In this tutorial Dave explains how to precisely measure and calculate the remaining (or wasted) energy capacity in a battery using the graphical analysis technique using a spreadsheet.
The differences between constant power and constant current loads and when to use them is discussed.
Also, the importance of ESR and measuring the battery voltage under load is demonstrated.
This is a particularly relevant to the Batteriser product and proof is provided that shows that even over the entire current range, the wasted energy in a battery can be no better than a few 10's of percent for a 1.1V cutout voltage. Blowing the batteriser claim of 80% wasted energy out of the water.
Forum: http://www.eevblog.com/forum/blog/eevblog-772-how-to-calculate-wasted-battery-capacity/'>http://www.eevblog.com/forum/blog/eevblog-772-how-to-calculate-wasted-battery-capacity/
All of my battery related videos are here: https://www.youtube.com/playlist?list=PLvOlSehNtuHtj5Ubhx7govoBfNkyUO4Pp
DIY Constant current load: https://www.youtube.com/watch?v=8xX2SVcItOA
Battery Capacity Tutorial: https://www.youtube.com/watch?v=R8hTQXqURB4
AA Battery Capacity Measurement: https://www.youtube.com/watch?v=uT4hKd66UY4
Batteriser Debunking Video: https://www.youtube.com/watch?v=4iEshd6izgk
LiPo Battery Discharging: https://www.youtube.com/watch?v=ln8Mlz4NsW8
EEVblog Main Web Site: http://www.eevblog.com
The 2nd EEVblog Channel: http://www.youtube.com/EEVblog2
Support the EEVblog through Patreon!
http://www.patreon.com/eevblog
EEVblog Amazon Store (Dave gets a cut):
http://astore.amazon.com/eevblogstore-20
T-Shirts: http://teespring.com/stores/eevblog
Donations:
http://www.eevblog.com/donations/
Projects:
http://www.eevblog.com/projects/
Electronics Info Wiki:
http://www.eevblog.com/wiki/
The differences between constant power and constant current loads and when to use them is discussed.
Also, the importance of ESR and measuring the battery voltage under load is demonstrated.
This is a particularly relevant to the Batteriser product and proof is provided that shows that even over the entire current range, the wasted energy in a battery can be no better than a few 10's of percent for a 1.1V cutout voltage. Blowing the batteriser claim of 80% wasted energy out of the water.
Forum: http://www.eevblog.com/forum/blog/eevblog-772-how-to-calculate-wasted-battery-capacity/'>http://www.eevblog.com/forum/blog/eevblog-772-how-to-calculate-wasted-battery-capacity/
All of my battery related videos are here: https://www.youtube.com/playlist?list=PLvOlSehNtuHtj5Ubhx7govoBfNkyUO4Pp
DIY Constant current load: https://www.youtube.com/watch?v=8xX2SVcItOA
Battery Capacity Tutorial: https://www.youtube.com/watch?v=R8hTQXqURB4
AA Battery Capacity Measurement: https://www.youtube.com/watch?v=uT4hKd66UY4
Batteriser Debunking Video: https://www.youtube.com/watch?v=4iEshd6izgk
LiPo Battery Discharging: https://www.youtube.com/watch?v=ln8Mlz4NsW8
EEVblog Main Web Site: http://www.eevblog.com
The 2nd EEVblog Channel: http://www.youtube.com/EEVblog2
Support the EEVblog through Patreon!
http://www.patreon.com/eevblog
EEVblog Amazon Store (Dave gets a cut):
http://astore.amazon.com/eevblogstore-20
T-Shirts: http://teespring.com/stores/eevblog
Donations:
http://www.eevblog.com/donations/
Projects:
http://www.eevblog.com/projects/
Electronics Info Wiki:
http://www.eevblog.com/wiki/
Hi in this video, I'm going to show you how to calculate the remaining energy capacity in a battery like this. For example, you triple a double AC D sized alkaline batteries you might design into your product. How do you know how much of the remaining capacity you're actually wasting in eie the area under the curve. Well, let's take a look at it, because, well, some people just can't seem to understand the concept.
So let's say you're designing a product which runs on a couple of double-a batteries. Either a single one or a couple in series or whatever it is. How do you actually know how much of the energy that you're wasting in the battery? And yes, unless you design your product to work down to no point, eight volts cut off foliage as I've explained in several videos before, then you're not utilizing the maximum capacity of your battery now. I've actually done several videos on battery capacity before, what it is, how you calculate it, and how to do measurements and all that sort of things.
