Battery datasheets - Page 1
Forum section: electronics / projects

author: Fungus
author_rank: Super Contributor
timestamp: July 28, 2015, 11:52:45 pm
content: I'm trying to understand a battery datasheet - this one here:http://data.energizer.com/PDFs/cr1220.pdfI'm going to really abuse some and drive some little high-current gadgets.I just took one out of the packet and connected it up. I draw 30mA and the voltage under load was about 1.75V. That gives me a battery ESR of 40 Ohms and the device is using 0.025W.Ok, so far...Now I want to get a very approximate idea of how long one will last. The datasheet graph that's confusing me is the "Internal Resistance Characteristics" one, shown below.What does that graph show, exactly. In particular, what is the X scale? Is the X scale how much power I've used in total?eg. If I suck out 10mAh is the ESR going to be about 80 Ohms?Similarly: If I suck out 20mAh will it be about 100 Ohms?I guess that calculating the expected life will be difficult because I'm so far out of the typical usage scenario. I might have to just connect one up and time it.

author: John Coloccia
author_rank: Super Contributor
timestamp: July 29, 2015, 11:59:31 am
content: You won't be able to get any useful information from the graph. A 30mA load on that battery is so far out of intended use, the datasheet won't apply. My expectation, if you actually did a test, is that you'd see the voltage drop almost immediately, and it will probably just continually drop steadily. That's how batteries tend to behave when you ask them to source a lot more than they're designed to source.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 29, 2015, 12:07:25 pm
content: If I'm reading the chart correctly, when you have an internal resistance of 40 Ohm, you'll have a capacity of "undefined" since that part of the green line is not plotted.But, on 200 ohm, you'll have 35 mAh.And the red and blue line show the battery voltage at used battery capacity if the discharge conditions are as described above.But, drawing 30 mA is for penlite batteries, not coin cells.

author: Kalvin
author_rank: Super Contributor
timestamp: July 29, 2015, 12:22:56 pm
content: I would read the IR chart so that as the IR increases when the battery gets discharged, the IR gives an estimate of a used capacity when the battery is being loaded according the load specified in the chart.As the full battery capacity is specified to be 40mAh:- If the IR is 40 ohms, the battery is new and the battery will still provide 40mAh.- If the IR is 80 ohms, we have used approx. 10mAh thus the battery will still provide approx. 30mAh.- If the IR is 120 ohms we have used 20mAh and the battery will still provide 20mAh.- If the IR is 200 ohms we have used 35mAh and the battery is almost empty providing possibly 5mAh.

author: tszaboo
author_rank: Super Contributor
timestamp: July 29, 2015, 12:36:36 pm
content: The problem with all the assumptions here is that you think that IR is measured by a DC method. Apply a current, and measure the voltage drop, divide.That isnothow it is measured. They apply an AC signal, 1KHz or 10KHz and measure the RMS voltage voltage change and RMS current. That way the chemicals have no time to react to anything. It will change similarly, but you will end up with enormous errors.

author: Kalvin
author_rank: Super Contributor
timestamp: July 29, 2015, 12:52:42 pm
content: The IR can also be determined measuring the battery voltage when the known pulsed load is applied:http://data.energizer.com/PDFs/BatteryIR.pdfFirst a small background current I1 is applied and the battery voltage V1 is measured. Then a pulsed load with a known current I2 is applied for a brief moment and the battery voltage V2 is measured. The battery's internal resistance can be determined using the Ohm's law R = - (V2 - V1) / (I2 - I1).The loaded measurement described above will indicate the total effective resistance of the battery seen by the load. The AC measurement will provide only the battery's electrical resistance, without the chemical resistance (ie. the ionic resistance which will increase as the battery is discharged).

