Measuring the impedance of a coin cell

If you want to construct a building, you have to start with the foundation. Humans have been digging holes and mixing concrete for millenia now, and these humble tasks remain the all-important starting point for the most advanced of modern structures.

A proper foundation is not a glamorous thing: you can't see it and it doesn't actively DO anything. The measure of a good foundation is the very fact that NOTHING happens for a very long time. But, cut corners at the beginning and... well, everyone knows about the tower in Pisa. Here's a domestic example with a dramatic ending.

The same is true in electronics. At the end of the project, software is often called upon to patch deep flaws in the system architecture or hardware, because "it's too late (or expensive) to go back and do it right".. The beginning of any project is the time to ensure that the underlying hardware architecture will perform as desired; fixing it later will only increase the cost, often exponentially.

Battery impedance is not a spec. which gets much thought. I recently purchased a new cordless drill, and I assure you that I did NOT ask about the battery's impedance. Consumers definitely know their battery chemistry: Lead-Acid for cars, Alkaline AA's, toxic NiCd rechargeables, and of course the modern workhorse Lithium. They've even learned a bit of electrical terminology: higher Voltage makes a more powerful drill, and cell phones with more mAh won't die mid-day.

Often, neglect for a battery's source impedance is reasonable. If relatively small currents are drawn from a relatively large battery, at moderate temperature, then source impedance will be negligible. But, when those two "relatives" or one "moderate" no longer apply, source impedance suddenly becomes a conversation topic.

For my present project, I'm hoping to use a CR2032 coin cell as the power source.

Panasonic lists the standard drain for their cell as 200uA.
This project's circuit is known to draw tens of mA for short periods.
Energizer provides this helpful graph showing that pulse currents reduce the total capacity (mAh) of their cell.

Because this project's anticipated current draw will be LARGE relative to the battery's ratings, source impedance warrants a closer look.

To measure source impedance, I used the following circuit.

For this measurement, the battery is modeled as an ideal voltage source and a series resistor R(source). As noted in the image above, Open Circuit Voltage (OCV) is measured with SW1 open, and Closed Circuit Voltage (CCV) is measured with SW1 closed. Note that an oscilloscope is used for measurement. A simple multimeter could be used, but the oscilloscope adds value because it can be set to trigger on a falling edge, capturing the CCV value an instant after SW1 is closed. Because batteries rely on chemical reactions, the CCV will decay as SW1 is engaged for longer durations. This decay is shown in the Energizer graph shown below.

I created a spreadsheet to record measurements and compute R(source).

The result: R(source) was nearly 14 ohms for the CR2032 I tested, which agrees with data provided by Texas Instruments and Energizer. The cell used for these measurements must be relatively fresh/new because values for R(source) are predicted to rise to 100 ohms or even 1k ohms as a CR2032's capacity is consumed through use.

After measuring the source impedance of the CR2032 cell, I know that it is significant and must be considered in my product design.

In future posts, I'll discuss the current consumption profile for my circuit in greater detail, and I'll also present a strategy for smoothing out short-duration current spikes to reduce stress on the battery.

My Excel spreadsheet is available here.
My measurement circuit is available in pdf format here.