So I won't go over too much of that detail again here. You can watch those in depth here, so I'll link them in. click here if you haven't seen those now. I Won't go over all that detail again.
Recap very quickly here. Only the energy that you've got available in one of these batteries is measured in watt hours. It's not measured in, necessarily in milliamp hours. It's not measured in voltage.
It's not measured in current. It's measured in what hours it changes over time. And if you've got a discharge curve which looks like this, this is a typical constant current curve. Then the energy that you have in your battery is all this under this discharge curve here.
and your discharge curve drops off extremely quickly. After Naught point. Eight Volts. It's a industry standard figure to use anything out of nought Point Eight Volts.
It just basically drops off like a rock that is pretty much the lowest cut out voltage you can have. Now, if you've got a product which you design with say a 1 volt per cell cutoff voltage, which is fairly typical as I've shown in a previous video, then if you draw a line like that and down like that, Ok, this is the amount of energy and the curve here that you are wasting in the battery. So this is how much is remaining in this battery if you have that particular cutout voltage. And if you're designing a product that uses nickel metal hydride rechargeable batteries for example, as well as Alkaline, then you basically must design your product to have a cut-off voltage of at most at absolute most one point one volts.
Otherwise, it's not going to work If you set it for one point to well, you're going to be wasting most of your capacity in your battery. And this is why, as I showed in a previous video, I've done tests on products and basically almost any decently designed product on the market that works with both rechargeable batteries and primary cells must have a dropout voltage of 1.1 Typically, it's between 1.1 and 1 volts here. So take this rechargeable battery here. If you set your cutoff voltage to one point one volts and you draw a line across there where it intersects. Bingo! That is how much capacity in your battery that you've wasted. As you can see, not a huge amount, but how much is actually wasted. That's what this video is about. And when you look at data sheets for batteries like this, one critical thing to remember is this: cutoff voltage.
Here all these characteristic discharge curves. This is the battery voltage under load. Now, it is absolutely and totally incorrect to measure the battery voltage when it's not under load. ie.
to take your battery back out of your product and then measure it with a multimeter. It is fundamentally the wrong measurement because batteries will actually regain their voltage when you actually take them out of the product. ie. you remove the load because of the internal resistance of the battery.
That is why every single datasheet for every single battery on the planet specifies they cut out voltage here which is under load. And I can actually demonstrate that here with this solid I've got a brand new battery. Let's take it out of the packet. Here we go.
Let's hook it in, hook it in here. haven't turn the load on yet. Nope. there we go.
1.5 1.5, 8 volts. It's brand new. If we switch the thing on I've got it set to a constant current of half an amp. so let's just draw that and will instantly see the voltage drop right down.
That's due to the internal ESR the equivalent series resistance in the battery. so now it'll You know it's got a fair amount of energy in there, so it can actually generate that voltage for quite some time. It'll slowly discharge. curve.
There we go. Went down a little digit there, but when we switch the load off, the voltage will actually jump back up. So that is why you never measure the battery voltage unloaded like this when you're talking about or specifying capacity of the battery. It's just meaningless in that respect.
It's an OK measurement to see whether or not that battery is still relatively fresh. But it is not the correct way to do it when you're talking about discharge curves and remaining capacity. It's just not. But some people just get this basic fundamental thing wrong about battery voltage.
In the case that I've done a previous video on Cliquey if you haven't seen the Batterer Riser which is an IndieGoGo campaign for this side DC to DC converter battery clip-on thing. they claim that it's one point three volts cutoff voltage and it's absolutely not because they measure and they show this and demonstrate it in their videos that you measure the voltage of the battery when it's not under load. Fundamentally incorrect. That's Battery 101 stuff.
This voltage must be measured under load. So this video is going to be about Well, let's say we designed our product with one point one volts cutoff voltage and it was drawing for example, a hundred milliamps constant current from the battery and I'll explain that in a minute. There's the characteristic discharge curve and we're using the Duracell copper top battery. We draw a line across. there. We draw it down like that and this is the amount of capacity that we're wasting in your battery. And well, there might be a reason why you set it to one point one Volts for various technical reasons in your product and you just have to live with the fact that you're wasting that amount of capacity inside your battery. Now as I said before, the total amount of energy in this battery is the area under that this characteristic discharge curve here.
