Coin cell voltage droop
After discharging about 100 CR2032 coin cells using a number of power profiles I have collected millions of data points, and have shared the results here over the last year or so. These articles include:
- Sleep current is mostly irrelevant
- How much energy can you really get from a coin cell?
- Why a capacitor can’t help a battery in long-lived applications
- Why the vendors’ advice to run the MCU fast, so to go to sleep quickly, is naïve
- Voltage scaling probably offers little benefit to systems running for years off a battery
- Reverse battery protection
- UL coin cell requirements, and why you can’t parallel two batteries to get more mAh
I finished getting some more data some months ago but have been too backlogged to reduce it to useful information. Finally that’s done!
The question I had was: suppose one applies a fixed load to a coin cell for a short period of time. Does the battery voltage change? That’s a special case of a broader question: everyone uses internal resistance (IR) to characterize these cells. Is IR really an accurate way to model their behavior?
For this experiment I discharged 9 CR2032s. Most of the time there was only a 0.5 mA background load to run the batteries down, but every two hours the test jig applied either a 10 mA or a 30 mA load for one second. That is, at the 2 hour mark the cells saw a 10 mA load; at four hours it was 30 mA. The system read voltages immediately after applying the load, every 10 ms until 100 ms went by, and then at 100 ms intervals.
Here are the results for 30 mA. The horizontal axis is time, but I left it unlabeled as it could be weeks, months or years depending on the discharge profile. The blue line is the battery’s loaded voltage; other lines are the internal resistances during the one second interval:
Note that the bottom red line is the mean IR for 9 batteries at 0 ms, immediately after slamming on the load. All of the other data points are pretty tightly grouped. In other words, the IR goes up significantly (about 10%) milliseconds after the load shows up, but there’s little change after that.
In other words, IR is not an accurate model of coin cell behavior. It’s not bad; for an engineering analysis it’s probably close enough. But there is some other effect from the battery chemistry going on.
The results are clearer with less data. Here the red line is the IR at 0 ms; grey is at 10 ms, and yellow at 1 second:
Especially nearing the end of life we can see a big increase in IR from 0 to 10 ms, but not much more from 10 ms to 1000 ms. Yes, the effect is different when the battery hasn’t run down too much, but then the voltage is higher and IR is so low the increasing IR isn’t particularly important.
With a 10 mA load the results are about the same:
The bottom line is that the voltage the MCU sees when it first wakes up is not that which will be there mere milliseconds later. Figure on another 10% drop, on top of all of the serious losses I’ve detailed in those earlier articles.