标签: cell

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.

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.