# Measuring a battery's voltage and current

#### danny davis

May 9, 2012
306
Hi,

At work today, I had to measure a batterys voltage

We used a 100 ohm resistor, Why is that value 100ohms? and not 10 ohms or 50ohms or etc.

The Battery was a 3.0 volt
I put the 100 ohm resistor in series
The Volt meter was in series

What did I Test?

If I measure the Batterys in circuit , the tech said that the Resistor would have to be a 10K resistor because the circuit is draw small current

But when I put a 100 ohm resistor in series with the battery, what kind of test is this? the impedance voltage?

Putting a 100ohm resistor in parallel with the 3 volt battery I understand

I'm confused about measuring the battery's voltage/current with a 100ohm resistor in series

Block diagram is:

3v battery---> 100ohm in series----> Volt meter

What kind of BATTERY test is this?

Is this the right resistor value to do a series battery test?

I'm not talking about a parallel battery test, I understand that

I just don't understand a series battery test

#### davenn

Moderator
Sep 5, 2009
14,264
you measure currect with an ammeter in series with the load ( the resistor) and voltage with a voltmeter in parallel with the load

like this....

cheers
Dave

#### Attachments

• Batter resistor and meter.GIF
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#### danny davis

May 9, 2012
306
But If you measure the voltage with a series resistor with the battery, what does that equivalent when compare the series resistor to a circuit design?

The Series resistor is equivalent to a circuits design

A Parallel resistor is equivalent to a circuit design also

What is the differences? between the series resistors equivalent VS a parallel resistor equivalent? relating to it to a circuits design

#### CocaCola

Apr 7, 2012
3,635

But, it begs to ask, why are you not asking these questions and getting the answers when you are actually doing these test at work? I'm sure your company is doing them to show you something, maybe it's time you started asking questions when you don't understand why, and when the tech that is supposed to be schooling you is right there to ask...

#### danny davis

May 9, 2012
306

1.) to reduce a complex network of sources and resistances to a single voltage source and single series resistance (Thévenin equivalent).

2.) to reduce a complex network of sources and resistances to a single current source and single parallel resistance (Norton equivalent)

The battery's equivalent load resistor can either be in series or in parallel

When you measure the voltage of an equivalent series load resistor ( Thevenin Equivalent ) connect to a battery.different than the voltage of an equivalent parallel load resistor ( Norton Equivalent )

The Equivalent series load resistor is doing what to the battery?

Most people think a load is in parallel connected to ground and you will only get a voltage drop

But Thevenin equivalent is a series load , the voltage you measure with a DVM of a Thevenin equivalent is tell you what about the Battery? is the voltage telling you about the current? the batterys life? the discharge of the batttery?

A 3 volt battery connect to a 100 ohm resistor is a Thevenin Equivalent circuit

This voltage you measure with a DVM meter tells you want about the battery under test?

#### danny davis

May 9, 2012
306
Open Circuit battery Test:
1.) Connecting a DVM volt meter across the battery direct

1.) Connecting A low resistance value resistor ( battery's load ) in Parallel to the battery
2.) Measure the voltage output

1.) Connecting a Resistor ( battery's load ) in Series to the battery
2.) Measure the voltage output

What is the difference between the Thevenin VS Norton voltage output and what other information does measuring the voltage output Thevenin?

The Battery is under test so what information do you get from using the Thevein equivalant way?

#### CocaCola

Apr 7, 2012
3,635

Why should I read your partially cut and paste paraphrase from another website?

#### danny davis

May 9, 2012
306
you don't understand the difference between a series load vs a parallel load

the battery is under test, when you measure the voltage output of a series battery load, what information does it give you?

#### CocaCola

Apr 7, 2012
3,635
you don't understand the difference between a series load vs a parallel load

LOL, as if... BTW I most certainly do...

May I remind you that it's you that are asking the questions not me...

#### danny davis

May 9, 2012
306
but u can't answer the question

and I remember i asked u in the past what value should i use to measure the voltage of a battery and u couldn't answer that, you just said a battery tester doesn't do anything

The answer was you have to use a 100ohm resistor in parallel to measure the batterys voltage , low resistance means high current

If the battery is bad then the ESR is higher and you will measure a larger voltage drop on your DVM meter

Open circuit it will measure 3 volts Good battery
Open circuit ti will measure 2.8 volts bad battery
100 ohm resistor in parallel will measure 2.4 volts, why because the ESR is higher when the battery is bad

why couldn't you answer this in the past if you know this simple basic question on how to measure a battery?

