EYE ON NPI - Texas Instruments BQ27427 System-Side Impedance Track™ Single-Cell Battery Fuel Gauge
Channel: Adafruit Industries
Date: 21/06/2023

This week's EYE ON NPI is going to fuel up your portable battery-powered designs with the Texas Instruments BQ27427 System-Side Impedance Track™ Single-Cell Battery Fuel Gauge (https://www.digikey.com/en/product-highlight/t/texas-instruments/bq27427-single-cell-battery-fuel-gauge) a powerful and inexpensive way to add high quality battery monitoring for charge-rate and aging tracking at the system side.

Lithium Ion / Polymer batteries (https://www.digikey.com/en/products/filter/batteries-rechargeable-secondary/91) - you love them for their low cost, high power density, and 3.7V nominal voltage, but keeping them recharged and letting people know exactly how much battery life is left is non-trivial. Like most batteries, common LiPoly/LiIon's start with a high peak voltage right after charging of about 4.2V, drop quickly to about 3.7V and then slowly drift down till they get to 3.0V at which point the cut-off circuitry disconnects until they are recharged. It's really easy to tell when the battery is 90% or greater charge, just look for a voltage of 3.9V or higher. It's also pretty easy to tell when they're 10% or less, the voltage will be 3.3V or lower. But that in-between section is tough because the slope during discharge is very shallow and the voltage changes quite a bit with aging, temperature, and discharge rate (https://www.ti.com/lit/an/snva533/snva533.pdf)  So while there are battery monitors that can use just the voltage across the battery terminals, they're not going to be as precise as a coulomb-counter design that takes into account both voltage and current.

From TI's White Paper on the topic (https://www.ti.com/lit/wp/slpy002/slpy002.pdf):

 "An impedance-based battery fuel gauge, as the name implies, uses the measured impedance of the battery’s cells as a key input to its remaining capacity predictions. The gauge measures and stores in real-time the battery pack’s resistance as a function of state-of-charge. The real-time resistance profiles along with the stored battery open-circuit voltage tables (open-circuit voltage vs. state-of-charge) enables the gauge to predict the battery pack’s discharge curve (by adjusting for the IR drop) under any system-use condition and temperature. The algorithm uses current integration (coulomb counting) when the system is ON, and open-circuit voltage measurement when the system is OFF or in SLEEP to adjust remaining state-of-charge (RSOC) up or down (for charge or discharge) the predicted discharge curve. By using the predicated discharge curve, the gauge can accurately calculate the battery pack’s remaining discharge capacity (RM) and the system’s run-time to empty (RTTE).

Since an impedance track gauge continuously adjusts RM and FCC for impedance and the change in impedance, rate and temperature inefficiencies and aging are inherently taken into account, enabling the gauge to maintain a high level of accuracy throughout the life of the system."

Texas Instruments has a few chips in the Impedance Track family (https://www.ti.com/video/6287050244001) - the BQ27427 charger (https://www.digikey.com/en/products/detail/texas-instruments/BQ27427YZFR/17748369) is the latest to come out. It's very small, coming as a 9-pin 1.6x1.6 BGA with the center pad being a shared ground so you don't need plugged vias to use. It's also really easy to use, requiring only 2 small stabilization capacitors. Internal temperature monitoring means you don't need a separate thermistor. The high-side 7 mΩ current sense resistor also means one less part on the BOM. I2C pins are used to communicate with the monitor and set battery characteristics. There's also an optional input that can be used to connect to a mechanical or electrical battery insertion switch or to an external thermistor, and an optional output that can be used as a battery low indicator.

Because so much is integrated, and the chip is so small, its very easy to pop it into your design between the battery terminal and the VBAT lines that go to your battery charger and power supply. The peak battery voltage and capacity are easy to look up since they're written on the battery. For other configuration settings you can use BQStudio / GaugeStudio (https://www.ti.com/tool/BQSTUDIO) with the TI battery devboard to characterize and customize the learning cycle. Either way, you will need to write the configuration to the sensor on every boot.

Pick up the Texas Instruments BQ27427 (https://www.digikey.com/short/18pb1ftw) and other chips from the Impedance Track family (https://www.digikey.com/en/products/result?s=N4IgjCBcoLQCxVAYygMwIYBsDOBTANCAG4B2aWehA9lANrhwCsAnBALqEAOALlCCAF8hQA0) stocked at DigiKey for immediate shipment. Order today and you'll get precision lipoly monitoring that will work with any setup or battery configuration by tomorrow afternoon!