As is well known, rechargeable batteries have four chemical reaction processes: nickel-cadmium, nickel-metal hydride, lithium-ion, and lithium polymer. While each of these four battery processes has its own characteristics for portable electronic devices, the advantages of lithium-ion and lithium polymer batteries in terms of energy density and safety have made them ideal choices for small, long-running devices, such as laptops and hard drive-based PMPs. For portable electronics engineers, correctly selecting and applying battery technology in portable electronic devices is crucial. This article will discuss this topic and provide application examples and analyses.
1. Battery charging algorithms for fine-current charging, fast charging, and steady charging: Depending on the energy requirements of the end application, a battery pack may contain up to four lithium-ion or lithium-polymer battery cells, with various configurations, and comes with a mainstream power adapter: a direct adapter, USB interface, or car charger. Aside from differences in the number of cells, cell configuration, or power adapter type, these battery packs share the same charging characteristics. Therefore, their charging algorithms are also the same. The best charging algorithm for lithium-ion and lithium-polymer batteries can be divided into three stages: fine-current charging, fast charging, and steady charging. *Fine-current charging: Used to charge deeply discharged cells. When the cell voltage is below approximately 2.8V, it is charged with a constant 0.1C current.
*Fast Charging. When the cell voltage exceeds the threshold for low-current charging, increase the charging current for fast charging. The fast charging current should be below 1.0C. *Voltage Stabilization. During fast charging, once the cell voltage reaches 4.2V, the voltage stabilization phase begins. Charging can then be interrupted using the minimum charging current, a timer, or a combination of both. Charging can be interrupted when the minimum current drops below approximately 0.07C. A preset timer triggers the interruption.
Advanced battery chargers typically come with additional safety features. For example, charging will pause if the cell temperature exceeds a given range, usually 0°C to 45°C.
Aside from some very low-end devices, most lithium-ion/lithium polymer battery charging methods on the market now integrate or have external components to charge according to charging characteristics. This is not only for better charging performance, but also for safety.
*Example of lithium-ion/polymer battery charging application - Dual-input 1.2A lithium-ion battery charger LTC4097
The LTC4097 can be used to charge single-cell lithium-ion/polymer batteries using an AC adapter or USB power supply. Figure 1 shows a schematic diagram of the dual-input 1.2A lithium-ion battery charger LTC4097. It uses a constant current/constant voltage algorithm for charging, with a programmable charging current of up to 1.2A when charging from an AC adapter and up to 1A when using USB power, while automatically detecting the presence of voltage at each input. The device also provides USB current limiting. Applications include pDAs, MP3 players, digital cameras, lightweight portable medical and test equipment, and large-screen cellular phones. Its performance characteristics include: no external microcontroller required to terminate charging; automatic input power detection and selection; programmable charging current up to 1.2A through an AC adapter input; programmable USB charging current up to 1A through a resistor; 100% or 20% USB charging current setting; 120mA drive capability for the output and NTC bias (VNTC) pins with input power presence; NTC thermistor input (NTC) pin for temperature-qualified charging; preset battery float voltage with ±0.6% accuracy; thermal regulation to maximize charging rate and prevent overheating. The LTC4097 can be used to charge single-cell lithium-ion/polymer batteries using either an AC adapter or USB power supply. It employs a constant current/constant voltage algorithm for charging, with a programmable charging current up to 1.2A from an AC adapter and up to 1A from a USB power supply, while automatically detecting the presence of voltage at each input. USB current limiting is also provided. Applications include pDAs, MP3 players, digital cameras, lightweight portable medical and test equipment, and large-screen cellular phones.
2. Lithium-ion/Polymer Battery Charging Methods The charging methods for lithium-ion/polymer batteries vary depending on the number of cells, cell configuration, and power supply type. Currently, there are three main charging methods: linear, buck (step-down) switching, and SEPIC (boost-buck) switching.
2.1 Linear Method When the charger input voltage is greater than the open-circuit voltage after a full charge plus sufficient headroom, the linear method is best, especially when the 1.0C fast charging current is not much larger than 1A. For example, MP3 players typically use only one battery cell with a capacity ranging from 700 to 1500mAh, and the full-charge open-circuit voltage is 4.2V. The power supply for MP3 players is usually an AC/DC adapter or a USB interface, with a standard 5V output; in this case, the linear method charger is the simplest and most efficient method. Figure 2 shows the linear method for charging lithium-ion/polymer batteries; its basic structure is the same as that of a linear voltage regulator.
*Example of Linear Charger Applications - Dual-Input Li+ Charger and Smart Power Selector MAX8677A The MAX8677A is a dual-input USB/AC adapter linear charger with a built-in Smartpower Selector for portable devices powered by rechargeable single-cell Li+ batteries. This charger integrates all the power switches needed for charging the battery and external power source, as well as switching loads, eliminating the need for external MOSFETs. The MAX8677A is ideal for portable devices such as smartphones, pDAs, portable multimedia players, GPS navigation devices, digital cameras, and digital camcorders.
