Small-size and low-cost netbooks, ultra-mobile PCs (UMPC) and mobile internet devices (MIDs) are becoming more popular and gaining wide user acceptance. The Lithium-Ion (Li-Ion) battery charging system used in these portable devices is much more sophisticated than that used in cellular phones. Understanding their battery charger requirements becomes critical for improving system safety. This article discusses Li-Ion battery charging requirements such as charge system safety and performance optimization between charger and system. A design example of a synchronous switching standalone battery charger IC controller with dynamic power management for optimizing the adapter power rating and fast charging the battery for netbook applications is provided.
Netbooks, UMPCs and MIDs Introduction
This year notebook computer shipments have exceeded that of desktop computers and are starting to dominate in personal computers. Meanwhile, a variety of low-cost, small-size portable computing devices are becoming popular and getting user acceptance.
Netbooks are a fast derivative of small, light and inexpensive laptop computers mainly for general computing and accessing web-based applications. They feature smaller screens and keyboards, and offer reduced specifications and computing power. Most netbooks use the Intel Atom microprocessors and two to three Li-Ion cells in series battery pack.
Ultra-mobile PCs (UMPCs) feature a powerful processor and support the latest connectivity standards. It offers a display of four-to-seven inches and touch capabilities, all in a package that weighs less than two pounds. But, it can run all of the same Windows Vista-compatible software with which you are already familiar. Ultra-mobile PCs can also feature global positioning system (GPS) devices, webcams, fingerprint readers, stereo speakers, TV tuners, and memory card readers. It is powered with two to three Li-Ion cells in series battery pack
A mobile Internet device (MID) is a multimedia-capable, handheld computer providing wireless Internet access. They are designed to provide entertainment, information and location-based services for personal rather than corporate use. They allow two-way communication and real-time sharing. MIDs are larger than Smartphones, but smaller than UMPCs. They usually use one Li-Ion cell battery pack.
System and Battery Charger Performance Optimization
The battery charge voltage and current are crucial for battery life and battery capacity. The higher the battery charge voltage, the more battery capacity. Figure 1 shows the relationship between the battery charge voltage, cycle life and battery capacity. The battery has 10 percent more initial capacity with 4.3V versus 4.2V. However, the battery capacity will be lower after 200 cycles due to its faster cell degradation at higher cell voltage.
It also shows that battery degradation is much slower with low-charge versus high-charge voltage. The battery charge voltage can be set to a low level such as 4.1V for maximizing the battery cycle life by sacrificing capacity such as in back-up applications. If the battery charger has about a two percent charge voltage tolerance, the battery could be either over charged up to 4.284V, or under charged down to 4.116V, which has 10 percent less capacity. Therefore, high-voltage regulation accuracy is critical for safety and maximum capacity.
Dynamic power management
The overall power demanded in these portable devices is lower than laptop computers because of their low-power microprocessors. The adapter power used is typically below 40 Watts, while laptop computers usually use 60-Watt and 90-Watt adapters. However, the adapter is still required to power the system while charging the battery simultaneously to minimize the adapter power rating. Due to the nature of the high-pulsating power from the microprocessor, the total power required for charging the battery and providing the maximum power to the microprocessor easily exceeds the maximum power available from the adapter. Alternatively, the adapter must be redesigned to supply maximum power for the charger and system, which increases costs.
To optimize the system and battery charger, dynamic power management (DPM) is used by introducing the maximum adapter current regulation loop. If the input adapter current reaches the regulation threshold, the battery charger automatically reduces the effective charge current while giving high priority to powering the system so that it will not exceed the adapter’s maximum power limit. The remaining power is used to charge the battery after supplying the system. Once the pulsating power has passed, the charger automatically resumes the fastest charging mode for reducing charge time (Figure 2). An important specification is input current regulation accuracy. The higher the input current regulation accuracy, the more power can be taken out of the adapter, and the faster a battery can be charged.
