Monday, September 17, 2012

Battery Selection for Zigbee and Low Power Wireless

Low Power Wireless Standards include Zigbee, Bluetooth Low Energy, Dash7, you name it. Often times the newest wireless standard makes a claim like "lasts 4 years on a coin cell battery!" This is usually in the earliest days of the standard before anyone has actually tried it.

Battery selection for low power wireless standards is very important. These systems typically need to run for a long time (months, if not years) and have many demands. Our ideal battery for these systems:
* Lots of capacity (measured in amp-hours)
* Small size
* Excellent peak current capability (a wireless node can pulse up to 100mA)
* Fairly flat voltage curve
* Very low self-discharge
* Dimensional stability (doesn't swell)
* Low cost
The challenge is that we're asking quite a lot from a battery. We want it to discharge very little while the device is sleeping but then also to discharge in large pulses when we transmit. Usually batteries that have excellent peak current capability have a higher self-discharge rate, and vice-versa.

When starting a new project, people will often leap into creating a fancy battery lifetime spreadsheet, showing how based on estimated current consumption the battery will last something like 13.7 years or so. If only it were so! Unfortunately reality is quite a bit different. Battery lifetime is usually quite a bit shorter and often people don't quite know why.

One of the biggest reasons why is internal resistance. This causes the battery's output voltage to drop under load, limiting the effective amount of current that can be delivered. In general, the larger the battery, the smaller the internal resistance. This is one reason why coin cells don't work well in low power wireless devices - they have too high of internal resistance and therefore cannot supply enough peak current. They'll work fine for data-logging or applications where the peak current is low (below 10mA) but for wireless devices that pulse at 30-100mA they will start to fade. Most coin cells are not specified at all for pulse current, let alone that much. Even worse, the pulse current capability will be different for different vendors' batteries. That means that Energizer may be able to handle a 15mA pulse but Maxell might be able to do 20mA pulses. It all depends on the coin cell size, geometry, etc.

My favorite battery for low power wireless devices is the Energizer L91 (AA size) or L92 (AAA size). These lithium batteries have low self-discharge, nice voltage curve, good peak capability, and you can buy them in most grocery stores.

Next I'll talk about how to test different batteries...


  1. Thanks for giving such a great information about Dash7. It is an indispensable tool for development, integration, installation, monitoring, and troubleshooting wireless sensor networks and products based on the DASH7 Mode2 protocol.

  2. Could you use a parallel cap to fix this or would that introduce too much leakage?

  3. Parallel capacitance can alleviate the peak power somewhat, but capacitors have their own leakage currents. It depends on how often the device will be in sleep mode vs. active mode.