The MAX8677A can operate with either a standalone USB or AC adapter power input, or either input. When connected to an external power source, the intelligent power selector allows the system to operate without a battery or with a deeply discharged battery. The intelligent power selector automatically switches the battery to system load, charging the battery using unused portions of the input power, making full use of the limited USB and adapter input power. All necessary current sensing circuitry, including an integrated power switch, is integrated on-chip. The DC input current limit is adjustable up to 2A, while both DC and USB inputs support 100mA, 500mA, and USB suspend mode. The charging current is adjustable up to 1.5A, supporting a wide range of battery capacities. Other features of the MAX8677A include thermal regulation, overvoltage protection, charging status and fault outputs, power good monitoring, battery thermistor monitoring, and a charging timer. The MAX8677A is housed in a space-saving, thermally enhanced, 4mm²4mm, 24-pin TQFN package and is specified for operation over an extended temperature range (-40°C to +85°C).
2.2 Buck (Step-Down) Switching Method When the 1.0C charging current exceeds 1A, or the input voltage is significantly higher than the fully charged open-circuit voltage of the battery cell, the Buck or step-down method becomes a better choice. For example, in hard drive-based PMPs, single-cell lithium-ion batteries are typically used, with a fully charged open-circuit voltage of 4.2V and capacities ranging from 1200 to 2400mAh. However, PMPs are now often charged using automotive kits, with output voltages between 9V and 16V. A relatively high voltage difference (minimum 4.8V) between the input voltage and the battery voltage reduces the efficiency of linear methods. This inefficiency, coupled with a 1C fast charging current exceeding 1.2A, leads to severe heat dissipation problems. To prevent this, the Buck method is employed. Figure 3 shows a schematic diagram of a lithium-ion/polymer battery Buck charger method; its basic structure is identical to that of a Buck (step-down) switching voltage regulator.
2.3 SEpIC (Boost and Buck) Switching Method In some devices using three or even four lithium-ion/polymer cells connected in series, the charger's input voltage is not always greater than the battery voltage. For example, laptops use 3-cell lithium-ion battery packs with a full-charge open-circuit voltage of 12.6V (4.2V x 3) and capacities ranging from 1800mAh to 3600mAh. The input power source is either a 16V AC/DC adapter or an automotive kit with an output voltage between 9V and 16V. Clearly, neither linear nor buck methods can charge this battery pack. This is where the SEpIC method comes in, as it can operate when the output voltage is higher than the battery voltage and also when the output voltage is lower than the battery voltage.
3. Battery Capacity Detection Algorithms Many portable products use voltage measurements to estimate remaining battery capacity. However, the relationship between battery voltage and remaining capacity changes with discharge rate, temperature, and battery aging, resulting in an error rate of up to 50%. The market demand for products with longer lifespans is increasing, thus system designers need more accurate solutions. Using a battery sensor to measure battery charge or discharge provides more accurate battery capacity estimation across a wide range of application power levels. 3.1 Example of Battery Capacity Detection Algorithm Application: Fully Functional Single/Dual Battery Portable Application Battery Pack Design *Battery Capacity Detection Principle. A good battery sensor should at least have battery voltage, battery pack temperature and current measurement methods; a microprocessor; and a set of industry-proven battery capacity detection algorithms. The BQ2650X and BQ27X00 are fully functional battery sensors with an analog-to-digital converter (ADC) for measuring voltage and temperature and an ADC for measuring current and charge sensing. These battery sensors also have a microprocessor that executes Texas Instruments' battery capacity detection algorithm. These algorithms compensate for factors such as self-discharge, aging, temperature, and discharge rate of lithium-ion batteries. The microprocessor embedded in the chip saves the host system processor from these computational burdens. The power sensor provides information such as remaining power status, and the bq27x00 series also provides remaining runtime to empty. The host can query this information from the power sensor at any time and then notify the user of the battery information through LED indicators or screen display. The power sensor is very easy to use; the system processor only needs to be configured with an I2C or HDQ communication driver. *Battery pack circuit description. Figure 4(a) shows a typical battery pack application circuit with an optional IC for identification function. Depending on the power sensor IC used, the battery pack should have at least three to four external terminals.
The VCC and BAT pins are connected to the battery voltage to power C and measure the battery voltage. A low-value sense resistor is connected to the battery ground terminal, allowing the high-impedance SRp and SRN inputs of the fuel gauge to monitor the voltage across the sense resistor. The current flowing through the sense resistor indicates the amount of charge or discharge from the battery. Designers must ensure that the voltage across the sense resistor does not exceed 100mV when selecting its value; excessively low resistance values may introduce errors at low current levels. The board layout must ensure that the connections from SRp and SRN to the sense resistor are as close as possible to the sense resistor terminal; in other words, they should use Kelvin connections.
