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A nice simple device for a change. This is a battery or cell tester that I've been using for a while.
It's extremely ungenerous in its test, so I've always used it as a rough guide as opposed to being an accurate indication of cell state.
Since Grant sent me a new one, I thought I'd take the old one apart, and decided to make a video about it.
I didn't remove the PCB from the meter in the new unit, but I did go back and measure the combined circuit resistance. So if you're up for a puzzle....
3 resistors in parallel measure 162 ohm. If the known resistors are 3820 ohm and 564 ohm, then what is the value of the unknown resistor. (The meter's coil impedance.)
Answer is at bottom of description. I think this may be a 2mA FSD coil.
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Mystery resistance is around 242 ohms.

This is my normal battery tester, and this is one that grant rennie sent me in amongst uh some other stuff he sent recently, and i have to say his not only has a bigger, bolder display, but it's much more generous to cells. Let me show you what i mean here is an alkaline battery and i've always just treated this just as a very rough guide. But if i put this fairly decent alkaline selling, it never really shows much more than just bordering on good. It's a bit wobbly and makes bad connections, but it barely goes into the sort of good region with a new cell.

Likewise, if i stick this pp3 batch in this little tiny, 9 volt battery, it again shows that it's barely good. So it's not that generous uh. The one that uh grant sent, on the other hand, if you put this battery in it, says it's well into the good region and likewise, if i get this battery and hold across the contacts at the end, it shows yeah. It's pretty good.

It's showing itself fairly good capacity, so so i've already did this one apart, but for your pleasure i shall take it apart again, because i stuck it together with sticky tape and, to be honest, it's such a disenchanting thing, i'm just as i say, i've never really Taken it too seriously um, but uh i've decided well, let's sacrifice it for knowledge and see how these things work. So, let's get the tape off that i put on to hold it together because it was a bit destructive when i took it apart and when we take it apart, we find inside that it's just some resistors and a little moving coil meter uh right at this Point i should take a picture of this uh. I should take a picture of this and then we'll explore in greater detail. One moment please and explore so this thing uses this moving car meter and the reason they call it a moving car meter is that if i bring this right up to here, let's zoom in on this and then i'll focus on it.

So it's actually in focus. If you look at it - and i shake it, a little bit you'll see that it's actually a little copper coil that actually moves around a static magnet. The reason for that is because it's a very low mass, it means it can respond easily and sensitively. Let's go back down to here and focus on that, so the coil itself has a resistance in this case, well, impedance, technically speaking, because it's a sort of inductive component, but it's 736 ohms and it's a one milliamp full scale, deflection uh meter.

What that means is that, from here with zero current, if it you pass one milliamp through it, it will deflect right up to the end. I could potentially demonstrate that if i actually cut these resistors off, this thing is toast anyways, it's no longer going to be used because it's rubbish, that's why and if i actually get my meter and set it to diode test that passes about a milliamp. So, theoretically, this should make the meter either slam into the end or hover in that direction. Let's try this polarity.

First, it will either go one way or the other. It went down the opposite direction. So let's turn the other way around and pass the one milliamp through the other direction. Fumble fumble - and you can see it's pretty much just with that one milliamp is passing through it.

It's just gone up to the end, and that is it at this full scale. Deflection anymore and it just slams into the end of the limit. Now we know what happens uh to actually work out how to get this to display the battery voltage. We have to add resistors, um and there's a couple of resistors per battery.

The nine volt battery has um. This resistor has a load 220 ohms to give it. If you don't load the battery down, it can give a wrong indication. So when you put the batteries into these testers, they actually do put a resistor across it to actually give it something resembling a load.

In the case of 1.5 volt cell, that's higher capability, it's a 4.7 ohm resistor, the 9-volt battery, because it's a higher voltage to limit the current. Through this coil. It uses an 8.2 000 ohm resistor and the 1.5 volt battery uses a 1.2 000 ohm resistor. So let me show you the schematic for this here's the schematic.

I shall zoom down a little bit closer. So here we have the meter coil with its 736 ohms, it's quite an odd value, and there is the 1.5 volt battery connection and here's the nine volt battery connection. If you look at the 1.5 volt side, it's got that 4.7 ohm resistor to provide it. That load to try and pull the voltage down a bit to give it an to emulate that it's actually driving something.

What current would that be? Let's work that out, i equals v a freshly charged battery 1.5 divided by the 4.7 ohms. It would be 320 milliamps. So i'll just ride 320 milliamps there. Okay, it's going to be a lot less for the 9-volt battery.

