How to maximize battery life? You must master these power design elements!

【Introduction】Remote patient monitors (RPMs) are constantly evolving to include more and more features that allow physicians to gain a deeper understanding of their patients’ health. These features place higher demands on the single battery that powers the monitor. This article provides a power solution for ECG (electrocardiogram) remote patient monitoring patches designed to extend battery life and maximize monitoring. This article also describes strategies for accurately estimating RPM battery life and ways to extend battery life before RPM is powered up.

The Internet of Things (IoT) revolution has created a paradigm shift in the way healthcare organizations use technology to provide real-time care to patients. Today, remote patient monitoring is one area where this new medical device is changing the way doctors interact with patients. Smaller integrated circuits and wireless communications allow decades-old devices to be updated with enhanced functionality, improving patient compliance and outcomes. Replacing the bulky Holter devices of the past, the current remote patient monitoring patch contains a variety of sensors capable of collecting heart rate, temperature and accelerometer data. The patches transmit patient data to the cloud, where both patients and doctors can access the data in real time.

While these devices help physicians improve care, they also present challenges for power designers, who must balance system performance with battery life requirements. These challenges are further exacerbated as second-generation patches employ multimodal sensing to improve accuracy and effectiveness, which in turn places higher demands on the power supply.

In this article, we will refer to the ECG RPM patch example shown in Figure 1. The patch continuously monitors the ECG and accelerometer while checking the temperature every 15 minutes. Data is transferred every 2 hours via Bluetooth® Low Energy (BLE) for a total of 12 BLE transfers per day. The patch has three different modes, each with a different load mode: standard monitor, temperature monitor and transfer mode. In standard monitoring mode, only the ECG and accelerometer are monitored. In temperature monitoring mode, another temperature sensor is also monitored. In transmit mode, the BLE radio transmits data while simultaneously monitoring ECG and accelerometer data.

How to maximize battery life? You must master these power design elements!

Figure 1. Schematic diagram of an ECG patch power supply. A 235 mAh CR2032 lithium coin cell battery powers the regulator, microcontroller, ECG front end, temperature sensor, and accelerometer.

power challenge

Designing RPMs, such as ECG patches, is a multifaceted challenge for power supply designers. Designs are often space-constrained, and patches with multiple sensors may require multiple power rails. Because RPM patches are typically single-use products, designers typically opt for a cost-effective power source such as a coin cell battery. Designers must also consider the efficiency of the power subsystem if only using a coin cell battery to power the patch.

One challenge that power supply designers often overlook is how to extend the shelf life of their products. Shutdown current and battery self-discharge can shorten the life of any system. Therefore, the designer must determine whether the RPM patch can meet the runtime requirements after a typical shelf life, and if not, what can be done to preserve battery life before the patch reaches the end user.

Determining battery runtime

To accurately determine whether a power solution meets battery life requirements, a load curve must be determined. The load curve is a simple representation of the system load duty cycle. For the remote patient monitoring patch we use, we will consider the three different operating modes introduced earlier: standard monitoring, temperature monitoring, and transmission mode.

To accurately determine whether a power solution meets battery life requirements, a load curve must be determined. The load curve is a simple representation of the system load duty cycle. For the remote patient monitoring patch we use, we will consider the three different operating modes introduced earlier: standard monitoring, temperature monitoring, and transmission mode.

For load profile analysis, the time period of each operating mode of the day is used to determine the duty cycle calculation. Use Equation 1:

How to maximize battery life? You must master these power design elements!

The duty cycle of the patch can be obtained for us, as shown in Table 1.

How to maximize battery life? You must master these power design elements!

Table 1. The duty cycle of the chip in different operating modes

Using the load curve in Figure 2, we can calculate the current consumption of the patch. Taking the effective current consumption in each operating mode, an approximation of the average current consumption per day can be calculated by Equation 2.

How to maximize battery life? You must master these power design elements!

Figure 2. Load curve graph.

How to maximize battery life? You must master these power design elements!

Here is an example calculation:

Standard monitoring mode current per day = standard monitoring mode current × standard monitoring mode duty cycle × 24 hours

Standard Monitor Mode Current = 1.88 mA Standard Monitor Mode Duty Cycle = 0.9956

Current per day in standard monitoring mode = 1.88 mA × 0.9956 × 24 hours = 44.92 mAh/day

Once the daily current draw for each operating mode is determined, the battery life can be determined by Equation 3.

How to maximize battery life? You must master these power design elements!

