Gas-gauging ICs for the high-stakes portable market are in the very-high-accuracy phase of their evolution, sparked by learning algorithms that enable the best of these "early warning" systems to determine remaining battery capacity to 1 percent. With power availability in portable devices perhaps the most critical factor in applications ranging from cell phones to medical monitoring, users and system designers want to know exactly what's left in the tank—to minutes and seconds in more and more cases.
It's not an easy task for vendors, one reason why new gas-gauge, or fuel-gauge, ICs have been sparse and not a consumer-class, one-fits-all type of chip. At the other end, several semiconductor vendors are quietly developing customized designs for OEM clients. "It's a matter of complexity," said Scott Eisenhart, business manager for battery management at Texas Instruments (Dallas). "Those in this business have found they need a true understanding of battery chemistry as well as the interaction between the battery and powered circuit." To accurately figure how much is left in the battery, the gas-gauge IC has to account for the power usage profile, operating temperature, and especially the battery's age, among other factors. Thus the most advanced gas gauges include some form of "learning algorithm" to better track battery parameters. Indeed, the quest for a better gas gauge increasingly invokes the essence of true power management, i.e., keeping dynamic and proactive, versus static, tabs on the power environment to short-circuit trouble before it starts and ensure power is there when the user expects.
Owing to the shortcomings of traditional voltage-based measuring, most of today's chips embrace a current-based measurement scheme. Also in keeping with the smart battery specification (SBS), they're often designed for systems that are microcontroller-based. TI cites its bq20z80 family of gas gauges (coulomb counter, on-chip algorithm, on-chip communications bus) as a breakthrough, part of a two-chip system with the company's bq29312 analog front end (24-pin TSSOP). It touts 1 percent accuracy over the life of a multi-cell lithium-ion battery in determining remaining capacity. The bq20z80, which integrates a/d current and voltage converters, two temperature sensors, flash memory and an SMBus interface in a 38-pin TSSOP, uses the company's Impedance Track technology to calculate the impedance of the battery on an on-going (real-time) basis. Incorporating the most critical component of such a system, the learning algorithm, it's designed to keep up to date on usage patterns, temperature, and age, which if not suitably addressed can lead to a 50 percent error in deriving actual battery capacity (see "Impedance no obstacle to accuracy"). Its dynamic modeling algorithm learns how much the battery has degraded through these parameters, then accurately correlates the chemical properties of the anode/cathode system in the battery's cell, independent of brand. The system can potentially support nickel-metal hydride (NiMH) and nickel-cadmium (NiCd) batteries as well.
Previously, the company released its bq2084, a precursor to the Impedance Track chip, also for use with the bq29312 analog front end. The bq2084, basically the bq20z80 without the learning algorithm (such devices constitute a class of chips often called "battery monitors"), provides 8 to 9 percent accuracy. The bq2084 was designed as a step improvement over traditional gas-gauging ICs, whose accuracy is generally no better than 20 percent, according to the company.
In somewhat of a parallel development, but for stand alone (non-microcontroller-based) versus SBS-class applications, Maxim Integrated Products (Sunnyvale, Calif.) is just announcing its DS2780 for the low-cost, single-cell portable (phones, digital cameras) market. The DS2780 is billed as a precision voltage, temperature, and current-measurement (coulomb-counter) system with basic learning-cycle algorithm and 1-wire interface (as generally the case with DS2xxx devices thus far) that can deliver as high as 1 percent accuracy; worst-case accuracy is said to be ±3 percent. Another chip, a counterpart to TI's bq2084 "battery monitor," Maxim's DS2751 uses a small EEPROM to characterize aging and the chip is said to provide a nominal accuracy of ±5 percent.
