标签: coin

Now that I have discharged about 100 CR2032 coin cells using a number of power profiles, I have come to collect millions of data points, and have shared the results here over the past months. These articles include:   How much energy can we generate from a coin cell? Implementing 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.

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 past months. These articles include:   How much energy can be derived from a coin cell? 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.

Alert reader Bob Snyder informed me of the UL rules for using coin cells in products . There are a number of requirements that must be met to obtain UL approval in devices with user-replaceable batteries. For instance, the device must be marked "Replace Battery With (Battery Manufacturer's Name or End-Product Manufacturer's Name), Part No. ( ) Only. Use of another battery may present a risk of fire or explosion. See owner's manual for safety instructions." I have never seen this on any product that uses coin cells. UL mandates that it’s either impossible to install a cell backwards, or that preventative safety measures of the type I explored previously be used. In many applications coin cells are used just to maintain RAM’s contents when the mains power is down. UL is very concerned that the battery cannot be reverse-biased when the power supply is feeding memory, so requires that either two series diodes or a diode with a current-limiting resistor be placed between the battery and the rest of the circuit, as follows:     Why two diodes? UL’s ultraconservative approach assumes one may fail. The resistor is to limit current if the diode in that circuit dies. In most cases the first circuit won’t work; even two Shottky diodes will drop about 0.8 V (as I showed last week), so RAM likely won’t get enough voltage to maintain its contents. The second circuit requires a resistor that limits reverse current to a UL spec of 25 mA for most coin cells. But how is one to compute the proper resistance? You need to know the battery’s internal resistance (IR) when reverse-biased, and I cannot find any documentation about that. There is some crude published data on IR when forward-biased, and in this series I’ve shown lots of empirical data from my experiments. One could assume the battery’s IR is zero, which would certainly give a nice worst-case result, but I decided to explore this a bit more. After discharging about 100 coin cells I’ve found that, forward biased, a CR2032 has 5 to 10 ohms of IR when new, increasing to hundreds as it is discharged. If we assume the reverse-bias IR is about the same as when forward biased, then the worst case situation is with a new, fully-charged, cell, since the IR is then at its lowest point. I applied a power supply to a CR2032 and measured the current flow when the supply’s voltage was 0.5, 0.75, and 1.0 volts above the battery’s unloaded voltage. The battery was contained in an explosion-proof container. Well, actually, an old coffee can, but it sure would have been fun to hear a boom. Alas, nothing exciting happened. From that it was easy to compute IR, which is displayed in the lower three lines on the following graph:     I took data every minute for the first 8 minutes, and at 5 minute intervals thereafter. The dotted lines are trendlines. Strangely, the internal resistance spikes immediately after the power supply is applied, then quickly tapers off. In a matter of minutes it falls to six to eight ohms, very much like my data for forward-biased batteries. The data is very similar for when the power supply was 0.5, 0.75, or 1 volt above the cell’s unloaded voltage; that’s not unexpected if one assumes this really is resistance and not some complexity arising from the chemistry. I have other data I’ll present soon that suggests that while modeling the cells using resistance is a good first approximation, there’s something else going on. However, for this discussion five ohms is a safe bet for the IR when computing the series resistance needed. The top three curves are the battery’s temperature. Unsurprisingly, temperature goes up with the voltage difference. Given UL’s dire warnings about catastrophic failure I expected more heat, but the max encountered was only about 50 C, far lower than the 100 C allowed by UL rules. This data is for a single battery so be wary, but it does conform to the IR characteristics I measured for about 100 forward-biased cells. This leads to another question: to get more capacity, can we parallel two or more coin cells? UL is silent on the subject. I suspect that since their argument is that reverse-biasing a battery is bad, they would require diode isolation. As we’ve seen in this series of articles, diodes eat most of the effective capacity of a cell, so should be avoided. From a non-UL, purely electronics standpoint, what would happen? This is a debate that rages constantly in my community of ocean-sailing friends. The systems on our sailboats run off large, often lead acid, batteries. On my 32-foot ketch, the fridge sucks 50 Ah/day, the autopilot another 50 Ah, and the radar, well, at 4 amps we don’t have the power to leave it on all of the time. All of this comes from two 220 Ah six-volt golf-cart cells wired in series. After a day or so of running the systems we have to fire up the engine to recharge, which everyone hates. Can we wire two banks of golf cart cells in parallel? I have heard all sorts of arguments for and against, but many do wire their systems that way and get good results. What about coin cells? My experimental data shows that the maximum difference in unloaded voltage for fresh CR2032s is about 0.25 volt. This is true for a single brand and between brands and lots. With two paralleled cells of unequal initial voltages, the lower-voltage battery’s small IR will discharge the higher-voltage cell rapidly until both batteries are at the same voltage. LiMnO2 cells have a very flat discharge curve till they approach end of life. Discharge one by a quarter volt and you have lost around 200 mAh of capacity, or about 90% of the cell’s 220 mAh rating. So the battery with the higher voltage will quickly run down to 10% reserves. Most of its capacity is thrown away. But it gets worse. Once heavily discharged the battery’s voltage is at a knee on the curve and falls rapidly. The one that seemed better, with a higher voltage when first installed, now acts as a load on the other! They essentially suck each other dry. So don’t put these in parallel.