Here is an example calculation:

Battery capacity = 235 mAh

Current per day in standard monitoring mode = 44.92 mAh/day

Current per day in temperature monitoring mode = 0.01 mAh/day

Current per day in transfer mode = 0.79 mAh/day

Battery life (days) = 235 mAh/(44.92 mAh/day + 0.01 mAh/day + 0.79 mA/day) = 5.14 days

These calculations show that the device will meet the 5-day operating time requirement with a battery life of over 5.1 days. However, this result is deceptive because the shelf life of the system is not taken into account. In the medical device industry, it is best to design for a 14-month shelf life (12 months for shelf life and two months for shipping).

Shelf Life Considerations

Summing up the shutdown currents of the devices in the system, while using the typical self-discharge rate of 1% to 2% per year of the CR2032 battery, it can be seen that after 14 months, the battery capacity is not sufficient to support 5 days of work, and it is necessary to carry out The battery is sealed.

How to maximize battery life? You must master these power design elements!

Table 2. Battery capacity after 14 months

After 14 months on the shelf, the battery capacity will decrease significantly. When the CR2032 is idle on the shelf, nearly 40% of its energy will be dissipated through shutdown current and battery self-leakage. Substituting this battery capacity into Equation 3 gives a more accurate run time:

Battery life (days) = 146.66 mAh/(standard monitoring mode + temperature monitoring mode + transmission mode)

Battery life (days) = 146.66 mAh/(44.92 mAh/day + 0.01 mAh/day + 0.79 mA/day) = 3.21 days

On shelves for more than one year, battery capacity will be affected by battery self-discharge and system shutdown current. Battery self-discharge is related to the battery chemistry and environment. The chemistry of the CR2032 battery is lithium manganese, which has a self-discharge rate of 1% to 2% per year. After a year, a coin cell battery can lose 2% of its capacity in hibernation. In contrast, the BR2032 battery chemistry is a lithium-fluorocarbon polymer with a self-discharge rate of 0.3% per year. We often think that the best battery chemistry for an application is the one with the lowest discharge rate, but this is not the case. Although the BR2032 battery has a lower discharge rate, it also has a lower capacity than the 200 mAh CR2032 battery. Recalculation using the previous formula can determine whether such a low-capacity battery has sufficient power.

In this ECG patch, when the system is powered off, the IC shutdown current is the biggest factor reducing battery life. Shutdown current occurs when the IC is disabled and there is no active load. These currents are usually due to leakage in the IC and ESD protection devices within the IC, which consume small amounts of current even when there is no load. These currents are typically small (under 1µA) but have a large impact on battery life. In this RPM patch, the shutdown current can reduce battery capacity by as much as 40% within a year. Using a battery seal limits the system’s ability to draw excessive current from the battery during shutdown.

Battery seals are typically done in two ways: mechanical battery seals in the form of Mylar pull tabs, and electrical battery seals in the form of load switches. Mylar/plastic pull tab is a mechanical battery seal where the plastic pull tab is located between the battery and the system. When the device is ready to use, the user simply pulls out the plastic tab and the battery starts powering the system. This is a simple, low-cost, proven battery mechanical seal that has been in use for many years. For medical devices, however, this solution is not always possible. For ECG patches that need to be waterproof, the protruding grooves in the mylar make the patch vulnerable to water damage. In addition, the small plastic sheet may not work well for the less dexterous end user.

Simple load switches, such as the Vishay SiP32341, are a good choice for electrical battery sealing. The device is a field effect transistor that, when turned on, isolates the battery from the rest of the system, making the SiP32341’s off current the only current drain on the battery. The load switch has a logic control line that can be turned on by a push button when the device is ready to use. The shutdown current of the SiP32341 is typically 14 pA, a significant improvement with battery sealing compared to the current consumption of the entire system without battery sealing. If SiP32341 is used as the cell seal, the CR2032 primary cell can maintain 99.97% capacity for 14 months. Without the cell seal to protect the cell from the ECG patch turn-off current, the CR2032 primary cell can only hold 62.39% of its original charge. Eliminating this 37% capacity difference allows the ECG patch to meet the 5-day lifetime requirement after a 14-month shelf life.

How to maximize battery life? You must master these power design elements!

Table 3. Battery capacity after 14 months with battery seal

The battery seal preserves battery capacity by preventing all devices in the system from drawing battery shutdown current. After 14 months of idle RPM patch, the remaining battery capacity remained above 99.9%.

Substituting this battery capacity into Equation 3 gives a more accurate run time:

Battery life (days) = 230.25 mAh/(standard monitoring mode + temperature monitoring mode + transmission mode)

Battery life (days) = 230.25 mAh/(44.92 mAh/day + 0.01 mAh/day + 0.79 mA/day) = 5.04 days

in conclusion

Battery analysis while the system is active and in shutdown/low power modes is critical to designing a power supply that meets all the requirements of a medical device. While this article specifically discusses ECG patches that collect heart rate, temperature, and acceleration data via BLE communication, the analysis and principles presented here can also be applied to any number of medical device systems powered by primary batteries.

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