Measuring Battery Life on Battery Powered Medical Devices

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1 Measuring Battery Life on Battery Powered Medical Devices By Bob Zollo, Keysight Technologies, Inc.* *Keysight Technologies Inc., formerly Agilent Technologies electronic measurement business Battery powered devices are everywhere in medicine, from pacemakers to patient monitoring telemetry to operating room tools. For all of these devices, battery runtime is a key requirement. Any medical professional can easily understand the impact of battery life of the electronic device they are using. Insufficient battery life easily dissatisfies users. In some cases, it could be a matter of life and death. Therefore, extending battery life by lowering power consumption is a main driver for all aspects of the design of medical electronics. Therefore, to know the run time of your device is important as a means to know if improvements in power management within the device are successful at increasing battery run time and if your device will meet the demanding needs of its end user. Considerations for Battery Run Down Testing Battery run time is determined by a battery run down test. In this test, you run your device, starting with a full battery and measure the time it takes until it stops working. The time that is measured is the battery run time. While this is easily said, it is not so easily done, as there are many variables in this test. Battery run down test variable The battery The battery s charge state The use case of the device Considerations There can be inconsistencies between batteries, even the same type from the same manufacturer, due to batteries coming from different manufacturing dates or different factories. To get the right run time, you need to use a fully charged battery. If the battery is partially discharged or old (reduced capacity), the runtime will be shorter. During a run down test, you will run the device, but what does run mean? Different tasks will pull different amount of current Solution Perform the run down test several times with different batteries. Be sure to use fully charged batteries. Condition the battery before the test by using a battery cycler to fully discharge then fully recharge the battery. Determine a standard use case. During the run down test, apply the same use case to

2 How to measure or determine when the device stops working and therefore run down the battery at different rates. In more advanced devices, this moment when the device stops could be determined by when the device gives a low battery notification. In a more basic device, like a flashlight, this moment could be when the light goes out. hold that variable constant across each test run. Use the battery voltage as a proxy measurement; i.e., measure the run time until the battery voltage reaches some low voltage threshold and consider reaching that threshold voltage to be the indication of when the device will stop working. Battery Simulation Some engineers have considered using a power supply to simulate the battery during a run down test. To date, this has not been practical. While it may be a hassle to ensure battery state consistency (type and charge), using a power supply to simulate a battery introduces additional variables and test errors. A standard power supply will not act like the battery as it will never run down, so the run down test will never reach an end condition. Instead, the power supply will need specialized features to be a battery emulator. Part of the emulation is to have a controllable output resistance and to have excellent transient response with respect to current pulses being drawn by the device. But to fully emulate a battery, the power supply s output voltage will need drop off as charge is pulled from the power supply into the device during the run down test. This simulation of draining the battery from full to discharged is challenging and sophisticated battery models must be employed. If the battery model is not adequate, the results obtained when using the power supply will not match the results obtained when using the battery. Until a good battery modeling solution becomes available, the best way to do a run down test is to use the real battery, as this will give the exact same results that the end user will experience. Engineers Want to See More than Just Run Time When designing a device, you will want additional insight into what is happening during the test, beyond just measuring the run time. This will additionally mean measuring the current being drawn from the battery and the voltage on the battery simultaneously. By plotting voltage and current versus time, a complete picture of a battery run down is achieved (figure 1).

3 Discharge Current Battery Voltage End of Discharge Voltage T 0 Run time Figure 1 Battery run down test results To simultaneously measure battery voltage and current flowing between the battery and the device, you will need two DMMs, 2 channel data logger, or a 2 channel digitizer. To measure the battery voltage is almost trivial, as the voltage will change slowly, so a DMM or data logger making measurements as slow as once per second should be fast enough to capture the slowly decaying voltage waveform. But measuring the current can be a much bigger challenge. In many battery powered devices, sophisticated power management schemes are used to increase run time. These schemes turn subsystems on and off for hundreds of microseconds as needed within the device to conserve power. The result is a rapidly changing current waveform that can range from microamperes to amperes. DMM s make integrated measurements that can take hundreds of milliseconds so they cannot capture a rapidly changing current waveform. Another issue with using a DMM is burden voltage. When a DMM is configured as an ammeter, the current to be measured is flowed through a calibrated current shunt inside the DMM. The DMM measures the voltage drop across the shunt and calculates the current. The voltage drop inside the DMM reduces the available voltage at the DUT and hence places a burden on the circuit. This burden voltage can be hundreds of millivolts (figure 2).

4 Ammeter V meas shunt Typical ammeter shunt values: 0.1 Ω for 1A and higher 5 Ω for 100 ma and lower V burden V bat I bat V DUT DUT V DUT = V bat V burden Example calculation of burden voltage of ammeter: On 100 ma range, 50 ma of I bat yields V burden of 250 mv, so a 4.2 V battery would be reduced to 3.95 V at the DUT. Figure 2 DMM presents burden voltage when measuring current When trying to measure a rapidly changing waveform over a long period of time, a digitizer seems like the best choice. Digitizers have sufficiently wide bandwidth to capture rapidly changing waveforms, but digitizers don t directly measure current so a current shunt must be used. If the dynamic range of current to be measured is microamperes to amperes, what size shunt should be selected? If the shunt is sized to measure the lowest current accurately, there will be a large voltage drop across the shunt during the high current events and this will place an unbearable burden voltage on the circuit. If the shunt is sized for the high current, at low currents there may not be enough voltage for the digitizer to measure accurately. Therefore, you may need to compromise on low level current measurement accuracy by selecting a shunt that is suitable for high currents (with acceptable burden voltage drop) and low currents (with a voltage drop that may be at the low limits of the digitizer s ability to measure). A Solution for Battery Run Down Testing Keysight Technologies offers the N6781A Battery Drain Analyzer and turnkey software for performing run down tests (figure 3) for battery powered devices requiring up to 3A of current. This N6781A can be configured as a zero burden ammeter, meaning there is zero voltage drop across the instrument as it measures current flow between the battery and the device. It offers a unique feature called seamless ranging, so that it can instantly and automatically change range and measure currents from microamperes to amperes at a speed of 100,000 samples per second without losing any data during the range change, making it ideally suited for measuring dynamic currents during run down tests. Furthermore, it can simultaneously measure the voltage across the battery. With

5 the Keysight Control and Analysis Software, a battery run down test can be quickly setup and run down measurements captured and plotted without writing any software. Figure 3: The Keysight N6781A Battery Drain Analyzer and 14585A Control and Analysis Software are a turnkey solution for battery run down tests on battery powered medical devices.