Alert reader Bob Snyder led me to the UL rules for using coin cells in products . There are a number of requirements that must be met to obtain UL approval in devices with user-replaceable batteries. For instance, the device must be marked "Replace Battery With (Battery Manufacturer's Name or End-Product Manufacturer's Name), Part No. ( ) Only. Use of another battery may present a risk of fire or explosion. See owner's manual for safety instructions." I have never seen this on any product that uses coin cells. UL mandates that it’s either impossible to install a cell backwards, or that certain preventative safety measures be used. In many applications coin cells are used just to maintain RAM’s contents when the mains power is down. UL is very concerned that the battery cannot be reverse-biased when the power supply is feeding memory, so requires that either two series diodes or a diode with a current-limiting resistor be placed between the battery and the rest of the circuit, as follows:     Why two diodes? UL’s ultraconservative approach assumes one may fail. The resistor is to limit current if the diode in that circuit dies. In most cases the first circuit won’t work; even two Shottky diodes will drop about 0.8 V (as I showed last week), so RAM likely won’t get enough voltage to maintain its contents. The second circuit requires a resistor that limits reverse current to a UL spec of 25 mA for most coin cells. But how is one to compute the proper resistance? You need to know the battery’s internal resistance (IR) when reverse-biased, and I cannot find any documentation about that. There is some crude published data on IR when forward-biased, and in this series I’ve shown lots of empirical data from my experiments. One could assume the battery’s IR is zero, which would certainly give a nice worst-case result, but I decided to explore this a bit more. After discharging about 100 coin cells I’ve found that, forward biased, a CR2032 has 5 to 10 ohms of IR when new, increasing to hundreds as it is discharged. If we assume the reverse-bias IR is about the same as when forward biased, then the worst case situation is with a new, fully-charged, cell, since the IR is then at its lowest point. I applied a power supply to a CR2032 and measured the current flow when the supply’s voltage was 0.5, 0.75, and 1.0 volts above the battery’s unloaded voltage. The battery was contained in an explosion-proof container. Well, actually, an old coffee can, but it sure would have been fun to hear a boom. Alas, nothing exciting happened. From that it was easy to compute IR, which is displayed in the lower three lines on the following graph:     I took data every minute for the first 8 minutes, and at 5 minute intervals thereafter. The dotted lines are trendlines. Strangely, the internal resistance spikes immediately after the power supply is applied, then quickly tapers off. In a matter of minutes it falls to six to eight ohms, very much like my data for forward-biased batteries. The data is very similar for when the power supply was 0.5, 0.75, or 1 volt above the cell’s unloaded voltage; that’s not unexpected if one assumes this really is resistance and not some complexity arising from the chemistry. I have other data I’ll present soon that suggests that while modeling the cells using resistance is a good first approximation, there’s something else going on. However, for this discussion five ohms is a safe bet for the IR when computing the series resistance needed. The top three curves are the battery’s temperature. Unsurprisingly, temperature goes up with the voltage difference. Given UL’s dire warnings about catastrophic failure I expected more heat, but the max encountered was only about 50 C, far lower than the 100 C allowed by UL rules. This data is for a single battery so be wary, but it does conform to the IR characteristics I measured for about 100 forward-biased cells. This leads to another question: to get more capacity, can we parallel two or more coin cells? UL is silent on the subject. I suspect that since their argument is that reverse-biasing a battery is bad, they would require diode isolation. As we’ve seen in this series of articles, diodes eat most of the effective capacity of a cell, so should be avoided. From a non-UL, purely electronics standpoint, what would happen? This is a debate that rages constantly in my community of ocean-sailing friends. The systems on our sailboats run off large, often lead acid, batteries. On my 32-foot ketch, the fridge sucks 50 Ah/day, the autopilot another 50 Ah, and the radar, well, at 4 amps we don’t have the power to leave it on all of the time. All of this comes from two 220 Ah six-volt golf-cart cells wired in series. After a day or so of running the systems we have to fire up the engine to recharge, which everyone hates. Can we wire two banks of golf cart cells in parallel? I have heard all sorts of arguments for and against, but many do wire their systems that way and get good results. What about coin cells? My experimental data shows that the maximum difference in unloaded voltage for fresh CR2032s is about 0.25 volt. This is true for a single brand and between brands and lots. With two paralleled cells of unequal initial voltages, the lower-voltage battery’s small IR will discharge the higher-voltage cell rapidly until both batteries are at the same voltage. LiMnO2 cells have a very flat discharge curve till they approach end of life. Discharge one by a quarter volt and you have lost around 200 mAh of capacity, or about 90% of the cell’s 220 mAh rating. So the battery with the higher voltage will quickly run down to 10% reserves. Most of its capacity is thrown away. But it gets worse. Once heavily discharged the battery’s voltage is at a knee on the curve and falls rapidly. The one that seemed better, with a higher voltage when first installed, now acts as a load on the other! They essentially suck each other dry. So don’t put these in parallel.  

For about a year now, I have on-going experiments to identify 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. 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.)