The HDQ pin requires an external pull-up resistor, which should be located on the host or main application side so that the power sensor can activate the sleep function when the battery pack is disconnected from the portable device. A 10kΩ pull-up resistor is recommended.
*Battery Pack Authentication. The problem of cheap counterfeit batteries is becoming increasingly serious. These batteries may not contain the safety protection circuitry required by OEM manufacturers. Therefore, genuine battery packs may include the authentication circuit shown in Figure 4(a). When authenticating a battery, the host sends a challenge to the battery pack containing the IC (bq26150, used for Cyclic Redundancy Check (CRC)). The CRC value in the battery pack is calculated based on this challenge and the CRC polynomial built into the IC. The CRC is performed based on the host's query command and the CRC polynomial with its secret meaning in the IC. The host also performs its own CRC value calculation and compares it with the battery pack's calculation result to determine if the authentication is successful.
Once the battery passes the test, the bq26150 will issue a command to ensure normal data communication between the host and the fuel gauge. The entire test process will repeat if the battery connection is interrupted or reconnected.
*Dual-battery application. Figure 4(b) shows a typical application circuit using the bq26500 to support a dual-cell lithium-ion battery. To support multiple batteries, an adjustable regulator is added to this circuit. The BAT pin of the fuel gauge is connected to the negative terminal of the bottommost battery cell to measure the variable battery pack voltage.
The host device must be able to read the variable battery pack voltage measured by the power meter to determine the discharge end threshold and the charging end threshold. As for the remaining state of capacity, it can be used directly without interpretation.
The aforementioned bq2650x and bq27x00 power sensors provide battery manufacturers with a simple and easy-to-use selection method [that is much more accurate than simply measuring battery voltage]. These power sensors can be used with various battery architectures and support battery qualification and dual-battery applications.
3.2 Example of power detection algorithm application: a new IC that can be applied to various general-purpose power meters.
Many manufacturers today offer a wide variety of fuel gauge ICs, allowing users to select suitable functional components to optimize product cost-effectiveness. Utilizing the battery parameters measured by the fuel gauge, this discrete architecture allows users to customize the fuel metering algorithm within the host unit, thus saving the cost of an embedded processor in the battery pack. This analysis focuses on the DS2762 chip from Dallasesemicconductor, a novel discrete fuel gauge IC, whose structure is shown in Figure 5(a). *DS2762 Application Characteristics
The DS2762 is a single-cell lithium-ion battery fuel gauge and protection circuit integrated into a tiny 2.46mm x 2.74mm flip-chip package. Its space-saving design is due to the integrated high-precision resistors for power sensing. Its small size and unparalleled integration make it ideal for mobile phone battery packs and other similar handheld devices such as pDAs. The integrated protection circuit continuously monitors the battery for overvoltage, undervoltage, and overcurrent faults (during charging or discharging). Unlike standalone protection ICs, the DS2762 allows the host processor to monitor/control the conduction state of the protection FETs, enabling system power control through the DS2762's protection circuitry. The DS2762 can also charge a deeply depleted battery, providing a current-limiting recovery charging path when the battery voltage is below 3V.
The DS2762 accurately monitors battery current, voltage, and temperature, with dynamic range and resolution meeting the testing standards of any common mobile communication product. The measured current is integrated over an internal time base to achieve power metering. Accuracy is further enhanced through real-time, continuous automatic offset correction. An integrated sense resistor eliminates resistance variations caused by manufacturing processes and temperature, further improving the fuel gauge's accuracy. Important data is stored in a 32-byte lockable EEPROM; 16 bytes of SRAM are used to store dynamic data. All communication with the DS2762 is conducted via a 1-Wire, multi-node communication interface, minimizing wiring between the battery pack and the host. Key features include: single-cell lithium-ion battery protector; high-precision current (power metering), voltage, and temperature measurement; optional integrated 25mΩ sense resistor, individually fine-tuned for each DS2762; 0V battery recovery charging; 32-byte lockable EEPROM, 16-byte SRAM, 64-bit ROM.
1-Wire, multi-node, digital communication interface; supports multi-battery pack power management and achieves system power control through protection FETs; power supply current is only 2µA (maximum) in sleep mode and 90µA (maximum) in operating mode; available in 2.46mm/2.74mm flip chip package or 16-pin under-SSOp package, both with or without sense resistor; evaluation board is included.
4. Conclusion: The application of appropriate battery technology in portable electronic devices is fundamental to the selection of lithium-ion batteries and lithium polymer batteries, as well as their chargers. The correct selection depends on the specific requirements of the portable electronic device.
