How much energy can you really get from a coin cell?

About a year ago I wrote of my on-going 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.

Conservative design means recognizing that ten years is the max life one can expect from a coin cell. In practice, even that will not be achievable.

There’s also a war raging about which MCUs have the lowest sleep currents. Sleep current is, to a first approximation, irrelevant, as I showed last year .

But how do coin cells really behave in these low-power applications? I’ve been discharging CR2032s with complex loads applied for short periods of time and have acquired millions of data points.

My CR2032 experiment. A small ARM controller applies various loads to batteries being discharged and logs the results.

The following results are for 42 batteries from Duracell, Energizer, and Panasonic. For each vendor I ran two groups of cells, each group purchased months apart from distributors located in distant states, in hopes that these represent different batches. (The devices are not marked with any sort of serial or batch numbers).

First, the weird part. Our local grocery store sells these cells for up to $5 each. Yet Digi-Key only wants $0.28 for a Panasonic and $0.40 for an Energizer – in singles. Duracells are harder to find from commercial distributors, but I paid about a buck each from on-line sources (e.g., Amazon).

I found little battery-to-battery variability (other than one obviously bad Panasonic and one bad Duracell), little vendor-to-vendor difference, and likewise different batches gave about the same results.

What parameters matter? Chiefly, capacity (how many milliamp hours one can really get from a cell), and internal resistance, which varies with capacity used.

All of the vendors say “dead” is at 2.0 volts.

The following graph shows the average voltage for the batteries from each vendor, as well as the worst-case voltage from each vendor, as they discharge at a 0.5 mA rate. The curve ascending from left to right is the cumulative capacity used. By the time 2.0 volts is reached the capacity variation is in the noise. I found it averaged 233 mAh with a standard deviation between all results of 5 mAh. Energizer and Duracell’s datasheets are, uh, a bit optimistic; Panasonic says we can expect to get 225 mAh from a cell, which seems, given this data, a good conservative value to use.

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.)

41 thoughts on “How much energy can you really get from a coin cell?

  1. I wonder whether BR-series coin cells would have performed any better (or worse). According to Panasonic, their BR-series coin cells have a lower self-discharge rate than their CR-series coin cells.

    I am planning to use a tabbed BR1632A/FAN to power the R

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  2. Correction: The PIC16LF1823 doesn't have an RTC. I am programming it to work as an RTC by clocking the 16-bit Timer1 at 32kHz.

    The timer uses only 650nA (typ) and 4uA (max). Every 2 seconds, when it overflows, the chip wakes up just long enough to increme

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  3. A BR2032 only has 190 mA of capacity, and that drops a lot if it is used in a cold environment. 190 mA over 10 years means the system's average draw cannot exceed 2 uA. It sounds like your system will be hot; you may have to assume the worst case power-dow

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  4. The datasheet lists “T1OSC Current” at 1.8V as 0.65uA typ, 4.0uA at +85C, and 7.0uA at +125C. It also says that the T1OSC figure includes the power-down base current (Ipd).

    Most of these data loggers will be used mostly on the workbench, with ra

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  5. 2 duff cells out of 42? I guess it really depends on your warranty policies as to whether they make a useful power source or not.

    I'm not sure why you'd design a coin cell driven system using a micro that chomps through 10mA active. Surely the vast bulk o

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  6. One of the best methods to get more energy out of a small battery with a pulse load is to bypass it with a large value, (10uF to 100uF) and low leakage, (i.e. ceramic) capacitor. This significantly reduces to power loss due to the battery's internal resis

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  7. A nice investigation, Jack. Thank you for sharing the data. It is notoriously difficult to estimate how much life to expect from batteries of all types, especially “consumer” types.

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  8. David…for what's it's worth, I found a TI white paper (Google “SWRA349”) which states that “adding a capacitor in parallel with a CR2032 coin cell is the most effective choice a designer can make to maximize battery capacity utilization in

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  9. Jack,

    Thanks for publishing the data. Really useful. Looking at the TI White SWRA349 white paper, it suggests that with 15mA loads in BLE type duty cycles, most batteries achieve around 90 to 100% of rated 220mA capacity. Quite a different result to your

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  10. Thanks, everyone, for your interest in this fascinating and fun subject.

