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pull tab and an electrical battery seal in the form of a load switch. Mylar/plastic pull tabs provide a mechanical battery seal where a plastic tab sits between the battery and the system. When the device is ready to be used, the user simply extracts the plastic tab and the battery begins powering the system. This is a simple, cheap, and proven mechanical battery seal that has been employed for many years. However, for medical devices, this solution is not always feasible. For an ECG patch where waterproofing is required, the slot that the mylar protrudes from can make the patch susceptible to water damage. In addition, the small plastic tab could potentially be difficult to use for an end user with low dexterity. A simple load switch, like the Vishay SiP32341, would make an excellent choice for an electrical battery seal. This device is a FET that, when open, blocks the battery from the rest of the system leaving the SiP32341 shutdown current as the only draw on the battery. The load switch has a logic control line that can be turned on via push button when the device is ready to be used. The SiP32341 has a 14 pA typical shutdown current, which is a dramatic improvement over the current draw of the entire system if no battery seal were in place. When the SiP32341 is used as a battery seal, a CR2032 primary cell retains 99.97 per cent of its capacity over the course of 14 months. When no battery seal is used to protect the battery from the ECG patch shutdown currents, a CR2032 primary cell only retains 62.39 per cent of its original charge. This 37 per cent difference in capacity is what allows for the ECG patch to meet the five-day requirement after a 14-month shelf life. A battery seal preserves battery capacity by preventing all the devices in the system from drawing shutdown currents from the battery. Over 99.9 per cent of the battery capacity is remaining after the RPM patch sits idle for 14 months. Inserting this battery capacity into Equation 3, a more accurate run time can be determined:
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
CONCLUSION
The battery analysis of a system when it is active and when it is in shutdown/low power mode is critical to designing a power supply that will meet all the requirements for a medical device. While this article exclusively discussed an ECG patch that gathers heart rate, temperature, and acceleration data with BLE communication, the analysis and principles within this article can be applied to any number of medical device systems powered by a primary cell battery.
Analog Devices
www.analog.com Instrumentation Monthly August 2023 11
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