Hi Somebody on the EV blog Forum asked what is the battery life of the Bm786 multimeter and well, the answer is I don't precisely know Um, and of course it depends on the type of batteries uh, you're using in it. Of course the uh, the specs in the manual are just that. it is just the current consumption I.E 8 milliamps in most normal functions, 10 milliamps in AC plus DC So anyway, I Thought we'd actually measure that and then actually look at how you would estimate the battery life of a product. If you can't, simply plug the batteries in and leave it on and have a timer there.

you know if you've got something that's got hundreds of hours battery life, uh, then you know that could be a problem. Sometimes you want to get a nice quick estimation of a battery life. so let's see how we do that. And I'll link a video up here and down below if you haven't seen it.

It's from my second channel from quite a few years ago on. uh, why the 121 GW multimeter uses uh, double A batteries instead of nine volt batteries and that has some interesting stuff in it if you're interested. So the Bm786 it uses three AAA batteries like this in a quite unusual, uh, vertical, uh battery configuration. Like that, it's got a spring and terminal down there, which makes it a real pain in the butt to get in there and actually, uh, measure So we could of course skip this measurement and just use the data sheet value.

But let's double check to see um, what Bryman actually say the power consumption is. They say eight milliamps. Well, let's actually measure it. Put some bar tape in there.

Just a temporary thing like that, and there you have it on DC volt mode. It's five milliamps. It's not 8 milliamps, so it's less than we thought. Uh, three AAA is, of course, a nominal 4.5 volts.

We can actually, uh, go above that and it's going to stay the same current and it should stay the same current. When we drop it like this, let me explain why. Let's go to Dave CAD here and I'll show you how this particular load ie. this multimeter actually works.

It's going to have a linear regulator inside. none of that switching regulator rubbish. It might have multiple linear Regulators but for the purpose of this discussion, it doesn't matter. We've got our battery over here.

Okay, And then we've got our current. uh, draw from the battery which is what we just start measured that five milliamps or so. And then we've got current which goes into the load, which is all the circuitry for the multimeter. Now, of course, the low can switch and you can have, like, you know, pulse currents and all sorts of things.

But a multimeter like this is like a fairly consistent five milliamp load there, as you uh, saw. So this linear regulator, of course, is going to have a little bit of quiescent current. Uh, it's called down here, which is why I've labeled it IQ Like this, but generally that's in like the micro amp region or something like that, whereas your load is in the milliamp regions. So as a rule of thumb in electronics, anything more than like two orders of magnitude.

so you know a hundred, one, one hundredth of something, you can pretty much rule it out. That's like one percent error doesn't matter, and if you get lower than that, well, it, you know it just becomes insignificant. So you can rule out IQ So for a linear voltage regulator, and this is something you should know as a basic building block thing for. Uh Regulators Like this is your output current, your load current is equal to the input current.

So effectively, because the linear regulator is a fixed voltage across, essentially a fixed load. Your effect essentially getting a fixed current or a constant current. Essentially so, as you'll see in a minute in the battery are data sheets we're drawing from the battery. essentially a constant current load.

If you're using a linear regulator, that's what you've got switching. Regulators Very different thing. anything with changing loads. Very different again.

But in this case, we've got essentially a simple constant current load. Which makes things really easy when we're trying to estimate battery life, as we'll see shortly. Just as an aside, the reason I'm using this rodent Schwartz NGA 100 power supply is because it's the only power supply I've got here that has a real low current mode. It's got like Precision measurement k capability ideal for measuring low current devices like this.

As you can see, it's got one micro amp resolution there which is absolutely incredible and I can actually go in there and modify that to like a fixed high range for example. and you'll see that even in the Uh 2 Amp range, it can get 10 microamps resolution. Terrific stuff. Not all power supplies have this and if your power supply doesn't very likely then you'll have to add a multimeter in the in series with the output to measure the current.

And here is where you can come a gutter Because here's where burden voltage becomes a big problem. Then this is why I developed specifically the microcurrent is so that it minimizes the burden voltage of your multimeter because the absolute last thing you want when you're trying to measure the voltage Dropout point of your product over here because we have to adjust this four and a half volts until we get down to where you know the low battery warning comes on or the product stops working in. So we need to know what that voltage is precisely. So I'll illustrate the problem here.

