Wednesday, September 19, 2012

Testing Battery Pulse Characteristics

Low power wireless devices typically sleep most of the time and then wake up to send a message and then go back to sleep again. In a previous post I discussed battery selection for low power wireless devices. One issue with using coin cells is that they have poor pulse characteristics, and this performance varies across manufacturers.

To figure out which battery will work for you (and get a feel for battery life) you will need to run a test. This test will involve pulsing the battery, waiting, and then pulsing it again; continuing until the battery voltage under load will no longer be sufficient for your product.

Pulse Waveform
First, you need to get the waveform of what a typical pulse looks like. The easiest way is to supply your circuit through a one-ohm resistor and capture a typical pulse on an oscilloscope. Below is an example.

To test battery life you will need to be able to reproduce this current draw. So, measure the current pulse. You don't need to get that fancy; just measure the width and height of the main pulse. For example, it may be 30mA for 5mSec. To reproduce the pulse you will need a suitable circuit. A simple MOSFET and load resistor will work. For 30mA at 3V use a 100 ohm resistor (V=IR).

Pulse Generation
To generate the pulses you can use an arbitrary waveform generator, or if it's just simple on/off, you could even just write code to do it on a microcontroller development board if that's easier. For the most accurate estimate of battery life as measured by number of pulses you will want to allow as much "recovery time" as possible for the battery between pulses. Obviously you can't wait too long though; if you pulse every 6 seconds and your battery lasts for 45,000 pulses then this will take 3 days to measure. Typically we will run a "slow" test and "fast" test simultaneously to see if wait time has any affect.

Remember that you need to measure the voltage of each pulse too to ensure that it it above your threshold voltage. This can be done in a few ways:
  • Analog DAQ, like those from National Instruments, although often these are not fast enough
  • Multimeter with output, like an Agilent 34410A DMM. This is what we used with good results. Some custom processing of the output may be required though.
  • Custom code on a microcontroller. Come to think, it might just be the easiest way, especially if you are using the microcontroller to generate the pulses too.
Whatever option you select, record how many pulses it took before failing. It might also be good to record a typical voltage measurement (like voltage readings every millisecond of the pulse) so you can see how it fades.

Test Strategy
For good measurement accuracy, perform the test on a sample of identical batteries, not just one. Five or so is sufficient. If you see lots of variability then you know to definitely not use that vendor. Test batteries from a few different vendors and use this information to determine which battery is best for you.

You also now have a good indication of real battery life and which vendor's battery is best for you. When calculating total battery life be sure to include sleep current consumption as well as the current consumed by receiving any data too.

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