Inevitably when designing battery powered devices, someone asks “how long will the batteries last”. Usually a few calculations are made followed by liberal amounts of hand waving to get an answer. At Xology our goal is to scientifically prove the battery lifetime. To do this we go through the following steps:
Device Current Profiling
Static - while sleeping
Dynamic - while active
Once the firmware has a reasonable approximation of what the actual current profile will be, we profile the device. This includes measuring static current and dynamic current. To measure static current, we typically put the device into its low power mode and then measure quiescent current with a milliammeter. This will typically measure in the microamperes, down to 2µA for very low power sensors.
Next we measure active current consumption for each mode. For example, in a BLE device there are at least 3 modes:
We determine the current profile for each mode. Current profile is current vs. time and is used to calculate the battery life and load the battery characterizer with the accurate profile. For example, the screen capture below shows the current profile for a BLE advertisement. The X axis is time and the Y axis is current.
Next, we use the measured current profile to perform accelerated testing on the battery. This is done with our Battery Test BoosterPack, shown to the right. This device is custom made for battery testing, and allows us to test eight batteries in parallel. We load the current consumption profile into this device, defining it exactly down to each milli-second and milli-ampere. We’ll repeat the profile at an accelerated rate, for example once per second instead of once per day. This device measures battery voltage and test chamber temperature once per minute.
To control temperature the entire test assembly is placed in a temperature and maintained at a steady 21C +/-1C. This ensures repeatability across multiple test cycles.
The end result of this process is a graph of battery voltage over time, such as the one shown below for CR2450 batteries.
Finally, with this measured data, we are able to statistically define our device’s battery life, including:
How long the battery will last
How to accurately predict when we should set low battery thresholds
If you are designing a battery powered device, this information is vital to the success of the product. With our method we can help you stop guessing and start knowing. Contact us at the link below to get started.