Computing system and battery charger safety
a. Adapter input and battery over-voltage protection (OVP)
Using 19-V and 16-V adapters are very common for laptops, while 5-V adapters are popular for Smartphones. netbooks, UMPCs, and MIDs usually use these adapters for saving development costs, though it usually does not require 19V for charging one to three cells in series battery pack. At minimum, the system should not be damaged with these commonly used adapters. Moreover, the IEEE P1725 requires the system to include input adapter and battery OVP. If the over input voltage is applied to these portable devices, it should prevent you from turning on the system. If the battery is over-charged, turn off the battery charger immediately. Also do not turn on the system if a reverse adapter voltage is applied.
b. Battery charging safety
It is dangerous to charge the Li-Ion battery at very low- or high-cell temperatures. The Li-Ion battery with LiCoO2 cathode material could explode when the cell temperature reaches 175oC with 4.3V. Industry battery charger safety guidelines such as the Japan Electronics and Informational Technology Industries Association (JEITA) has been released to safely charge the battery by reducing the battery charge current and voltage at low- or high-cell temperatures.
To initiate the charge process, the typical battery charge temperature range is between 0-40oC. Therefore, the cell temperature must be monitored through either a fuel gauge or charger. A safety timer is another level of protection in case the battery charging system fails. The battery stops charging when the safety timer expires.
c. Battery charger output short and over charge current protection
For computing applications, the most commonly used Li-Ion battery is the 18650 Li-Ion cells with 2200-2600mAh capacity. The charge current is around 2-4A with 0.7oC charge rate from a 12-V or 19-V adapter. A synchronous switching buck-based topology is required for high-efficiency charging. It also requires a smokeless charging system with any component failure or abnormal operation conditions such as charger output short or inductor short. The charger needs to have such a protection mechanism to prevent fire or smoke.
Battery Charger Solution for Netbooks, UMPCs and MID
Based on system optimization and safety requirements, Figure 3 shows a standalone high-efficiency synchronous switching Li-Ion battery charger with dynamic power management for netbook applications. This design example charges a two-cell Li-Ion battery with 200mA pre-charge current and 2A fast charge current, and three hours safety timer. The DPM function is achieved by monitoring the voltage drop across the input current sense resistor R1. The synchronous switching charger operates in 600kHz switching frequency for optimizing the efficiency and solution size. The external resistor dividers R11 and R12 are used to set the desired battery charge voltage. For maximum battery capacity, set the external resistor divider to charge voltage at 4.2V per cell.
For maximum battery life cycle, set the battery charge voltage at 4.1V per cell. This scenario can charge one to six Li-Ion series cells up to 10A with external power MOSFETs for fitting many different battery charging applications without a host controller. It also provides other protections such as input over voltage, battery charge over-voltage, battery short, over charge current protections, and automatically monitors the battery temperature for safe charging.
As portable computing devices evolve and include more features, battery charging and system designs become the most important design factors for achieving a high-safety and high-performance system. Battery charge requirements, safety, and system optimization were discussed. As an example, a standalone high-efficiency synchronous switching battery charger solution is presented for netbook applications.
1. Download the bq24610 datasheet and other technical documents at: www.ti.com/bq24610-ca.
2. “How does battery management electronics enhance battery safety,” by Jinrong Qian and Simon Wen, Power Management DesignLine, Nov. 15, 2006: http://www.powermanagementdesignline.com/194400185
3. “Using Dynamic Power Path Management to Achieve Proper Battery Charge Termination and Prevent False Safety Timer Warning” by Jinrong Qian, Battery Power Products & Technology Magazine, March/April, 2006. http://www.batterypoweronline.com/bppt_edhighlights.htm
About the Author
Jinrong Qian is a Sector Manager of Battery Charge Management and Distinguished Member of the Technical Staff for the Portable Power Battery Management group at Texas Instruments. He has published a myriad of peer-reviewed power electronics transactions and power management papers, and holds 22 U.S. patents. Dr. Qian earned a BSEE from Zhejiang University, and a Ph.D. from Virginia Polytechnic Institute and State University. Jinrong can be reached at firstname.lastname@example.org.