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 29, 2015, 01:05:48 pm
content: Using AC is general practice for capacitors to define Ri, but I doubt battery manufacturers will do that. Mostly they use a simple delta-U/delta-I scheme. Usually they are not very generous with small details. Anyway, small Li/MnO2 coin cells are not intended/designed to deliver more than say 1 mA. You will not find much about data out of specs in a data sheet. If you want more than made for, you are on your own to figure out what happens or you have to consult the manufacturer.

author: Fungus
author_rank: Super Contributor
timestamp: July 29, 2015, 01:36:53 pm
content: Quote from: John Coloccia on July 29, 2015, 11:59:31 amYes, I get that. I'm just trying to read the graph to understand what it's actually trying to say.eg. How would I use the graph if my load was only 0.5mA.

author: Fungus
author_rank: Super Contributor
timestamp: July 29, 2015, 01:50:51 pm
content: Quote from: Jeroen3 on July 29, 2015, 12:07:25 pm:-)The problem is I need somethingsmall. CR1220 are about the only size apart from CR2032 that it'seasy/cheap to get battery clipsfor. CR2032 are too big, so that's what I'm stuck with.Those batteries are cheap and if the gadget can run for half an hour then that's plenty.

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 29, 2015, 01:58:04 pm
content: If that is your goal then forget about Li/MnO2 cells. Silver oxide (more expensive) or even Alkaline coin cells are a better option.

author: technix
author_rank: Super Contributor
timestamp: July 29, 2015, 02:50:06 pm
content: Your use case seem to be better suited with a small LiPo. Adding a small LiPo into your project, a microUSB charging port (also double as a debug/firmware update port if you want to do so) and the TP4056 all-in-one LiPo charge controller chip would not be difficult I think.

author: Fungus
author_rank: Super Contributor
timestamp: July 29, 2015, 03:06:04 pm
content: Quote from: technix on July 29, 2015, 02:50:06 pmTotal overkill. This is just a throwaway thing that needs to be very small and cheap to make.

author: ralphd
author_rank: Frequent Contributor
timestamp: July 29, 2015, 03:06:12 pm
content: TI has a paper on coin cell battery life that looks at current draw and IR.http://www.ti.com/lit/wp/swra349/swra349.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: July 29, 2015, 03:16:46 pm
content: Quote from: Pjotr on July 29, 2015, 01:05:48 pmA modern battery characterization equipment has better current control than the best 6,5 digit multimeter can measure, like 50 ppm voltage measurement accuracy, AC measurement, and it can store millions of datapoints for tests going for moths and hundreds of channels for parallel measurement and testing.Now, on a closer look, I see that energiser is using the DC method, I guess they had some ancient equipment doing these tests.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 29, 2015, 03:44:33 pm
content: Quote from: NANDBlog on July 29, 2015, 12:36:36 pmHorribly wrong.Internal resistance isvery oftenthe DC resistance, because it is the type of resistance that makes any sense for most of the system designers reading the datasheet. Although, it is true that often 1 kHz (or maybe 10 kHz) AC resistance is given. Sometimes both separately. Proper datasheets define what is being measured, and the frequency used for the AC measurement.Without specifying whether the number is AC or DC resistance, the datasheet is utterly useless, since both are common industry standard practices and the term "resistance" doesn't give any real hint which one it is. (Sometimes "impedance" is used for AC.)Generally, the 1kHz AC resistance is a useless number (although it may be useful for the manufacturer's internal quality control), while the DC resistance matters. For many batteries, the DC resistance is somewhat higher, making the AC reading more appealing, but there is no good correlation between the numbers.

author: Fungus
author_rank: Super Contributor
timestamp: July 29, 2015, 07:45:45 pm
content: General consensus seems to be that that datasheet makes no sense to anybody else.Good to know it's not just me being a bit thick.

author: tszaboo
author_rank: Super Contributor
timestamp: July 30, 2015, 08:21:15 am
content: Quote from: Siwastaja on July 29, 2015, 03:44:33 pmSince when do they explicitly publish information for your convenience? The 1KHz method is the industry standard. Energizer by the looks of it doesnt have the equipment to measure it, so the marketing department makes claims that the DC method is better. Well it is not.The reason that they dont publish how they do the 1 Khz is because it is standardized. You apply an 1 KHz AC current, measure the voltage, and then increase the current to the point when the AC voltage reaches a certain voltage, I believe it is 100 mV (from the top of my head).This way you can compare batteries, with different size and different chemistry. How would you do this with the DC method? Measure it with 5 different currents? 5mA to be measured on a 18650 lithium cell or 5A on a coin cell? Proper datasheets give you a different characteristics for you:http://www.rayovac.com/technical-oem/~/media/rayovac/files/product%20guides/pg_lithium.ashxFigure 18. "Effect on load resistance on operating voltage", while the internal resistance is measured with the AC method.

