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I'm making a voltmeter for household batteries, what resistor values should I use on the input? Answered

So I've made a voltmeter, a basic outline is it has a different amp as the input, this feeds into an ADC DCA feedback system to find the accurate value. It works fine as it is but the purpose of it is to find the voltage of household batteries. I have heard that for this I will need to use large input resistors on the difference amp so that the voltage measured is more accurate when the battery is near depletion or something like that. Is this true? If so could someone explain it to me or send me a link to somewhere that does because everything I've found so far seems to be vague. I link would be great regardless as I need to show evidence of what I've found.

Thanks in advance.

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Best Answer 3 years ago

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TomV48

3 years ago

Here is the rough circuit diagram, it might not be entirely accurate so if there's any questions just ask.

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icengTomV48

Answer 3 years ago

It looks like a clumsy ADC Analog-to-Didital-Converter using two NE555 timers lots of gates counter and latch to deliver a differential voltage into a PIC microprocessor.

Why don't you just use a PIC that has an ADC already built into the IC ?

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TomV48iceng

Answer 3 years ago

Unfortunately this is a school project so I'm limited as to what I can use.

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icengTomV48

Answer 3 years ago

What is the mystery pic microprocessor ?

You are actually going to build this ! ?

The op-amp works by maintaining the inputs at the same voltage, the input protection diodes are perfect protection for accidental attempts to measure over 30V.

When a uP, 555 or any digital IC switches it generates a transient that travels on the power bus and trips delicate analog devices before they should... So a diode resistor capacitors try to absorb digital chatter for smooth analog DC.

You should have two sets of power bus lines that join at only one point..

How is the pic going to display the result ?

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TomV48iceng

Answer 3 years ago

Yes I have already built this and it is working fine so I don't need any further advice thanks though.

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TomV48iceng

Answer 3 years ago

Thanks but the point was to make one myself, using a chip would defeat the purpose, sorry if that was not clear.

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3 years ago

Measuring voltage alone is useless when it comes to batteries. I once had a healthy but dead lipo battery for a quadcopter, I wasn't sure if it was a dead or not. The nominal voltage for that battery was 11.1v, 12.6v when fully charged. It was measuring about 12.1V, I assumed it was all good, until I tried to fly my quadcopter. It simply did not have the kick to it I was expecting, and it only flew for for a couple minutes. Looked at the battery again, it was down to 9V, which is the danger zone for LiPo batteries, anything below 9V and the cells are permanently damaged, and regular chargers would refuse to charge them. Point is, it is not the voltage that you need to measure, it is that, as well as the output resistance ESR. Ideally you should track the capacity entering or leaving the cell at all times so you have an idea of how much energy it absorbed and depleted. That is not easy to do. When a battery is dead it acts like a worn out runner. If you let it rest a little, it can recover a little bit and it will push onward and forward a little more, but it will quickly die again under any load.

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TomV48-max-

Answer 3 years ago

Okay thanks I'll bare that in mind.

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Jack A Lopez

3 years ago

I think the word is "differential amplifier"; i.e. an amplifier with two inputs, whose output is some constant gain multiplied by the difference of the two signals at its inputs.

I mention this just to make sure we're discussing the same thing here.

Yeah. A resistor across the inputs of your differential amplifier is a good idea for a couple of reasons.

One reason is that the amplifier inputs themselves might have crazy-huge input impedance. So a moderately big resistor, like 10K or 100K ohms or so, will discharge any DC voltage present on the stray, unintentional, capacitance that might exist between the amplifier inputs.

The second reason for placing a resistor across those inputs, would be to act as a load for the battery. Here I am assuming that maybe you want to put a small load on the battery as you test it.

The effect of measuring the voltage on the battery, under load, is you get to see the battery voltage while the battery is doing work on the load. Essentially what this test does is look at the effect of an internal resistance inside the battery. I mean, assuming the battery can be modeled in such a simple way (as an ideal voltage source in series with a resistor), there is a term - I*Rinternal. So the measured voltage of a battery is highest at no load (when I=0), and lower by an amount I*Rinternal when the battery is doing work (when I>0)

https://en.wikipedia.org/wiki/Electrical_load

This is the same thing Seandogue is referring to when he mentions the Thevenin equivalent source.

But you're probably still wondering, what might be an appropriate sized load for a "household batteries"? I don't know what you mean by that, but I am guessing you mean alkaline, or carbon-zinc cells, sized AA, AAA, C, D, etc.

It turns out, I've got an old-school Radio$hack brand battery tester toy, on my desk here. So I decided to measure the resistance seen looking into its inputs, and my (HarborFreight(r) ) voltmeter is telling me that it sees about 10 ohms.

So it might mean 10 ohms is the resistor the designer of that tester decided was an appropriate load for single AA, AAA, C, D sized cells. Also guessing this device is not doing anything tricky; i.e. it is using the same sized load for all those cells.

I am willing to upload more details about this tester toy, like maybe a picture of it, if you think that would be helpful to you.


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TomV48Jack A Lopez

Answer 3 years ago

Thanks, yes they're the same thing. That would be great though if you wouldn't mind, I'm not really sure what I'm doing as you might have guessed.

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Jack A LopezTomV48

Answer 3 years ago

Attached to this post are some close-up, explicit pictures of my old-school Radio$hack battery tester toy. Almost everything ever sold by Radio$hack had a catalog number, and the number for this toy was: 22-098.

The toy itself does not use batteries, which is convenient. It is powered by the battery it tests. Its display is an analog galvanometer, and the scale on this is divided into two sections: the low side is labeled "BAD" and the high side is labeled "GOOD". Pictures 1 shows just the toy alone. Picture 2 shows me testing a AA battery, which is apparently good enough to make the needle reach the second "O" in GOOD.

Pictures 3 and 4 show me using an ohmmeter (a HarborFreight(r) multimeter) to sort of probe into those inputs, to sense how much resistance is there.

Pictures 5 shows the toy from the back, with its cover off, so you can see the circuit inside, essentially a network of resistors, plus the galvanometer.

Picture 6 is my interpretation of the circuit, in diagram form. The resistor values are guesses based on the colored stripes on each resistor. Every resistor has at least one red stripe on one end, and I am guessing this means they are all 2% tolerance resistors. There are five resistors total. In the circuit diagram, going clockwise, these are: 10 ohms, 10K, 240 ohms, 10K, 3.3K

It looks to me like the smallest resistors in this circuit are those placed in parallel with the battery inputs. These are 10 ohms, for the 1.5 cell tester, and 240 ohms for the boxy "9-volt" style battery tester. These values, 10 ohms and 240 ohms, agree approximately with the resistance values seen by testing with the ohmmeter.

So basically, the designer of this tester toy, made the decision to use a load of 10 ohms for every size (i.e. AAA, AA, C, D) of 1.5 cell.

However, to me this "one size load fits all" approach seems kind of weird.

I mean, if you actually look up the data sheets for these different batteries, they'll have some numbers describing typical sized loads that they expect their batteries to work with.

Anyway, I am hopeful this close up look at a battery tester will maybe give you some clues about the art.

battery-tester-01.JPGbattery-tester-02.JPGbattery-tester-03.JPGbattery-tester-04.JPGbattery-tester-05.JPGbattery-tester-06.JPG
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seandogue

3 years ago

For some head-banging fun, research "Thevenin equivalent circuits" Batteries are often modeled as Thevenin sources...

Hint. think votlage divider.

Also search for the topics about accurate measurement and this term >> non-intrusivity.

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iceng

3 years ago

Would you be so kind as to provide a circuit diagram so an Engineer can select a resistor for you.

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TomV48iceng

Answer 3 years ago

That would be amazing thank you.