    The TI paper looks at loss of capacity due to discharging the batteries relatively quickly. Their sleep times are very short. My data is all for the opposite situation, where dischar

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  11. I have been thinking about a protocol for testing the specific type of battery I am planning to use to power an RTC circuit.

    Suppose I purchase a dozen identical batteries and connect each one to a different resistance. For example, 100 ohms, 200 ohms, 40

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  12. The harder you load a battery, the less capacity it has. Pull 100 mA from a CR2032 and you will get much less than the rated 220 mAh.

    Some of my other experiments confirm published data that for a load of 0.5 mA on average or less you can get the full 22

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  13. That paper is part of what initially spawned my interest in the subject. Unfortunately there's no data about how many batteries were tested or other aspects of the experimental setup. The graphs look awfully smooth, like someone drew them with a crayola. I

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  14. What do think is a reasonable lifetime estimate for a Nordic nRF51822 running Bluetooth Low Energy on a coin cell? Say it is doing something very simple like broadcasting temperature once every few minutes (or when it changes). They have detailed charts of

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  15. Jack, I have a little kidney shaped alarm/timer that was given to me by an employer as a Christmas gift in 1987. I used it once and thought it was quite a quaint device probably having something like an RTC and a micro that maybe only gets powered while se

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  16. Jack,
    Very informative and educational article. I often face the challenge of convincing people that IR exists and a given CR2032 is far away from an ideal voltage source approximated by a line between 3V and 2V. I will reference this article going forward

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  17. Not sure why my comments didn't stick. Just wanted to mention that Energizer L91 (AA) and L92 (AAA) Ultimate Lithium cells have a 20-year shelf life, according to the datasheets. Of course, they're not coin cells, but 20 years is achievable if you have roo

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  18. AAAs are a great solution as they have around 4x the capacity of a CR2032. But, you'll either need two to get enough voltage to power most MCUs, or a boost converter. Some people are put off by the required inductor, but often those are surprisingly inexpe

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  19. I removed it on purpose as the numbers aren't important. It's time, assuming some constant-ish rate of discharge. For a system with a ten year life, consider the left side 0 years and the far right 10 years.

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  20. While boost converters are efficient at high currents (high relative to what we're talking about here), they are hopeless at sleep currents.

    At sleep currents boost converters are going to have way worse loss than anything we've talked about so far.

    Cons

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  21. A further note on choosing between C and B chemistries: yes, the B has less capacity for the same size, but it tolerates higher temperatures at up to 80 C, vs. only up to 60 C for the CR. This is significant insofar as self-discharge increases with tempera

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  22. “Great job compiling this very important data set!nnWould you also like to discuss the hardware setup that you used to collect this data? The loading of the cells, or the inaccuracies in measuring the cell internal resistances would be something people w

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  23. “I will put together a piece about this. Basically, an MCU activated transistors to put various loads on the cells. The voltages were measured, as was the Vce drop on the transistors. All was fed through analog muxes to an A/D. A precision voltage referen

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  24. “Please note that battery manufacturers do not recommend 30mA load. See Energizer test cases. The recommended operation modes pretty much eliminate the use case above. nhttp://data.energizer.com/PDFs/cr2032.pdf . The CR2032 will do ~0.2 mA continuous

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  25. “The battery chemicals are actually having some of the “healing affect” from my understanding, so if you can give enough time for the chemical to recover, say 1 millisecond at 20mA and 1 second time to recover(low current drain),the battery should still

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  26. Jack…can you comment on putting a large capacitor in parallel with the coin cell to give a bit more “oomph” during short periods of power use in systems that are effectively off for a large percentage of the time?

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  27. “You probably saw how Bitcoin increased over 900% over the course of last year.nIt was wild but not totally unprecedented if youu2019d been watching crypto currency over the last several years.nAnd hereu2019s the crazy thing:nThere are many other coin

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  28. “”Itu2019s pretty hard to build a useful system that needs only 10 mA.”nMy battery powered microprocessor controlled proximity sensor and 433MHz transmitter, operating at 35mA at a 0.00326 duty cycle (0.326%) operates at an average current of 0.114 mA

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