So how much of a percentage is this area compared to all the rest of it? Well, you can kind of sort of eyeball it and go on. and I was maybe 20% or something like that. But what this video is going to be about is how to actually calculate this properly using proper engineering, not just estimates. Now, if you look at the data sheets here, of course, you'll find that you typically might get characteristic discharge curves like this that are both constant current.
And If you flip it over here, you also get constant power ones as well. And you might also get constant resistance. Which one do you use for what products? I'm glad you asked. I Just so happen to have a Dave Kerr drawing here which shows your basic two different ones: your constant power and your constant current.
I've gone over this in the previous video. There's a lot more detail in there if you want to check it out. I'll recap briefly here if your power in your product. this is the battery power and the product.
And you use a typical linear regulator like a 7805. Not very efficient, but hey, simple and you know quite common this is a constant current. It's drawing a constant current from the battery. Why? Because there's basically no current flowing through the control pin of the linear regulator here.
So the output current is going to be equal to the input current. So if you've got a consumed it's a constant resistance, load, constant voltage, then you actually going to have a constant power in the low. But don't confuse that with constant power from the battery as we'll see in a second. So what we're actually drawing there is constant current from the battery.
So in this particular case, you would want to use the constant current discharge curve here, not the constant power one. But if you had a DC to DC convertor inside your product, then you're going To actually get a constant power from the battery. How does that work? Well as the battery voltage discharges like it did up here. Your the DC to DC converter might be saying you know, 80% efficient.
Ninety percent. Whatever it is, you're actually going to get a bit of power wasted in your DC to DC convertor. So you've got constant of power in your load. But as this battery voltage drops because you're going to have assuming a constant efficiency in your DC decant DC converter, the input current ie. the current from your battery is going to change as the battery voltage drops even though you've got a constant voltage on the output here. So it's actually different to your linear regulate up here which draws a constant current. You're going to have a constant power in this case. So you want to go over to your datasheet and you want to use your constant power discharge curve.
That's for any product which uses a DC to DC converter. Now of course, I won't go into the concept of how DC to DC converters aren't perfect as the input voltage drops. They actually don't have a completely flat response. ie.
the efficiency is not constant over the entire input voltage range. But for the purposes of today's talk, we'll assume it is. and for product evaluations and things like that, it's not a bad approximation to make. It depends on your design of your DC to DC converter.
And if you're wondering about the constant resistance graph here, this is like, you know, real old-school one. There's not too many cases of product design these days. We had actually have a constant resistance up load. so these are two major ones: constant current and constant power.
That's why most artists sheets most good ones will have both of those in there. But that being said, the constant resistance graph is actually kind of like, you know, a bit of an industry our standard test. So that's why they include the current characteristic curves in here so that you can actually compare different battery types. But of course, the problem with most data sheets for these batteries is that you only get a limited set of characteristic discharge curves for constant power to 50 milli watts, 500 up to 1 watt for example, constant current Here you? Well, this is actually not a bad set.
You get 5 milliamps 10, 25, 50 you can go over here. you get, you know, up to an amp. but what if your product is in the middle here? Well, you can kind of sort it, you know. draw your own curve in there and sort of guesstimate and that's not bad for you know, back-of-the-envelope type battery capacity calculations, how long your products going to last, how much you know energy you're wasting in your battery, and things like that.
But hey, we want to do it the proper engineering way. I'll show you how so rather than just draw your own curving there, you actually want to measure it and I won't actually show you the technique for measuring. I've done a previous video on which I'll link in below, but you basically want to get your own measured discharge curve like this. and of course it is the best way to do that. And most accurate of course is to do it based on your real product. So what you would do is you would have your real product here which includes your DC to DC converter. You'd stick your voltmeter on there like that and you would actually log the discharge curve and you'd end up with a curve like this: a B at a constant current or a constant power. or in the case of your real product, your actual real product cuz your product might be switching going into different modes.
It's not going to be constant power things like that. So ideally best-case you want to actually do it with your real product. But hey, if you can't do it with your real product, if you haven't built your prototype yet or whatever, or it's just not convenient for some reason, then you can actually use a constant current or a constant power dummy load. And I've done a previous video on this.
Click here if you haven't seen it. It's a very popular video. A lot of people are making these themselves. It's your own constant current dummy load.
You don't need much, you need an Op amp, a couple of resistors, and you need a MOSFET and that's pretty much it. Build your own. No problems whatsoever. Very common thing to do in the industry.
Just roll your own dummy load. Or you could use a commercial dummy load like these. This one, for example, does constant current and constant resistance. This one does both constant current, constant resistance, and constant power.