You've got your power supply. You've set it to precisely 4.00 volts. Okay and then, but it doesn't have an accurate current meter so you have to put an external multimeter in here and the current shunt inside the multimeter has a resistance and that's going to have a voltage drop when the current flows through it. It's just simple Ohm's law.

So depending on the current range you choose in the multimeter depends on which value shunt resistor I have. You're going to get a voltage drop when you pass current through your device under test. Now, if you're at very low currents, it may not matter, right? but then you don't get the resolution on your current range on your multimeter like you might want. So you might think that you're at four and a half volts.

uh, battery. And that's where you see your battery uh Dropout sign. Come on. But you're actually it's like three volts or three and a half.

Some multimeters can have like a one volt uh, burden voltage on them or more. Now of course, to overcome this, you can put in a second Digital model meter in here and you can actually measure the voltage uh, actually right at the input terminals. but you're going to need two multimeters for that. As I said, always have two multimeters so just be careful.

You can really come a gutter with that and your measurements won't be accurate. Now you could of course have a low burden voltage multimeter like the 121. Uh GW. There's not many on the market that have low burden capability, but it just be aware.

just because it's low burden, it's it's lower than a regular multimeter, but might not be low enough for your particular requirements. Just be aware of that. Read the manual, know what your burden voltage is and for those playing along at home in AC plus DC mode, Here, we're measuring about nine milliamps and if you want to know what happens if I turn the backlight on, there you go backlight. Oh, it's a killer 42 milliamps.

Oh, anyway, let's measure and then estimate the battery life of this product. uh, without having to time the thing. So we'll take that as five milliamps and we start out at the maximum voltage there: four and a half volts. It could go above that, but you know it won't stay that uh far for long.

for Alkaline R batteries. for example, if you're using Lithium, it could go higher. but as I said, we can actually go in there. and if we adjust the voltage 5 volts for example, there you go.

It's still only going to be drawing five milliamps because as I said, it's a constant current load. Then now lower this until our battery uh icon comes on. or however, you determine that your product stops working. Now, just be aware there's maybe some lag on the time it takes for the low battery uh symbol to come up.

So I know it's not going to be four volts so there's no need to muck around there, so it's going to be somewhere below that. There we go I saw it come on there at 3.5 something. So at 3.6 Oh yeah, 3.5 out. Now there could be some hysteresis here I.E when it actually switches back off, but that doesn't really seem to be.

Yeah, so let's definitely say 3.58 volts there. Okay, and we're going to assume that the meter is still accurate just at that point where it comes on. Now, of course, you could go into more advanced stuff, like actually measuring the performance of your instrument, but that should have been part of your design process rather than just, you know, uh, like estimating the, uh, battery life, which is what we're doing here now. 3.58 divided by three.

Get the confuser out. Uh, that's a smidge under 1.2 volts. So we definitely aren't extracting all of the energy from these AAA batteries because I've done a ton of battery videos and I'll link in the playlist. Uh, down below, and batteries do contain usable energy right down to 0.8 volts.

But in products like this that use a linear regulator like this, or multiple linear Regulators then So yeah, it is common to weigh some energy in your batteries like this, and that's just an unfortunate side effect of a product like a multimeter that you don't want to have a switch in power supply in here because that can screw things up. You know, this is a Precision measurement device, so the last thing you want in this is a switching converter to power your like dual slope a to d inside this thing, you know Precision measurements? not yeah, no thanks. So it's common in products like multimeters like this to sacrifice uh, some of the energy in the batteries for measurement performance and uh, simplicity. So let's look at some data sheets.

