Texas Instruments has developed a ‘gas gauge’ technology that calculates remaining capacity in lithium-based battery packs with up to 99% accuracy throughout the entire life of a battery.
TI’s Impedance Track technology precisely gauges changes in impedance, or resistance caused by battery age, temperature and cycle patterns, to accurately predict run-time of two-, three- and four-cell battery packs.
The technology, which sits inside TI’s Flash-based bq20z8x gas gauge chipset, analyses precise state of charge when a battery pack is in a relaxed state by correlating between a battery pack’s open circuit voltage and the current state of charge and temperature.
An exact ‘starting position’ is determined for instant state of charge, and total capacity is calculated from the amount of capacity that exists – eliminating the need for a full charge and discharge cycle. For certain applications such as heart defibrillators or back-up battery packs used in telecom systems that never fully charge and discharge, Impedance Track will ensure instant and accurate capacity information on a continual basis.
Impedance Track relies on a dynamic modelling algorithm to learn how much a battery has degraded through age, temperature or usage, and then correlates typical chemical properties of the anode/cathode system in the battery’s cell – no matter what brand of battery cell used. It also allows for the mixing of different manufacturers’ cells in a single pack.
Many of today’s gas gauge IC technologies depend on static and unreliable modelling techniques that require the creation of large databases to measure each attribute of hundreds of available battery parameters. The Impedance Track technology significantly reduces development and implementation set-up time required by original equipment designers and manufacturers to ensure proper characterisation because those databases are no longer necessary.
The two-chip bq20z8x gas gauge reports capacity information to the system host controller over a system management bus (SMBus) interface. A host controller, such as a TMS320C55x digital signal processor, manages remaining battery power to extend system run time. The bq20z8x chipset also features TI’s protection analog front end (AFE) chip, the bq29312, with integrated 3.3-V, 25-mA linear dropout (LDO) regulator.
Initially targeting applications with multi-cell, lithium-based battery packs, Impedance Track also is capable of supporting other types of battery chemistries, such as nickel-metal hydride (NiMH) or nickel-cadmium (NiCD). In addition, TI plans to target single-cell Li-Ion powered applications, such as smart phones, digital still cameras and PDAs.
The bq20z8x gas gauge chipset with Impedance Track is sampling today from TI and its authorised distributors. Volume production is scheduled for fourth quarter 2004. The chipset is packaged in a 38-pin thin shrink small outline package (TSSOP), and the bq29312 in a 24-pin TSSOP package.
Suggested resale pricing in 1,000 piece quantities is $4.35 each for the bq20z8x and $1.20 each for the bq29312. A second-level protection IC, the bq29400 is also available from TI in an eight-pin DCT package priced at $0.42 each in quantities of 1,000 units.
Evaluation modules of the bq20z8x, complete reference designs, application notes and technical documentation are also available.