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The results of these calculations show that the device will meet the five day runtime requirements with a battery life of over 5.1 days. This is deceiving, however, as this does not consider the shelf life of the system. In the medical device industry, it is best practice to design for a 14-month shelf life (12 months on the shelf, two months in transit).
SHELF-LIFE CONSIDERATIONS Summing the shutdown currents of the devices in the system while using the typical one to two per cent per year self-discharge rate of a CR2032 battery, one can see that, after 14 months, the battery does not have enough capacity to support a five-day run time and a battery seal is required. The battery capacity after 14 months on the shelf will be severely diminished. Nearly 40 per cent of the CR2032’s energy will be consumed by shutdown currents and battery self-leakage while sitting idle on the shelf. Inserting this battery capacity into Equation 3, a more accurate run time can be determined:
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
While sitting on a shelf for over a year, the battery capacity is impacted by battery self-discharge and system shutdown currents. Battery self-discharge is a function of the battery chemistry and environment. A CR2032 battery has a lithium manganese chemistry and a self-discharge rate of one to two per cent a year. At the end of a year, the coin cell battery can lose two per cent of its capacity while sitting dormant. Meanwhile, a BR2032 battery has a lithium carbon monofluoride chemistry and a self- discharge rate of 0.3 per cent per year. It is easy to assume that the best battery chemistry for the application is the one with the lowest discharge rate but that is not necessarily the case. While the BR2032 battery has a lower discharge rate, it also has a lower capacity than a CR2032 battery cell with 200 mAh. By recalculating using the previous formulas, one can determine if such a lower capacity battery is adequate.
In this ECG patch, the IC shutdown currents are the greatest contributors to reducing battery life while the system is powered off. Shutdown current is drawn when an 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 that draw small amounts of current even when there is no load. These currents are
usually small (under 1 µA) but can have a massive impact on battery life. In this RPM patch, shutdown currents can reduce battery capacity by as much as 40 per cent in a year. A battery seal can be used to limit the system from drawing too much current from the battery while in shutdown. Two common options for battery seals are mechanical battery seals in the form of a mylar
10 TABLE 2. BATTERY CAPACITY AFTER 14 MONTHS
2% Battery Capacity Leakage (mAh) Standby Current Consumption (mA) Shelf Life—Hours Shelf Life—Days Shelf Life—Years Capacity After 14 Months (mAh) Percent Capacity Left After 14 Months
230.30 0.0082 28085.37 1170.22 3.21 146.66 63.68
TABLE 3. BATTERY CAPACITY AFTER 14 MONTHS WITH A BATTERY SEAL
2% Battery Capacity Leakage (mAh) Standby Current Consumption (mA) Shelf Life—Hours Shelf Life—Days Shelf Life—Years Capacity After 14 Months (mAh) Percent Capacity Left After 14 Months
230.30 0.000005 46060000.00 1919166.67 5257.99 230.25 99.98
August 2023 Instrumentation Monthly
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