Let's just take a Duracell ultra power AAA here and we know we've got a constant current load here and this will go into the different varieties and sure enough, look at this constant current graph. They've got two constant current graphs here, but one one one Murphy's Law Wouldn't you know they've got one milliamp and 10 milliamps. They don't have 5 milliamps and as you can see, it's quite a large discrepancy between these, so you could actually like, try and guesstimate it like by sort of matching that but like it's probably not going to be in the middle, it's more likely to be like further over to here it's going to be a non-linear thing going from one milliamp year to 10 milliamps here and then. as I mentioned, there are different load types.

like if you had a DC to DC converter in your product for example, then all things being equal. it's essentially a constant power thing. but of course it has to do with the draw like the efficiency of your DC to DC converter over the load range and all sorts of things right? But essentially if you've got a decent diesel converter, you'd be looking more at the constant power graphs than you would for the constant current graphs. Now, unfortunately, we can't just use Ohm's lowering.

You remember the voltage is dropping like that. So yeah, um, it's not the correct thing to use. And also a thing to note that is these: All these things will not only depend on the manufacturer of the battery, they'll depend on the batch of the battery, they'll depend on the type of the battery, they'll depend on the chemistry of the battery, and they'll depend on the temperature as well. And you can see here for the same uh draw they don't.

They assume it's like constant, uh, Power draw. Look, there's quite a difference in service hours from like three hours up to nine hours here of battery life from -10 to 21 degrees. So that's only a 30 degree. well, 30 degrees.

There's a lot of temperature differential, but that is a huge difference in battery life. Two and a half hours to nine hours just for temperature, right? So we go over to an Energizer over here. Well, they've got a milliamp hour capacity here. Continuous, uh discharge at a constant discharge current.

So constant current. But why only 25 milliamps? That doesn't help us. We need five milliamps and then down here on the industry standard tests, these are actually a resistive load. 5 ohms, 24 Ohms.

They've got one down here, which is a constant current 250 milliamps. They've got another one which is 50 milliamps down here. But they're like milliamp hours per day and stuff like that. So yeah, nah, it's not what we want now.

Of course, if we want the ultimate battery life in a product like this, we would use a Lithium primary battery. Check it out our discharge profile. We don't get five milliamps. We only get one milliamp and 10 milliamps.

But look at the differential right in these graphs. Unfortunately, it's not rendering. My PDF viewer is not odd there. It didn't render that properly I Found a bug in drawboard PDF which is a software I Use A lot of people actually ask me which software I use Anyway, you can see that it is somewhere between these two lines here, right? And as you can see the height, the longer up you go, the more battery you discharge.

There's essentially no difference between 1 milliamp and 10 milliamp draw. Essentially, so we're essentially good to go. We can accurately use this. So where did we, uh, determine? It was just under 1.2 volts.

If you remember per cell, you remember we've got three cells in series. Uh, so we were 3.58 volts divided by three. so you can see down at 1.2 If we extrapolate that over here, you can see that we're really extracting pretty much at the absolute maximum energy out of a Lithium battery. Beauty We're not wasting any, as we'll see that we will in an alkaline.

So yeah. Fantastic. So let's just call that like 1200 milliamp hours. So we get the confuser out.

Uh, 1200 milliamp hours divided by 5 milliamps there. 240 hours of battery life for the Bm786 on uh, using these Lithium primaries, so that's going to be reasonably accurate and does and what the temperature should have lesser effect on these as well, which is one of the advantages that they actually have and you can really see. Look at this, you can really see the ESR just like go through the roof right there. This is how batteries die.

Their internal series resistance just suddenly goes and yeah, and it just doesn't give you the volts anymore. It Ohm's law Like, well, you're talking uh, Kirchhoffs now and they do have a nice graph over here. and as I mentioned before, that non-linearity right? They actually show you it's they use a log graph here. This is constant Uh current here.

So so Two, three, four, five, so five up there. Uh yeah, we're talking about that 240 odd hours we were talking about before. so you know that that Line's a bit Rough and Ready. But yeah, you can get an estimate from that.

And yep, they give us a temperature effect on capacity for constant current. Unfortunately, only at uh, 25 milliamps there. but you can see that the 25 milliamp is the thick one there. so at the lower currents and five milliamps, that'd be like flat as attack, right? So there's effectively no change uh, in the battery life over temperature with these Lithiums, But that's what you expect.

that's why you pay a huge premium for it. Anyway, We want uh, Alkaline or Manganese um, dioxide, Zinc Manganese dioxide here for those uh playing along at home. So let's go to the Energizer again. but this is the Energizer Max.

