|Nickname: Jack Ganssle
|Jack Ganssle is a lecturer and consultant specializing in embedded systems' development issues. He has been a columnist with Embedded Systems Design for over 20 years.|
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Posted: 07:12:22 PM, 24/04/2014
How much energy can be derived from a coin cell?
I worked on some experiments to determine how coin cells behave. This was motivated by what I consider outrageous claims made by a number of MCU vendors that their processors can run for several decades from a single CR2032 cell. Some vendors take their MCU’s sleep currents and divide those into the battery’s 225 mAh capacity to get these figures. Of course, no battery vendor I’ve found specifies a shelf life longer than a decade (at least one was unable to define “shelf life”) so it’s folly, or worse, to suggest to engineers that their systems can run for far longer than the components they’re based on last.
My CR2032 experiment. A small ARM controller applies various loads to batteries being discharged and logs the results.
Battery discharge data
But in practice you won’t get anything near that 225 mAh.
As cells discharge, their internal resistance (IR) goes up. Actually, this is not quite correct, despite the claims of all of the published literature I have found. Other results I’ll report on in a later column shows that there’s something more complex than simple resistance going on, but for now IR is close enough.
The next chart shows average IR for each vendor’s products, plus the IR’s standard deviation.
Internal resistance and its standard deviation
So what does this all mean to a cautious engineer? The IR grows so quickly that much of the battery’s capacity can’t be used!
First, the average IR is not useful. Conservative design means using worst case figures, which we can estimate using the measured standard deviation. By using three sigma our odds of being “right” are .997.
The following graph combines the IR plus three sigma IR to show what voltage the battery will deliver, depending on load.
Voltage delivered from battery depending on load
If a system, when awake, draws just 10 mA, 88% of the battery’s capacity is available before the voltage delivered to the load drops to 2.0. It’s pretty hard to build a useful system that needs only 10 mA. Some ultra-low-power processors are rated at 200 uA/MHz with a 48 MHz max – almost 10 mA just for the CPU.
With higher loads, like any sort of communications, things get much worse. Bluetooth could take 80 mA, and even Bluetooth LE can suck nearly 30 mA. At 30 mA only 39% of the battery’s rated capacity can be used. An optimist might use two sigma and suffer from 5% of his system not working to spec, but that only increases the useful capacity to 44%. The battery will not be able to power the system long before it is really “dead,” and long before the system’s design lifetime.
And long before the time MCU vendors cite in their white papers.
(Some MCUs will run to 1.8 volts, so vendors might say my cutoff at 2.0 is unfair. Since battery vendors say that 2.0 is “dead”, I disagree. And, even if one were to run to 1.8V there’s less than a 5% gain in useful capacity.)
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