You never told me about the ESR effect and how to measure a battery

So if you know the answers , answer the simple basic questions

#### GreenGiant

Feb 9, 2012
842
but u can't answer the question

and I remember i asked u in the past what value should i use to measure the voltage of a battery and u couldn't answer that, you just said a battery tester doesn't do anything

The answer was you have to use a 100ohm resistor in parallel to measure the batterys voltage , low resistance means high current

If the battery is bad then the ESR is higher and you will measure a larger voltage drop on your DVM meter

Open circuit it will measure 3 volts Good battery
Open circuit ti will measure 2.8 volts bad battery
100 ohm resistor in parallel will measure 2.4 volts, why because the ESR is higher when the battery is bad

why couldn't you answer this in the past if you know this simple basic question on how to measure a battery?

You never told me about the ESR effect and how to measure a battery

So if you know the answers , answer the simple basic questions

While a lot of your statements are true they are not always true, USUALLY a battery's ESR will be higher when it is bad, but the results that you are talking about can also mean that the battery's capacity has dropped to a very low level, what are the 100% State Of Charge (SOC) and 0% SOC voltages? if you have a 4.2 volt battery then 3V the battery is probably dead but the voltage is floating back to about 3.

Measuring a battery with no load will yield a higher voltage than not under load yes, but when it comes to a 3 volt battery 100 ohms does not mean high current. (3 milliamps... 3v/100ohms) it should not affect the voltage at all, even still the internal workings of the DVM have a MUCH higher resistance than that anyways, usually up in the Mega to Giga ohm region.

#### GreenGiant

Feb 9, 2012
842
Hi,

At work today, I had to measure a batterys voltage

We used a 100 ohm resistor, Why is that value 100ohms? and not 10 ohms or 50ohms or etc.

The Battery was a 3.0 volt
I put the 100 ohm resistor in series
The Volt meter was in series

What did I Test?

If I measure the Batterys in circuit , the tech said that the Resistor would have to be a 10K resistor because the circuit is draw small current

But when I put a 100 ohm resistor in series with the battery, what kind of test is this? the impedance voltage?

Putting a 100ohm resistor in parallel with the 3 volt battery I understand

I'm confused about measuring the battery's voltage/current with a 100ohm resistor in series

Block diagram is:

3v battery---> 100ohm in series----> Volt meter

What kind of BATTERY test is this?

Is this the right resistor value to do a series battery test?

I'm not talking about a parallel battery test, I understand that

I just don't understand a series battery test

From my understanding of what you are asking here, when you have the battery and a resistor and nothing else series and parallel are talking about the same thing, the only difference is the placement of the DVM/Ammeter

You CANNOT measure the voltage of a battery with a DVM in SERIES with the battery and the resistor
Likewise you CANNOT measure current with the Ammeter in parallel with the resistor and the battery

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,738
Danny,
I think you have a misconception of the thevenin and norton theorems.

Forget (for a moment) the Voltmeter. Look at the circuit consisting of a battery and a load resistor. Now, just as a thought experiment) "open" the battery. YOu can model the battery as an ideal voltage source plus a series resistor (think of this resistor as the ESR). Now replace the original circuit (battery + load resistor) by the ideal voltage source, ESr and load resistor.
You can now either
1) model this circuit as a voltage source with a lower ideal voltage plus a lower ESR by applaying thevenin's theorem
or
2) model this circuit by an ideal current source with an equivalent ideal current and equivalent parallel resistance.
You need to understand that while these circuits do look very dissimilar, they in fact behave the same mathamatically and physically - provided the theorems have been applied correctly.