And there's a lot of these are types of dummy loads on the market. and actually one like this. You could actually hook up to the PC and do data logging as well. And you can actually get your battery discharge curve directly from it.
Now let's jump on over to our spreadsheet here and analyze the data. Let's just assume that we've got the data into a spreadsheet. Whatever mechanism you use as I said, you could use them data logger for example, multimeter or some other data logger to measure the voltage. Or you could do it old-fashioned way.
pen and paper every minute. Just you know, write down a voltage reading and so we've got the data into a spreadsheet. However way you want to do it and you can see down here we've logged the voltage. It starts out at one point six five volts and that's a fresh cell of course.
And then we're gonna have a we're gonna nominate a cut-off voltage of nought point Eight volts here. and we've got a test current here of naught point One amps. Now, you don't actually have to measure the current if you know you've got a constant current low. but in this case, the current has been measured and we've got the exact value here.
But you don't necessarily need that. All you need is the voltage value and the no one discharge current. If you're just measuring the product itself. Well, the product is the product that draws whatever power, whatever current that it actually does. all you care about is that voltage there. That's what you really want to log. and that's what we've got here on our graph. And just in case anyone's wondering, those little jaggies there are to do with that data decimation that I've done here because there was a lot more data than this Cells actually like a hundred thousand data points.
and I might actually do a separate video on that. how to actually decimate data. It'll be very simple anyway. that's you can see that we've got the typical characteristic discharge curve here.
and this is a real measured battery discharge curve under load. Remember that it must be under load. There's it's just pointless to specify a battery voltage that's not under load if you're talking about battery capacity, energy, or anything else. And that's where that battery's a product is just so wrong.
They use the open circuit voltage of the battery to talk about Energy, remaining energy capacity. It's just ridiculous. Totally meaningless. And if you want to know how good a fit real measured battery characteristic curve is to the datasheet curves, well, there we go.
There's one of the datasheet curves I Won't lie I Can muck around a little bit, but there you go. It's pretty darn close match. But that's exactly what you expect because the manufacturer measures it exactly the same way. I Promised I'd Show you a way to measure the true remaining battery capacity or the wasted battery capacity in your battery.
So let's do that. Now we need to calculate the energy so that what our energy now will do this as a the amount of energy used as the battery discharges first. Okay, just for simplicity sake. So we started out at zero.
Obviously at the first time period we measure, we haven't actually used any energy in the battery, so I will just say zero there for example. Now here comes the We have to calculate the accumulated energy at each point as we go down here. Now put what hours here, but you don't actually said the purposes of what we're doing today. You don't actually have to care about the exact units, it's just accumulated energy.
So it's not necessarily what hours it could be, what seconds or whatever, right? It doesn't actually matter so what we need, so you can just title that accumulated energy for example. So what we need to do here is we have to do a formula. Okay, we've got to go equals to the previous value. Okay, so let's choose the previous cell.
We started from zero, so in this case it's going to be cell d5 and then we've got to add on the current power that we're using. So to get that we of course power is that voltage multiplied by current. so it's 1 point 6 volts. the cell net at the cell in this case are C6 and then we've got to go multiplied by.
Now we could just put in point 1 ants because it's a constant current, but we actually measured it. So let's actually use the real measured value here and we go bingo like that. and we've got a figure. an accumulated energy figure here. Okay, and if you you know if you wanted watt hours and stuff you'd put like divided by 3600 and you know to convert seconds to hours and all that sort of jazz, right? But anyway, we've got our accumulated energy. Now we can just drag a formula down here all the way down. and this is where your spreadsheet does all the magic for you. It gets your total accumulated energies.
So we started out using 0 amount of veggies, 0 watt hours and then at each point each sample point it just. we're using more and more and more and more energy from the battery until we get the final figure right down here at our naught point 8 volt cutout Voltage Beauty. Now we can go and graph this and do some useful stuff with it Now of course, plotting the graph is trivially in Excel or in this case I've been using that LibreOffice slash OpenOffice and we do want to scale it. Okay to not point eight volts at the bottom end here and why we want to do this will be important as you'll see in a minute so we don't want for our minimum.
You know we don't want like a graph like that because it's just gonna be silly. So we want to make sure we set that minimum value down to nought Point 8 volts and the maximum value up there to our foot the scale for the Y-axis here our actual min and Max value. Now the next thing we want to do is we actually want to create another Y axes here on the right hand side. We want to have two different axes because we're going to have a battery capacity on this right hand axes and we're going to have as we've already got voltage on the left hand axes here and you can do these in spreadsheets if you haven't done it before.