Let's do the maths for that. So it's got a 220 ohm resistor across it uh, so excluding the 1 milliamp. That's going to pass through the meter. It's uh going to be um.

I equals v, which is going to be 9 volts for fully charged battery divided by the 220 ohms. So it's going to load it down with about 40 milliamps. So let's write 40 milliamps here. So when you connect the battery, it pulls the voltage down if it's uh say a well-used battery and that will determine then how much flows through this resistor in series with that resistor for the one milliamp full scale deflection and using these values.

Let's assume, then, let's work this out, i equals v over r. Let's see, what's that current we're going to get actually, we can work out what voltage if we work out uh. If we multiply the uh combined resistance by the 1 milliamp, it will give us the voltage required for full scale deflection. So let's get the kink calculator into action and uh work that out, so that would be uh 1 200 ohms, plus that plus 736 ohms, which is about 1 9.

Three, six, one thousand nine hundred thirty thirty six ohms times the one milliamp required point: zero: zero. One means it's going to have to be a 1.9 volt to actually make it go right. That's why it's not been very generous this from day. One has had the wrong values of resistor in it.

That's rubbish, okay, but it is what it is uh. So that's more or less it a resistor to load the cell down and another resistor in series to actually uh limit the current through that and just basic of what that resistor does. Is it sets how far the needle is going to go for a specific voltage? Now i do have another picture: it's the guts of this one and it's much simpler if it's using a very similar meter, but they've got a tiny little surface mount board across the back of this. Incidentally, if you notice a red dot of uh paint in this, it's because to get that at the zero position, they fine tune.

The zero position there's also this little thing for manually adjusting it, but the function is your position then put the red paint on to actually lock it in place. So it's just for the calibration of that, but this thing has very different values of resistors and i've noted i drew this the schematic down for this, so i can show you this in this case the newer one has a 3.9 ohm resistor. Let's see what that's going to do so, let's work out how much current is going to load the 1.5 volt battery down um i equals v over r. So that's a freshly charged battery one point well not charged with nickel metal hydride batteries.

That will always show a lower voltage because uh the inherent voltage is lowered. It's measuring the voltage so 1.5 volt for a fresh alkaline battery um divided by 3.9 ohm. So it's going to test it more at 385, 85 milliamps. In the case of this one, it's using the same 220 ohms.

So it's going to be the 40 milliamps again, which makes sense because the the 9v batteries are lots of tiny cells in series, so they're not really rated for the high current. It's worth also mentioning that these have a much lower range um for the button cells, because the button cells themselves have a very low sort of high internal resistance. So by when you put it across, it's never going to get up to the end with that resistor across it. It's going to be in this low end, but it still gives you a good indication, but in this case uh i'm not sure what the impedance of that coil is, but they've used a higher value resistance.

Is that harvard? No? It's lower they've used a lower value resistance um to actually fine-tune that, so it doesn't go up to the full end of scale when it's got a fresh battery. So it's interesting it's a very simple thing. Uh i get the feeling that uh it's a design that has just basically continued over the years. It's you know that old meter, i've just taken apart, is probably a very close relative of this one, but with sort of fine-tuned values, but there we go.

That is it that's, what's inside these simple battery testers, they all they do. Is they put a load over the battery and then measure the voltage, and by doing that, they can tell you roughly how much capacity is left in it.

17 thoughts on “Inside a simple battery tester with schematic”
  1. Avataaar/Circle Created with python_avatars Angel Delvax says:

    Are you shure the painted screws are for calibration? I was told that they are locked because they hold the coil on to the needle bearing, and they are locked in a position where the bearing has least friction while still holding on to the needle as precise as possible. Especially since you can easily change calibration, as you showed.
    (Not saying you're wrong, just questioning what explanation is the correct one..)

  2. Avataaar/Circle Created with python_avatars Tom Schmidt says:

    My go-to battery tester is an old EICO 585 kit I built years ago. Has a rotary switch for different battery voltages and load currents. Not much use these days for the higher "B" battery voltages. It has front panel terminal strips for custom voltage and load current resistors. Guess I could determine values to test button cells but we have so many flavors floating around probably not worth it.

  3. Avataaar/Circle Created with python_avatars Akinaro says:

    Its nice device to maybe make DIY version… I know it never gonna be really accurate and most of us have multimeters anyway, but its nice idea for small project… like for quick check of 18560 or something like that.

