In LDO mode, the output of Lp3946 is set to 4.1V. The BIpB pin can also be used for the battery in-place detection function, that is, when the battery is in place, it will be connected to ground through the battery's ID resistor. The LP3946 is a typical self-maintaining charger that is easy to use and requires only a minimal number of external components.

　　During the charging cycle, it virtually requires no user intervention. However, some applications require more interaction with the charger. The main reason for this is to modify the charging parameters so that they match the type of battery being charged. An example of this is a standard battery being interchanged with a heavy duty or high capacity battery. High-capacity batteries can be charged using the charging parameters set for low-capacity batteries, but this requires longer charging times. However, for security reasons, the reverse scenario is not recommended.

　　Also, to reduce the required PCB space, it uses a minimum number of external components. Users can communicate with the device through the I2C interface.

　　The default settings at the factory are: 500mA constant current, 4.1V termination voltage and 0.1mA EOC current.

　　When powered on, the charger uses these default values, but the user can program them to different values. The current range of constant current is 500mA to 950mA in 50mA step increments. Termination voltage options are 4.1V and 4.2V, and EOC options are 0.1C, 0.15C and 0.2C.

　　As long as the connection to the battery is maintained at the Vbatt pin and the battery voltage is higher than 2.85V, the newly set value will be used as the default value at startup. If the battery is disconnected or the battery voltage drops below 2.85V, the factory defaults will be used on subsequent charge cycles. In addition to changing the operating parameters of the battery, users can also read the status of the EOC and CHG registers through the I2C interface to query the status of the charging cycle.

　　The characteristics of the LP3945 can be fully exploited in applications with (controllers). Conclusion To power a rapidly growing range of portable products, batteries with wider operating temperature ranges, higher energy density and longer idle life are being developed .Battery charging technology continues to improve as our understanding of battery characteristics continues to improve. Additionally, new applications lead to new methods and create new needs.

　　For example, charging the battery from the USB port connected to the PC. In this application, the USB protocol requires that any device connected to the port must initially operate in a low-power mode, such as consuming less than 100mA. The charging process must start with a current of 100mA, and the input voltage of the charger can only be 4.5V.

　　Once communication is established between the host and the device, the host will allow high-power operation. Such applications require a certain level of intelligence, and this level of intelligence must be provided by the charger or the system utilizing the charger.

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