You can see that this Energizer Max data sheet is significantly different. It's got milliamp hour capacity and then constant power performance. constant current performance here. whereas before we got like the these industry standard tests.

So the max version is you know, significantly different data sheet. We do have constant current performance. What do we got here? What lowest value we've got is 10 milliamps here. So 5 milliamps is off the graph so our 1.2 volts cut out.

You can see how all of these manufacturers, like even the same manufacturers can give you totally different curves. You know you've got to be able to interpret all these different types of graphs. This one gives you a characteristic curve of voltage here. like this: a plotted against the Uh discharge in milliamps and giving you the surface hours.

So we would have to extend this graph right up here like this to try and get our five milliamps here. But as I said, it's a logarithmic scale anyway, so we do know the figure is somewhere above 100 and should at least do that so we can go to like a Phillips brand here. They don't give us anything. Let's try the Germans Vata Shall we discard? Does type load five items 24 Once again, this industry standard stuff here.

That's all they got. Let's go for a Panasonic jobby shall we? And once again, we've got like significantly different characteristic. We've got load in Milliamp like this is real old school. Like they've like plotted this like on an actual pen plotter or something.

But once again, the five Milliamps that we want if we extrapolate that up, the curves don't go that far. Once again, it's the end voltage per cell. So we're looking at the 1.2 volt curve here. So yeah, like eyeballing that it's somewhere between 150 hours and 200 hours? I'd say it's not more than 200 hours.

Ah, like we've got nothing that's absolutely precise yet. Then we've got this Panasonic Alkaline Handbook here. Once again, you can see that they stop at 10 Milliamps here, but you know once again, 1.2 volts, this is the lowest curve here I Don't know I that's a bit how you doing, isn't it? Five Milliamps go up there like that and yeah, we're talking 180 hours. So as it turns out on that same EV blog Forum thread which I'll uh Link in down below for the Bm786 uh Joe Smith uh, following his videos down below, you know it does Multimedia destruction.

Multimeter tense: extensive multimeter testing. That's pretty much all he does on his channel. Um, he measured 118 hours on the Bm786. I you know it's like 118 is like is down here I can't see a tail in off like that.

but as I said, huge discrepancy temperature battery type battery Brands Uh, slightly different electrochemistries between manufacturers. All sorts of things apparently. Uh, some people have measured like eight Milliamps on the Bm786, so there seems to be some variability in the actual meter itself. So anyway, it looks like it's easily going to get 100 hours.

We could split it, say 150 hours. Something like that. I Think mine at Five Milliamps will actually last probably 100 1380 hours. Something like that.

I can do another follow-up video. leave it in the comments down below if you want me to do that anyway. There's like more you can do to this and obviously nothing beats actually putting it in the product itself. especially if you get any sort of pulse current.

And as it turns out, uh, that. uh, Roden Schwartz Power Supply I just press the log button there. It is like the Five Milliamps because you can see just in DC Volts mode. There's not really a huge discrepancy in the current there.

and I think most other modes will probably similar when it flashes the backlight and stuff like that. The backlight seems to be you know a major thing, but easily get 240 hours for those Energizer Lithiums over here. But as I've shown you in many uh, other videos, the voltage that you set over here for like the minimum Dropout voltage for your product, that determines how much energy you actually waste and in this particular case, the energy. You have to extend this all the way down to there, all the way down to zero volts down here, this just drops off like a rock.

Yeah, but all of this under here is the energy. The area under the curve is the energy. So all that energy is wasted. So compare the area of that compared to the area under this side of the curve here.

and that's how much energy you're wasting in alkaline batteries Where you don't waste that in the Energizer Lithium ones you saw. There is no area under the curve, right? You're wasted practically nothing of that battery. It's it's fully discharged so your better eyes are ain't going to extract your 800 percent. It's not even going to extract an extra eight percent.

Really. So I hope you enjoyed that video and found it useful. If you did, please give it a big thumbs up. As always discussed down below, catch you next time.