  4. Avataaar/Circle Created with python_avatars Xeraser says:

    Hey Clive, I have the digital version of the tester on the right. Same exact thing, just digital instead. Is it any good? Any chance you might take a look at it? It seems to work well enough for me but I don't have an accurate tester so I have no idea if the measurements are correct.

  5. Avataaar/Circle Created with python_avatars Breeman says:

    My Dad had this exact same tester. I have many fond memories as a kid testing spare batteries to put in my toys and various electronics. I now have a different newer tester that I use for my kids toys, but I very much remember this one.

  6. Avataaar/Circle Created with python_avatars Adam Carmichael says:

    When I first saw the poor meter, I was reminded of a pot plant indicator my grandmother used to have, a very similar dial for dry and moist. I wouldn't be surprised if it's a very similar circuit inside and a metal spike sampled the soil's resistance as a function of dryness.

    Thanks for the trip down memory lane 😀

  7. Avataaar/Circle Created with python_avatars No music RC says:

    So sadly my 30-year-old radio shack battery tester died I've taken it apart looked at every connection everything looks fine nothing's broken but it refuses to work I tried to buy the exact samee one on eBay but nobody has it so I bought a different radio shack battery tester

  8. Avataaar/Circle Created with python_avatars MetalheadAndNerd says:

    Good modern battery testers which have a proper load unfortunately don't exist. I searched for years. The least rubbish one is a digital one from Westfalia but it has the stupid habit of going into standby exactly 2 minutes after it is turned on, no matter whether I am busy testing several batteries. This standby mode turns off the display but uses enough power to deplete the internal battery within a day. There is no indicator to tell this standby mode apart from proper off mode.

  9. Avataaar/Circle Created with python_avatars ESC crasci says:

    Interestingly, I was actually able to find one of these with a electronic display which is accurate upto 2 decimal places. That has actually been as accurate as my Mastech multimeter. But these analog ones, I wouldn't trust as much from the get go.

  10. Avataaar/Circle Created with python_avatars Ben Baselet says:

    I have the same dinky square meter. It sort of gets the job done somewhat for sorting random batteries.Then again I have now a programmable load which cost a few hundred bucks and I barely use it so maybe it's time to throw that old dinky thing in the bin 😀

  11. Avataaar/Circle Created with python_avatars gadgetman36 says:

    The issues with an unpowered tester like this are:- 1) The load can significantly drain the battery being tested. 2) It isn’t very accurate (as demonstrated in the video). 3) It doesn’t measure the actual battery voltage. I much prefer battery powered testers because they don’t have the drain/accuracy issues, and some also display the actual battery voltage on an LCD.

  12. Avataaar/Circle Created with python_avatars Dom Wright says:

    I was half expecting to see you "fix" the old tester by putting new resistor values in it.

    My old Maplin multimeter has battery test functions when plugged into the mA socket. It just gives a reading in volts while putting a small load across the battery.

  13. Avataaar/Circle Created with python_avatars Ranger Kevin says:

    My guess would be that the old tester was built that way so that people would buy more new batteries, since the tester would show them that the old ones are "on their way out".
    Or whoever designed it just did not do the maths.

  14. Avataaar/Circle Created with python_avatars devttyUSB0 says:

    It's a nice and easy to grok circuit. Good for the young'uns to get a grip on calculating voltages at certain points in the circuit. Perhaps you could have shown what 'loading the cell' does to the results, or perhaps that is homework for the young'uns? Thanks, Clive!

  15. Avataaar/Circle Created with python_avatars utubeuser1024 says:

    I had the square one growing up (in white though) and always wondered why it was so harsh on the battery capacity – a 680 ohm resistor in place of the 1.2K one would make it a lot more accurate – that way a fully charged NiMH cell would get a full deflection.

  16. Avataaar/Circle Created with python_avatars Edward Jones says:

    I got a larger type one for someone when they didn't wan to have to faff around with a multimeter to test batteries. Of course, it didn't do rechargeable batteries correctly. Surely someone will have made one that actually can do rechargeable cells, even if there's a manual selector to choose type.

  17. Avataaar/Circle Created with python_avatars matthew Beddow says:

    I've got the tester which is the same as the one grant sent, I've had it for many years and it's pretty good for a quick check if I can't be bothered to put them across one of my multifunction meters.
    I've also got a very good coin cell tester that tests many of the coin cell types and gives a number of LEDs rather than a voltage so it's good again for a quick test.
    Thanks for the video, it was interesting.

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