Overview
Abnormally high or low blood glucose can cause serious health risks, so monitoring glucose levels is critical. There are roughly 150 million people worldwide diagnosed with diabetes, creating strong demand for personal portable blood glucose monitors (BGM).
Continuous Glucose Monitor (CGM) Architecture
Figure 1 shows a continuous glucose monitor (CGM), which helps patients check glucose readings in real time and monitor glucose over extended periods. A CGM continuously tracks glucose and alerts the user when values reach dangerous levels. The device typically includes a sensor unit and an aggregator unit.
Figure 2: CGM sensor unit
The sensor unit uses a coin or button cell and connects to the body for a limited period (for example, 8 to 10 days). The aggregator unit is a battery-powered handheld device that reads glucose data over RF wireless technologies such as near-field communication (NFC). The aggregator's battery management subsystem includes a battery charger, a battery fuel gauge, and protection circuitry. A single 3.7 V lithium-ion cell can power a typical aggregator unit, and it can be charged through USB or DC input from a power adapter.
Why a Smart Battery Fuel Gauge Matters
A battery fuel gauge can predict and estimate remaining capacity, state of charge, time to empty, and overall battery health under varying load conditions, helping solve battery management challenges. With a smart fuel gauge, run time and cycle life can be extended.
Texas Instruments' Impedance Track measurement algorithm achieves more than 99% accuracy for capacity estimation, enabling precise analog measurement performance and battery modeling.
Figure 4: Extended run time with a TI fuel gauge
Implementation Options
This CGM supports several single-cell fuel gauge options that are compact, cost-effective, and very low power. The fuel gauge can be placed on the battery pack or on the system PCB, the latter being common in portable medical applications.
Figures 5 and 6 show typical system-side and pack-side fuel gauge configurations. A system-side gauge, such as the BQ27426, requires minimal user configuration and consumes very little current during normal operation. For tighter integration, some gauges include an integrated sense resistor, for example BQ27421-G1.
If the fuel gauge is inside the battery pack, a flash-based firmware solution with integrated 256-bit secure hash algorithms, such as BQ27Z561-R1, can provide high-accuracy estimation. Protection ICs like BQ2970 provide voltage, current, and reverse-charge protection.
Figure 5: Typical host/system-side fuel gauge configuration
Figure 6: Typical battery pack-side fuel gauge configuration
Benefits of Accurate Fuel Gauges
A battery fuel gauge increases the sophistication and intelligence of power management. Systems without an accurate fuel gauge can only shut down at a fixed voltage. Many devices choose a shutdown voltage of about 3.5 V to preserve reserve capacity for worst-case scenarios, but measuring only battery voltage with a microcontroller and ADC to generate a low-battery warning is not a reliable method for estimating remaining capacity because most applications have variable loads. A fuel gauge computes remaining capacity and adapts the shutdown voltage to meet required reserve capacity under any condition, thereby increasing run time.
Beyond preserving reserve capacity, some fuel gauges avoid reporting 0% charge under high transient pulse loads, preventing the battery voltage from falling below the termination voltage when there is still significant remaining charge. This behavior is beneficial when the battery has high remaining capacity but is subject to high transient loads that would otherwise force an early termination voltage.
Conclusion
Batteries are complex electrochemical systems influenced by aging, temperature, and impedance. Algorithms, compact devices, and advanced integration are key to improving system performance. What are the biggest challenges you face for battery-powered medical applications such as CGM? Share your perspective at the end of the article.
