Battery Technology for Portable Electronic Devices

Introduction

Today, battery technology in portable electronic devices includes various aspects such as power detection algorithms, battery charging algorithms, and battery charging techniques. Rechargeable batteries come in several types, including nickel-cadmium, nickel-metal hydride, lithium-ion, and lithium polymer batteries. While each of these battery types has its characteristics, lithium-ion and lithium polymer batteries have become the ideal choice for small, long-running devices like laptops and hard-disk-based portable media players (PMPs) due to their energy density and safety features. For engineers working with portable electronics, it is crucial to correctly select and apply battery technology, and this article will discuss and analyze these aspects with practical examples.

1. Battery Charging Algorithms: Trickle, Fast, and Constant Voltage Charging

Depending on the energy requirements of the final application, a battery pack may contain up to four lithium-ion or lithium-polymer cells, configured in various ways, and powered by a primary adapter: direct adapter, USB interface, or car charger. Despite differences in the number of cells, their configuration, or the type of power adapter, these battery packs share the same charging characteristics, and thus, the charging algorithms are similar. The optimal charging algorithm for lithium-ion and lithium polymer batteries can be divided into three stages: trickle charging, fast charging, and constant voltage charging.

Trickle Charging: Used for deeply discharged cells. When the cell voltage drops below approximately 2.8V, a constant current of 0.1C is applied to charge the cell.

Fast Charging: Once the cell voltage exceeds the trickle charging threshold, the charging current is increased for fast charging. The fast charge current should be less than 1.0C.

Constant Voltage Charging: During fast charging, once the cell voltage reaches 4.2V, the constant voltage phase begins. Charging is stopped when the minimum charging current drops below about 0.07C, or a timer triggers the interruption.

Advanced battery chargers typically come with additional safety features. For instance, if the battery temperature exceeds the specified range (typically 0°C to 45°C), the charging process will pause. Modern lithium-ion and lithium polymer battery charging solutions integrate or include external components to follow these charging characteristics, ensuring better efficiency and safety.

2. Lithium-ion/Polymer Battery Charging Solutions

The charging solution for lithium-ion/polymer batteries varies depending on the number of cells, their configuration, and the power supply type. There are three main charging solutions: linear, Buck (step-down) switching, and SEPIC (step-up and step-down) switching.

2.1 Linear Solution

When the input voltage is slightly higher than the open-circuit voltage of a fully charged cell, the linear solution is the best option. This is particularly effective when the 1.0C fast charge current is not much greater than 1A. For example, an MP3 player typically uses a single cell with a capacity ranging from 700 to 1500mAh and an open-circuit voltage of 4.2V. These devices usually use an AC/DC adapter or a USB interface with a regulated 5V output. In such cases, a linear charger is the most efficient and straightforward solution.

Example Application: MAX8677A Dual Input Li+ Charger: The MAX8677A is a dual-input USB/AC adapter linear charger with an integrated Smart Power Selector, suitable for portable devices powered by a single-cell Li+ battery. The charger switches between power inputs and optimally charges the battery. It also includes current-limiting, thermal regulation, overvoltage protection, and more, ensuring safe and efficient charging for devices such as smartphones, PDAs, cameras, and GPS devices.

2.2 Buck (Step-down) Switching Solution

For cases where the 1.0C charge current exceeds 1A or when the input voltage is much higher than the open-circuit voltage of the battery, a Buck (step-down) solution is a better choice. For instance, portable media players with a single lithium-ion cell and a wide input voltage range (9V to 16V) will benefit from this solution, as it is more efficient than linear charging when there is a significant voltage difference between the input and battery voltage.

2.3 SEPIC (Step-up and Step-down) Switching Solution

For devices with three or more lithium-ion/polymer cells connected in series, where the input voltage is not always higher than the battery voltage, the SEPIC solution is ideal. For example, laptops typically use a 3-cell lithium-ion battery pack with a fully charged open-circuit voltage of 12.6V. The SEPIC converter can handle both scenarios: when the output voltage is higher or lower than the battery voltage.

3. Power Detection Algorithm

Many portable products rely on voltage measurement to estimate the remaining battery capacity, but this method's accuracy can be significantly affected by discharge rate, temperature, and battery aging. To achieve more precise battery capacity estimation, power meters are used to measure the charge added or consumed by the battery, providing more accurate estimates across a broad range of application power levels.

3.1 Example Application of Power Detection Algorithm: Complete Single/Double Battery Portable Application Battery Pack Design

A good power meter for battery detection should at least measure battery voltage, temperature, and current. A microprocessor and a set of well-tested power detection algorithms are essential. For instance, the bq2650x and bq27x00 power meters come equipped with ADCs to measure voltage, temperature, and current, and integrate TI's power detection algorithms to compensate for self-discharge, aging, temperature, and discharge rate. These meters provide information on remaining battery capacity, and the bq27x00 series even provides estimated run time to empty.

3.2 Example Application of Power Detection Algorithm: New IC for General-Purpose Power Meters

Several manufacturers offer a wide range of power meter ICs, allowing users to select the most suitable device to optimize product cost-effectiveness. For example, the DS2762 from Dallas Semiconductor is a low-cost, highly integrated power meter IC suitable for mobile phones, PDAs, and other similar portable devices. It combines power detection with overvoltage, undervoltage, and overcurrent protection. The DS2762 also offers a current-limiting recovery charging path when the battery voltage drops below 3V, providing efficient power management.

4. Conclusion

Properly applying battery technology in portable electronic devices is crucial for selecting lithium-ion or lithium polymer batteries and their chargers. The choice must be made based on the specific requirements of the portable electronic device.