author: Kalvin
author_rank: Super Contributor
timestamp: July 30, 2015, 10:19:29 am
content: Quote from: NANDBlog on July 30, 2015, 08:21:15 amOr, Energizer has figured out that the AC measurements are useless for the primary batteries like the coin-cells. Why print something that is irrelevant and nonsense anyway. AC measurement may be very useful when characterizing lead-acid car batteries and determining the peak current capability, though.If you take a look at this paper "Comparison of Several Methods for Determining the Internal Resistance of Lithium Ion Cells", including the AC method:https://www.mdpi.com/1424-8220/10/6/5604/pdfFor example, see figure 3: Constant current is applied but the battery voltage drops as a function of time. That means the battery's internal resistance increases with time. The AC measurement doesn't take that into account, so AC method is not appropriate here. Chapter 4.8 discusses the AC method and its limitations more thoroughly.AC method gives too low value for the internal resistance, so it is basically useless when characterizing primary batteries. The step-load method gives more accurate estimate of the battery's internal resistance and it can be implemented even on the device itself in order to get an approximate estimate of the battery's state.

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 30, 2015, 11:18:37 am
content: Indeed AC resistance is of no use for OP needs and most battery makers doesn't even notice it in their data sheets for Li button cells. Li button cells are not suitable for OP needs anyway. Better look for LRxx alkaline cells. All those Chinese 50cts gimmicks/toys use 1 - 3 of those.

author: tszaboo
author_rank: Super Contributor
timestamp: July 30, 2015, 12:37:08 pm
content: Quote from: Kalvin on July 30, 2015, 10:19:29 amOf course the DC method will give you a higher internal resistance! It is fundamentally wrong. Look at figure 2. See, even they say, that this is inside the battery, with Rt, transfer resistance. How do you distinguish between these if you use the DC method? You cannot, that is why you measure Ri + some of the currents flowing through Rt1 and Rt2. With the AC method, at 1 Khz, Cd1 and Cd2 becomes a short, so Rt1 and Rt2 is insignificant. That is why the AC method result is smaller. It gives you the pure internal ressitance, not Ri plus some test method dependent resistance.Notice, how they dont mention Rt after figure 2, possibly they have no idea what they are doing (with all dues respect).But I can also send you links:http://batteryuniversity.com/learn/article/how_to_measure_internal_resistance"The pulse DC load method provides valuable readings for a DC application such as driving a motor or powering a light""valuable readings"<- not internal resistance. Also, the battery in question is a coin cell, not an electric car battery.Side note for the study: I dont know whoever in sane mind would say, that overloading a battery at 18x it's rated capacitance for 18 seconds will yield usable results. They put 48A, 150W load on a (guess) 18650 cell. And they claim, internal resistance 18 mOhm. Here is the problem: this method dissipated 40W inside your battery. Heating it up. And battery voltage changes with temperature by a lot, not to mention the internal chemicals. There goes the result.

author: John Coloccia
author_rank: Super Contributor
timestamp: July 30, 2015, 01:33:02 pm
content: Quote from: Fungus on July 29, 2015, 07:45:45 pmThink that one's confusing? Check this one out.http://www.professional.duracell.com/downloads/datasheets/product/Specialty/Specialty_Electronics_CR1220.pdfI'm not sure exactly how Duracell ran their tests, but someone made a boo boo somewhere.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 30, 2015, 02:09:32 pm
content: Quote from: NANDBlog on July 30, 2015, 08:21:15 amI see it happen more and more; however, batteries are still under-specified by the manufacturers, and most of the time, customers need to do their own tests, or sign contracts to get more information from manufacturers. Datasheets often show ACR, and application notes may use DCR.If I may draw an analogy, reporting only ACR is comparable to only reporting Vgs threshold for a swithing MOSFET; it's a useless number which will only give you ahintwhich gate voltage really turns the FET on. The battery DCR equivalent in this analogy would be the curve set which shows the real gate voltage needed. Though, this analogy sucks a bit because the ACR vs. DCR is much less correlated (and thus, gives less hint) than Vgs(th) vs. real gate voltage needed.QuoteYes it is, and has been for a long time - it's standardized and easy to measure, which is probably why it is so damn popular.The problem is that the resulting number is most often completely useless for the customer (designer), who really needs the DC resistance to do their efficiency calculations, thermal design and compare the current capabilities between different batteries.QuoteCompare WHAT? You can indeed compare the 1 kHz AC impedances of different batteries. What do you gain with this information? Please tell me, since I sincerely don't know, and I have never used this number in any of my design work. It issomewhatcorrelated to the current capability, efficiency or thermal properties of the said products, but not reliable enough for that purpose.On the other hand, "DC resistance" is not well specified, and there are many different ways to measure it, but all of them yield quite similar results, because they measure "the right thing". What "DC resistance" is, it is a simulation of the real-world properties of the battery, put in one number. It needs additional data with it, such as how it was measured. Temperature and State Of Charge affects the DC resistance wildly, but it also affects AC resistance! Both AC and DC resistances are multi-dimensional graphs with strong temperature, SoC and aging correlation, and they are never single numbers, but both can be given at certain points, for example, a new product, at 25 deg C, at 50% SoC.QuoteBasically, there are two different methods which, at least most of the time, seem to give comparable results within +/- 20%:1) Method of two full cycles: The battery is discharged twice, giving two characteristic curves. DCR curve is dV/dI at every point. It appears that most of the time, the numbers are very close regardless of the currents chosen, given that extremes are avoided. For example, I may use 0.5C and 0.2C discharges for a energy-type lithium ion cell, but using 1.0C and 0.5C gives the same result within a few percent.2) Pulse method: The battery is discharged once; my method uses alternative (lower or higher; now I'm using 0.8*average, IIRC) current every two minutes for about 10 seconds, and calculates dV/dI from the voltage drop or rise during that pulse. This seems to be what Energizer is using.The main difference between 1) and 2) is that 1) is affected by real-world effects of temperature; the higher current discharge heats up the battery more, reducing its resistance. Method 2) maintains the heating effect of (approximately) the average current used. The results of there methods seem to sometimes differ, but in any case, they are both useful numbers and reflect the losses and heating seen in real-world applications, unlike the AC resistance.ACR is basically a special case of method 2), but using such short "pulse times" that the result reflects the battery's capability to give very short (approx. 1 ms) pulses, which are rarely needed in real world - bulk electrolytic capacitance is typically used for that purpose.ACR measurement is the combination of the battery's battery properties and its capacitor properties.Capacitor properties are not completely insignificant in system design, but nearly so.The actual current levels used in testing are naturally dependent on the battery technology,but exactly the same is true for the AC measurement- it is taken into account in the standard (by using a fixed voltage drop, as you describe). The same could be done with DCR measurement, and it's one way of doing it!Typically, a properly specified battery has characteristic curves provided for several different currents. You can use this data to calculate the DCR using method 1, even if the manufacturer doesn't do it.Standardizing something does not automatically make it good or useful. Even if ACR measurement was formally more standardized, it doesn't help as the designer really wants DCR. Also, there is no reason why DCR measurementcouldn'tbe standardized similarly to ACR, to be independent of technology. It is what should be done. There may even be standards out there already, we are just not using them.And the datasheet in question is just fine; they use the useful DCR measurent, provide DCR curve for full SoC range, and specify their method of measurement. Everything in the datasheet makes sense for the system designer. This is wonderful, but sadly it's rare.I understand that a person who is not really involved in battery technology and battery system design, but has read battery datasheets before, might find this unorthodox way of presenting information inferior to the "good old industry standard".But I'd like to see much more useful information in datasheets in general, and especially battery datasheets, even if it makes the datasheet less concise or lessstandardized.I think I have said what I need to say; we can agree to disagree, but I'd really like to know what do YOU do with the ACR number, as you seem to find it more interesting than DCR. I haven't still found use for ACR as a battery system designer, but there can be some. Quality control is one, and checking the ACR can easily reveal that the battery differs from the specification, but that's about it.In the end, there is no such a thing as "DC" - everything changes, and batteries are drained in pulses. Even a completely smooth discharge is a cycle-long pulse. "ACR" measures the properties at 1 kHz, and when the frequency of the measurements starts dropping below a few Hz, we start calling it DCR. These numbers describe how well the battery provides current for said frequencies, or how much the voltage sags. Fromthermal and energy efficiency viewpoint, the average or DC value matters.I hope this helps.