Building on its expertise in microcontrollers, Microchip Technology (Chandler, Ariz.) just released its PS501, a fuel gauge that expands on the company's earlier PS401, PS402, and PS700 versions with more functionality and the capability to work with two-three, and four-cell lithium-ion/polymer, or six- to 12-cell NiMH and NiCD sources. It contains the company's PIC18 microcontroller core, with its Accuron algorithm stored in 16-kbytes of flash memory, a 16-bit sigma-delta integrating a/d converter, and EEPROM to store user-customizable and learned battery parameters. Twelve general-purpose pins (SSOP-28 package) support charge and safety control or user-programmable digital I/O. This system provides resolution of battery voltage, current, and temperature with better than 1 percent accuracy. It reports true battery capacity to within ±1 percent from cycle to cycle with repeatable results, according to the company. The company's PS516xEV development systems are available to speed designs for the various battery types.
Several other vendors are working in the background to develop their next generation of gas gauges. Among the active participants include Intersil (Milpitas, Calif.) with its just announced ISL6295, described as a low-voltage fuel gauge that's presently under a non-disclosure agreement. Previous entries leading up to the newest generation of gas gauges include Maxim's MAX1781 "smart battery-pack supervisor" for notebooks, which integrates an 8-bit microcontroller core, coulomb-counter-based fuel gauge, V/f converter and internal oscillator,16 kbytes of EEPROM program memory, a master/slave SMBus interface, and over-current protection to eliminate the need for a separate protection IC. It touts 0.5 percent accuracy for individual cell-voltage measurements. Other special products include Maxim's DS2770, released last year and billed as the industry's only battery charger with fuel gauge circuitry and an optional sense resistor.
Improved coulomb counters include Maxim's DS2740, for single-cells, which arrived last year. Linear Technology (Milpitas, Calif.) also introduced the LTC4150, a bi-directional coulomb counter (10-pin MSOP package) to support 1- or 2-cell lithium-ion and 3- to 6-cell NiCd or NiMH batteries.
Impedance no obstacle to accuracy
With continued concern over unexpected loss of power itself a topic of conversation among cell phone and other wireless users, vendors tend towards development of gas gauges that derive remaining run time using current integration (said to be fairly accurate when the battery is fully charged), versus techniques that rely on accurate measurement of the battery's terminal voltage (low cost, but usually low accuracy). The traditional voltage-based approach correlates battery voltage with capacity, a correlation that holds well under no-load conditions. In practice, however, to calculate remaining capacity as typically demanded by the user for a given load, the gas gauge needs to estimate the battery's true sourcing capability by first subtracting the voltage drop across the battery's internal resistance. But the internal resistance is continually changing, depending on the current drawn, operating temperature, the time-variant demands of the load (device's usage profile), and particularly the battery's age (largely a function of the number of charging cycles). Each operating parameter varies considerably from cell to cell, and for different battery brands. One solution is to incorporate the best fit of time- and load-varying parameters into a look-up table and to periodically reckon remaining battery time from those tables. But because of the difficulty in determining actual battery impedance, the accuracy of such a technique can degrade wildly even for new batteries that go through relatively few charging cycles.
Current-based systems using so-called coulomb counters, versus voltage-based systems, get around the voltage-drop issue and are used in most gas-gauge ICs today. But the method isn't perfect. Battery-aging issues do not disappear, and usage profiles vary, making measurement of, or modeling, such parameters as self-discharge very difficult. In general, the traditional algorithm, according to Texas Instruments, looks at the end points of the charge states and attempts to determine actual charge without the benefit of knowing exactly how changing parameters are affecting the charge state in between. TI's bq20z80, as one example, uses an improved algorithm to address those issues. The system, which combines both current- and voltage-based methods, initially provides a precise state-of-charge estimation (i.e., a starting point) from the open circuit voltage (the open-circuit voltage versus the state-of-charge has a high correlation, defined by chemical properties of the anode/cathode system, and not by brand specific electrolytes or separator materials). Thereafter, it performs and updates real-time measurements of the actual impedance, thus doing away with the typical table that relates nominal impedance to charge-delivery capability and temperature, and other tables such as those that make corrections for a battery's self-discharge characteristic. In short, once the starting position is determined, the chip can accurately calculate total capacity from the amount of existing capacity without an otherwise needed charge and discharge cycle, says TI. With this technique, the system can determine the change in state of charge between almost any two points with a claimed error no greater than 1.5 percent; typically, it will be below 1 percent throughout the life of the battery, says the company.
Maxim Integrated Products