A very powerful technique used multiple axes so we can just go in here like this and we can insert delete axes like this and we can have the primary axes or we could have the secondary axes. so we can have an extra y axis here and we've created one. Bingo! The next thing we want to do is we actually want to scale this axes here from zero to a hundred percent capacity. So we want to get rid of the automatic one there, change it from zero to a hundred percent like that, can just fix up the axes there.
We don't want that we. so we want ten for our minor and major there Bingo! So now we've got our remaining battery capacity from a hundred percent down to fully used here. Okay so what we want to do now is we want to go into this graph and we want to add a second line on here. that's a reference to our new right-hand y axes here so we can go in here and we can.
Helps If I go into the graph. Sorry, this is like how to use spreadsheet kind of stuff and it's It's kind of like telling you how to suck eggs I guess So excuse me if you already know about this stuff, but if you don't then here we go. Okay, we'll add a second data series and then we'll actually choose all the data here. Oh come on there we go. And Bingo! We now have a second graph. So now we want to make sure that this line here this data set is associated with the secondary axes. so we have to actually select secondary Y axes there. and it already is because I've already been marking, but there it is.
remain in energy capacity in percentage. but it doesn't quite look right, does it? Hmm, we can fix this now. Remember how I said before that all we care about is the zero to 100% scaling of our figure. But look, we haven't got zero to 100% We're going from zero to you know, almost twelve there.
And that's exactly what we're seen on the graph so that that is of no use to us, we have to actually scale this starter here from zero to a hundred percent. Now of course, that's really easy to do. We can just go equals like this the current cell which is D5 here and then we can divide that by the very last cell down the bottom. But I'll show you an old spreadsheet trick which we have to do here.
we have to go instead of which is cell D. Ninety Nine is the one we want a reference, but one do. Dollars D dollars. Ninety Nine.
Like this. This gives us an absolute value of the cell down the bottom. So when we drag this formula down like this, it won't increment D 99. It won't go D 100, D 101, etc.
like it will with the D five? Here, it'll go D 5, D 6. We want that absolute value cell to be exactly the same. It's like entering a constant in there and we just multiply that by a hundred and we've got our formula in there. No problems whatsoever.
Okay, and we scaled our data up from zero to 100% and then we can go back into our graph here. And then we can go. We can reselect our values here to be this column instead of the other one. and now we'll get a nice graph of zero to 100% scaled.
Beautiful. Look at that, but unfortunately, this is not in a usable form. It's actually going in the opposite direction. We don't want it increasing like this because it's remaining energy capacity.
We actually want to start out at a hundred percent on the left hand side here and then decrease down like that because that's what we want remain in energy capacity. This graph would be okay if we had energy if it was energy capacity you. but you'll see why in a minute. We want remain in energy capacity so we need to fix that.
Now you might be thinking that we could just go in here and flip these axes. for example. you can actually reverse direction of the scale like this and you can go like that and well. that's okay.
and our data is perfect. That's exactly what we want here. But how Axis doesn't make them? The labeling on our axes doesn't make sense anymore. It starts out at zero and what a hundred percent remaining right at the end? No, we want zero. So we have to go back into the data and actually fix this up. So what we need to do is actually a bit fancier formula than what we had before. We have to go Dollars D dollars 99 so reference that absolute the maximum value and then we have to subtract the current value. we've got D 5 and then we'll close brackets and then we divide by that maximum value again dollars D dollars 99 like that.
And then we've got to multiply that by a hundred to get our zero to 100% scaling. And Bingo! We're starting out with a hundred here, so if we drag this down, our graph will instantly. There we go, we're going down to zero. Tada.
And look, that's exactly what we got. Bingo, there's our money shot. There's our remaining energy capacity from a hundred percent down to 0% Beauty. Now we can do the fun stuff.
Now you remember how I said before that if we didn't scale this graph over here properly, For example, if we had this going all the way down to zero like this and these two graphs don't line up at these points here, then well, we can't actually do anything useful with this graph. So it's an old graphing trick data analysis trick that you actually when you align the data point at the start and the end like this and you have the correct axes that you want then you can actually directly draw stuff on this graph and intersect lines and do other stuff and read off both axes so as you will see this in a minute. why this is the case. But it's absolutely critical that you that you have the exact same start point up here and the exact same end point.
