Gas gauging solutions for Li-ion batteries: Part 1

Since it’s commercialization by Sony in 1991, lithium-ion (Li-ion) batteries have been widely adopted in portable devices for consumer, industrial and medical markets. The Li-ion battery is now making its way into many more sectors, replacing existing primary (nonrechargeable) batteries and lead-acid batteries due to its high energy density.

One thing that many system designers overlook is the importance of a battery fuel gauge integrated circuit in their system. Fuel gauge ICs were typically considered an optional feature for many applications. Yet this device provides fundamental information: when the battery will be empty and how much energy is left in the battery. Early devices had primitive solutions such as an indicator when the battery neared empty, or a number of bars to indicate relative capacity estimates. In some devices this might be enough, but as devices get more complex and integrated into people’s everyday lives, better indicators are definitely needed.

Texas Instruments has years of experience producing Li-Ion fuel gauges for rechargeable batteries. In this vast portfolio, it’s easy to get lost in the details, so in this post I’ll focus on only two of the premier low cost gas gauges. The bq27421-G1 and the bq27441-G1, are system-side gas gauges that aim to reduce cost for gauging secondary (rechargeable) batteries.

The bq27421-G1 with Impedance Track™ technology comes in a nine-ball chip-scale package (CSP). To reduce and simplify total solution size and cost, it has an integrated sense resistor inside the IC, thus saving sense-resistor cost. For long-term reliability of the solder bumps through which the current flows, current should be limited to 2.5A continuously and 3.5A peak. This device supports consumer devices where space is constrained and the system current does not exceed the data-sheet limits.

In addition to the 9-ball CSP version, the bq27441-G1 with Impedance Track technology gas gauge comes in a slightly larger 12-pin quad flat no-lead (QFN) package. The device functionality is the same as the bq27441-G1, but it uses an external sense resistor to eliminate the peak-current limitation.

Both devices aim to reduce cost by using read-only memory (ROM) memory instead of flash memory, as is used in other reprogrammable gauges like the TI bq27541 series. The devices have an on-board temperature sensor, eliminating the need for a negative temperature coefficient (NTC). Furthermore, there are fewer parameters to configure when compared to typical flash-based gas gauges. These low-cost gas gauges perform the gauging function in a completely stand-alone fashion, requiring only a microcontroller (MCU) to configure the random access memory (RAM) parameters upon power on reset (POR) or a hard-reset command.

These and other low-cost gas-gauging solutions from Texas Instruments give designers the fastest and simplest path to add gauging functionality to their systems, while still providing good accuracy at a reduced cost. If maximum flexibility in gauging features and performance is required, flash-based gas gauges are also available.

For a demonstration of how to use these two devices with their evaluation modules (EVMs), please check out these unboxing videos: