Smarter: How smart battery fuel gauges can improve battery life in continuous glucose monitors

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Smarter: How smart battery fuel gauges can improve battery life in continuous glucose monitors [Copy link]

 

High or low blood sugar levels can lead to serious health threats, so monitoring blood sugar levels is of utmost importance. Currently, 150 million people in the world suffer from diabetes, so there is a huge demand for personal portable blood sugar monitors (BGM).

The continuous glucose monitor (CGM) shown in Figure 1 can help diabetic patients check their blood glucose readings in real time or monitor blood glucose values over a long period of time. CGM can continuously monitor blood glucose levels and then alert the user when the user's blood glucose value reaches a dangerous value. This monitor usually includes a sensor unit shown in Figure 2 and an aggregator unit shown in Figure 3.

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Figure 1: Continuous Glucose Monitor (CGM)

This sensor unit uses a button cell or coin cell battery and is connected to the body for a certain period of time (e.g., 8 to 10 days). The aggregator unit is a battery-powered handheld unit that can read blood glucose data using wireless radio frequency (RF) technology such as near field communication. The battery management subsystem of the aggregator unit consists of a battery charger, battery fuel gauge, and protector. A single 3.7 V lithium-ion battery can operate a typical aggregator unit. It can be charged through the USB or DC input of the power adapter.

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Figure 2: CGM sensor unit

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Figure 3: Schematic block diagram of a CGM aggregator unit

Battery fuel gauges can help solve battery management challenges by predicting and estimating the remaining capacity, state of charge, time to empty, and health of the battery under different load conditions. With smart battery fuel gauges, users can extend the operating time (as shown in Figure 4) and battery cycle life. Texas Instruments' Impedance Track measurement algorithm achieves battery capacity prediction with an accuracy of more than 99%, giving it excellent analog measurement performance and battery characteristic modeling capabilities.

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Figure 4: Long runtime with TI fuel gauge

This blood glucose monitor offers multiple single-cell battery measurement options in a compact, cost-effective form factor with ultra-low power consumption. The fuel gauge can be located within the battery pack or on the system PCB, the latter being more common in portable medical applications.

Figure 5 and Figure 6 show typical system-side and group-side fuel gauge configurations, respectively. Fuel gauges on the system PCB, such as the BQ27426, require minimal user configuration and consume little current during normal operation. For a higher level of integration, some fuel gauges have integrated sense resistors, such as the BQ27421-G1.

On the other hand, if the fuel gauge is inside the battery pack, a solution with high accuracy can be provided through flash-based firmware and a 256-bit integrated secure hash algorithm such as the BQ27Z561-R1. Protection ICs such as the BQ2970 provide voltage, current, and reverse charger protection.

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Figure 5: Typical host/system-side fuel gauge configuration

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Figure 6: Typical battery pack-side fuel gauge configuration

Battery fuel gauges bring sophistication and intelligence to power management. Systems without an accurate fuel gauge can only shut down at a fixed voltage. Many devices shut down at 3.5 V to preserve the reserve capacity for the worst case (reserving power for shutdown), but as shown in Figure 4, simply measuring the battery voltage with a microcontroller and analog-to-digital converter to generate a low-battery warning is not a reliable way to measure the remaining capacity. This is because most applications have variable loads. A battery fuel gauge will calculate the remaining capacity and change the shutdown voltage to meet the required reserve capacity requirements under any conditions, thereby increasing the operating time.

In addition to the advantage of retaining reserve capacity, some battery fuel gauges are able to not report a 0% state of charge due to high transient pulse loads generated by the application, causing the battery voltage to drop below the termination voltage. This feature is advantageous when the battery still has a high charge, but high transient loads can cause the termination voltage to be reached prematurely.

Batteries are complex electrochemical systems that are affected by cell aging, temperature, and impedance. Algorithms, compact devices, and advanced device integration are all key features to improve system performance. What are the biggest challenges you face in medical battery-powered applications such as continuous glucose monitors? Please tell us your thoughts at the end of this article.

GuyGraphics

Thanks for sharing!

alan000345

GuyGraphics posted on 2019-11-14 13:30 Thanks for sharing!

You're welcome, hope this helps.