And the of course you have to have the same type of scale as well. They're both linear scales and those with the Kenai Might have noticed that this red data series here is not quite a straight line. There's a there's a slight bow in it. downwards bow in that.
Yes, this is actually deliberate. This is the way the math works out when you get changing voltage and maybe a little bit changing, but essentially a fixed current here and accumulated energy over time. you're actually not going to end up with a linear straight line with that. Now we could of course of just put in a straight line like that and not worried about you know they're just a minor discrepancy here, but hey, this is a tutorial is designed to show the correct engineering method behind it and there's no point going this far and then just fudging it by doing a straight line.
You've got real data. If you have real data, always use your real data so we'll get a really accurate. There's no guesstimate, there's no estimates, nothing it. We're gonna be a hundred percent accurate with the real data.
So yeah, you're actually not going to get a linear result there now. I've exported that graph as an image file and opened up in open view so that we can actually draw on it and do some fun stuff. Now first up, if you don't believe me that that red lines not linear, then we can actually draw a line in there. Boom There it is. It definitely deviates. I've actually deviates a fair amount and you could actually get quite significant error if you just assumed it was linear now. Sorry, this video is taken so long. but we've finally got into what we actually wanted to do here and use the graph to actually properly calculate the Romanian energy capacity in a battery.
And we can do this because we've lined up these axes as I said the start data point up here in the end data point. so we can actually draw lines on here and intersect things and working out. Now let's say that designed a product with a cutout voltage under load of course as I've said of one volt here. Okay, so we can actually go along here and draw a line a straight line across here until it intersects our voltage graph.
Okay, and then once at that point there, we actually drop a line down vertically until it intersects because it's the same time period. We want exactly the same time period. That's why we're dropping that line vertically drop it down until it intersects our remaining capacity graph, and then we can draw. And then we can actually extend that horizontally out to our new Y our right-hand Y axes here to get and read off directly our remaining energy capacity.
So as we designed our product with a 1 volt cutoff and we're using a Duracell Copper Top double A, it could be multiple ones in series at a hundred milliamps, constant current discharge, so one volt per cell, Then if our remaining energy capacity after the product shows low battery, look, it's not quite five percent. It's maybe four percent, but you know we could round it to five. But yeah, look, it's bugger or so. We've wasted hardly any of our energy there.
So if we go back to what we originally did at the start of the video and you remember the a wasted energy in our battery is the area under that curve at that point there. So all that that area there, would you have guessed that that was like four percent between four and five percent of that total, Or would you have picked 10 percent? See, you don't know, right? So until you actually go in here, draw the real graph, get the extra axes on here, and actually extrapolate that it's only four percent, so you know you're wasting bugger. Also, if you go design your batteries a product to extend your battery life, or you're building your DC to DC converter in your product a try. And you know, get down to nought point.
Eight Volts. You're only getting an extra four percent at that particular current. It's gonna change, you know, based on the current draw and things like that. But this is how you can get the all data so it's not worth trying to get that extra 5% out of the battery.
In most cases you know you've done the extra bill of materials, cost, or whatever you might have other technical issues going down to nought point, eight volts instead of one volts for example, and so on. So if we now go back to what batteries a claim for example that they have now admitted that it's one point one volts under load issue. You know your typical product. They say like one point three not under load. Well we can do that as well. But to show the fallacy of that, maybe in another video. But look, you extend one point one volts across here like this. Then you drop that down.
Look, you're only wasting like just barely over ten percent of your capacity. So that entire battery is a product they that they claim. Oh, it cuts out at one point one volts load voltage. Your average product look ten percent.
So much for you know, the claim of you know, eight times and all that sort of stuff. It's just it's just absolutely ridiculous. So if you would go and use something like the batteries are on your product that has a cutout voltage of one point one volts and it happens to be drawing 100 milliamps constant current like this, and you're like one point one volts per cell, you're only gonna get at best just over 10 percent extra capacity. And that doesn't include the efficiency of the DC to DC converter.
By the time you include that over the whole range, you might end up with a net result of zero Because you're you know, even the best DC to DC converters a barely pushing 90 percent at a spot figure. So you know, really it could be it could go into the negative region, it could be detrimental, and we can actually go backwards as well. And let's take their claim of like you know some Now they're saying some. They were saying all before, but now they're saying you know a significant number only at waste eighty percent of your battery capacity.
So we can actually go here and we can go backwards and draw that and then we can intersect once we drop it down here. same time period intersect here and bingo, we would have to have an under load under load cutoff voltage of just over one point four volts to get their claim. Now yeah, there might be the odd, old, ridiculously badly designed product out there that might have an under load cut out voltage of one point 4 volts. but ah, Jesus Burger.
Or they themselves have admitted that the typical product cuts out at one point one volts and my own testing and data and experience in the industry of course aback verifies that so that like they've completely changed tune and one point forward. it's just ridiculous. Like, absolutely ridiculous. So it's gonna be very few products out there that are wasting 80% of the battery.
it's just not gonna happen. So you can do that with any data curve you like at low currents at high currents, for example, it doesn't matter what it is, whether it's a resistive load, whether it's a constant power load, where it's a complex load which is most likely and most stark common electronic products that have DC DC convertors built in, and as the battery voltage drops, your efficiency changes all that sort of stuff. Once you have the real data, you can actually go in there and see how much energy you're wasting. Beauty. So this, of course is going to change depending on your discharge load and things like that. And it's also going to change fairly drastically for different battery chemistries as well. Lithium-ion for example, is a much flatter response going out here, so it's sort of. You know, easier to design cut-offs to maximize your energy use.
but you know things like Alkaline. You know they they have this sort of in a very sort of poor characteristic or non flat characteristic discharge curve and you can actually waste a significant amount of capacity in your battery. But as you saw, any product designed to use rechargeable batteries is going to have at least one point one volts maximum or less. And it's bugger-all And when you look at for example, all the different currents, for example, look even from five Milliamps all the way up to an ampere.
So it pretty much covers the entire rain here. You know, if you have that cut out at one point one volts or you know, or a volt or something which a lot of products are even at five milliamps? look Bangor Or one point one volts one? look, you're you know, cover the whole range right there. That whole batteries a concept is like. look, it's the entire range of currents.
Sure, constant care, a bit constant power is not going to be much different. so you're really only gonna be getting that. You know that ten per twenty twenty percent extra available energy? a wasted energy in your battery. For you know, one or one point one volts cut off.
That's absolute best case. I Mean that there's just no getting around it. So this, you know, eight times rubbish relies on the fact that the product is extremely poorly designed and has a cut off here up here under load of like one point Four volts, Do you? Yeah, eighty percent crazy. So I Know this was a rather lengthy video, so if you're still hanging in there, thank you for that.
But I hope you actually learn something here because I've actually never seen anyone actually show this technique of getting the remaining capacity using the extra y-axes on the graph here. So it's not something that you'd you know, typically learning your textbooks or something like that. it's you know, one of these applied electronics engineering techniques that you know practicing engineers actually figure out and use in the field and this is the most accurate way. It is a hundred percent accurate way to actually determine how much remaining capacity you're actually gonna waste in your battery.
So I hope you found that really useful. If you did, please give it a big thumbs up because that always helps a lot. If you want to discuss it, jump on over to the Eevblog Forum is the place where all the comment action happens, but it also happens on Youtuber Ii. Try and read all my comments and the blog website as well. Catch you next time you you.
In general, you explain how to compute an integral and basically, what you say is somewhat correct and it will certainly give you a result which is reasonably close to the precise data. But you made some systematic errors (which luckily won't change the result very much).
1. The capacity is the area under the full curve which goes right down to zero, not 0.8. That is why your first guess of 10 to 20% remaining capacity was a bit on the exaggerated side. If you see the full area, you would – right from the start – have gotten a better estimate.
2. When doing an integral with empirical data points, you should consider taking the previous data points into your sum. You can simply use a linear interpolation by just taking the average of the current and the previous point. Would be a tiny bit more accurat, but of course still based on assumptions. Does not make a huge difference, but just relying the current data points is not the best assumption.
3. You Do Not Divide by the last data point. That is wrong. You divide by the maximal value. Which you do not have measured. It will of course not be far off of what is writte in your last row but remember that the curve goes down to zero. There is a tiny amount of capacity. This does not show in your graph. You can guess it to be only about 1 or 2% of the full capacity, but nontheless, your graph is (strictly speaking) having systematic errors.
Overall, again, nothing new under the sun. Of course, for simple calculations, that is more than good enough. But I just like to be a smart-ass. 🙂
Holy crap, so many bot dislikes.
There is an error in the spreadsheet work. The first energy consumption number is 0, which is an apriori figure. But the second entry shows you taking previous energy consumed (t=previous) + V (t=current) * I (t=current) when in fact it should be energy consumed (t=previous) + voltage (t=previous) * current (t=previous). This isn't a grievous since energy consumed at t=previous and t=current are nearly equal but it does mean that you should have one additional entry, so you're total will be off by a little more than approximately 1/# of entries.
Eagerly awaiting the tutorial on how to suck eggs, especially with Easter coming up.
Hi EEVblog,
I have not seen all then videos in your sequence but will watch them as you also show how to use that spreadsheet. Thank you for your upload.
Take care
mrbluenun
Is that a line graph or a scatter plot? (More important if the X data is not at regular intervals…)
For constant power discharge, instead of constant current, the line would actually end up linear, since the capacity is also a percentage of energy… With the constant current example. the remaining energy is a power percentage, and the power of the load is decreasing as the voltage decrease…
(A normal (mostly) constant resistance load is a non-LED flashlight….)
Thumbs up!!
Real science, real engineering. Opposite of this would be buying 1k dislikes from Vietnam…
I only believe statistics, I forged myself.
it's 4% compared to a 0.8V cut off :/
don't you have to compare it to 0V?
1172 people are Batteriser backers lol
Just to get this right, when looking at your curves I must come to the conclusion that after these kind of batteries are discharged to 0.8V (under load of course) there is only a very very small amount of energy left in that cell?
Otherwise it would not be very engineering like to just put the zero point of energy left right there, or would it? I'm sorry for bothering you, but showing the complete curve (if even possible) would be a better way of debunking that silly product. I'm really keen to see the results of your long term battery test. I hope that it will show the complete curve way down to zero volts (if, as I said before, this is even possible).
Thanks for your great videos, both educational and entertaining, I love it! 🙂
Useful video, been aware of it for ages and I usually test the old alkaline battery pile of my parents every year by plugging some banana plug on one end and alligator clips on the other end leads then hook a resistor up between the two probes that way and measure voltage. If it drops quickly I discard them if they're still decent enough for use in low power applications I'll put them in the "half dead but still useful box".
An interesting fact to be aware of though is that there are some battery chemistries where the open circuit voltage is a sound indication of state of charge. With my 18650 lithium ion batteries its a rather good indication and while I'm no expert at lead acid batteries, if I'm not mistaken it applies to them as well to an extent?
Does the size of the load matter? if i put a 1k load or a 1M load what i measure?
Very interesting. I was kind of hoping for an old school area under the curve measurement where you make very large print on a strip chart recorder then cut out and weigh the relevant sections with a triple beam balance.
they're going to come back saying you stopped at 0.8v instead of going down to zero and throw bullshit all around that.
I really wish we had the current/voltage measurements down to zero volts.
While I'm pretty sure the falloff is so quick that it isn't significant, this experiment does not actually measure the total (theoretically) possible battery capacity; rather, it measures the capacity only as far as the designated cutoff voltage of 0.8V. I reckon there is another 5-10% in there. Not that you could get at it easily without a bunch of cell in series, and without battery leakage to ruin your day, etc. etc. 🙂
Does the wasted percentage of energy have a dependancy on the current drawn? Looking at those curves on the datasheet, it looks like you may waste more and more energy at higher current levels for a given cutoff voltage. It does make sense since higher current levels will drop more voltage across the ESR so therefore it will prematurely reach cutoff level.
I think that technically, you need to multiply that I*V by the (delta t) i.e. the time-step size, after adding the previous energy value.
So,
E = E(n-1) + V(n)*I(n)*{t(n)-t(n-1)}
Since your time resolution was 1 second, you don't get any errors (apart from being in Joules rather than Wh). But if you had any other value, your energy value will be wrong I think.
As right as you are abotu the Batteriser marketing I still have one argument to defend the usability of this thing.
I just made a test with an alkaline battery that had 1.35v with no load in the beginning. I intended to find out how much energy in Wh I can squeeze out of it when draining it from 0.8v to 0v. To first bring it down to 0.8 I put a load of 200mA on it which caused it to drop down to 0.85v under load. After draining 500mAh it had 0.64v with the 200mA load on it and 1.1v with no load. As you can see that were 500mAh and it's still going in the range where everything apart from solely resistive devices like flashlights or some dumb fans or so would've shut off. I can't think of many devices with microcontrollers and low voltage cut off that would put such high loads on the battery but that would be a situation where the batteriser could really help you.
Thanks Dave!
Just what i needed to know for the project I am working on.