Now the voltmeter comes back into the game. This is entirely different from the norton/thevenin problem.
A typical voltmeter has ~1MegOhm input resistance. If you put that in series with 100 Ohm, the additional resistance is 0.01% of the voltmeter's resistance, meaning it will not influence the measurement at all (unless the precision of your voltmeter is better than 0.01% - i doubt it). Therefore the current will be negilgible and you measure the battery's open-circuit voltage. As Green Giant already correctly stated: This gives you no information about the state of the battery.
If you connect the resistor parallel to the battery and the voltmeter parallel to both, then you can read the voltage of the battery under load. How much information that gives depends on many factors. If the voltage measured this way breaks down to less than e.g. 50% of the open-circuit voltage, you may take the battery for empty - or the battery may be obverloaded which may happen if the nominal load current of the battery is much smaller than the current you draw through the 100 Ohm resistor.
If the voltage dropes just a little (e.g. 5%) either the battery is still in good shape or the load is way too light compared to the battery's nominal load.
It all depends on the characteristics of the batttery. Without that information you can neither say 100 Ohm is to much load, is too little load or is just the right load.

If you've been told to use 100 Ohm by someone at your workplace:
1) they probably gave you this value from experience
2) you should ask the people at work why to use just this value - from your previous posts it looks like you seem to be afraid of asking the people over there. Why? A good question deserves a good answer and even respect for someone acknowleding that he doesn't understand something but is willing to learn. Or ist it that "work" is "school" and your questions belong into the homeworks section?

Harald

#### danny davis

May 9, 2012
306
What value resistor would you guys use in parallel to test the battery under test?

#### CocaCola

Apr 7, 2012
3,635
but u can't answer the question

Says who? You? You have absolutely ZERO ability to even toss up an educated guess as to what I can or can't answer, so please don't pretend to know anything in that regard, as you simply don't a have the slightest clue... In this thread it's more of a case of me not knowing WTF you are even asking in half of your post, as I clearly stated...

and I remember i asked u in the past what value should i use to measure the voltage of a battery and u couldn't answer that, you just said a battery tester doesn't do anything

That is because there is no one single value to give, I have explained this multiple times to you but you continue to ignore it and insist a single answer be given... You continue to ask for a single answer to a question that has multiple if not infinite correct answers, and then complain when someone doesn't give you what you want...

The answer was you have to use a 100ohm resistor in parallel to measure the batterys voltage , low resistance means high current

100 Ohms is not an answer that's a value chosen by your tech for convince, since we use a base 10 math system, and thus it's a math friendly number...

why couldn't you answer this in the past if you know this simple basic question on how to measure a battery?

Hardly a failure to be able to answer, more so as I repeat yet again there is no single answer to the question...

You never told me about the ESR effect and how to measure a battery

I'm not your private tutor, there is a world of things I have not told you about, and never will...

So if you know the answers , answer the simple basic questions

Who do you think you are that you have any authority to demand what I choose to answer?

#### CocaCola

Apr 7, 2012
3,635
What value resistor would you guys use in parallel to test the battery under test?

LOL, just a few post back you were spouting off as an authority and proclaiming that 100 Ohms was the answer...

Let me dig up the appropriate quote as it's all too fitting...

"...a little knowledge is apt to puff up, and make men giddy, but a greater share of it will set them right, and bring them to low and humble thoughts of themselves."

#### danny davis

May 9, 2012
306
When I measure the voltage of the battery using different load resistors ranging from 5 ohms to 100 ohms , I get different voltages drops because it changes the voltage drop across the internal series resistor inside the battery

So how do u know what is the best load resistor value? ranging from 5ohms to 100 ohms? i have tried them all in that range

#### danny davis

May 9, 2012
306
I tried different values of resistors ranging from 5 ohms to 1Meg , measuring the voltage in series. The battery's voltage didn't do much different until you went higher in resistance , but the voltage stays the same right? it just changes the current?

I did measuring the current with a 3 volt battery only had a current reading 80ohms and below. Anything above 100ohms I got no reading on my DVM meter in current mode, why?

Aug 13, 2011
1,114
I did measuring the current with a 3 volt battery only had a current reading 80ohms and below. Anything above 100ohms I got no reading on my DVM meter in current mode, why?

There are several possibilities. Since you don't know how to use the meter correctly, you may have it set on the wrong range or you may have a test lead plugged into the wrong jack or you may have blown the low current fuse in a previous misadventure. You may have even blown the high current fuse, come to think of it.

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Danny, you realise that a voltmeter can be modelled as an ammeter in series with a resistor?

Assume you have a DVM with an impedance of 10M (not unusual). If you have it set on a 20V range, it requires a current that will drop 20V across that 10M, that is 2uA.

Imagine you have a resistor in series with this. Anything up to 100k is barely going to make a difference (100k will make a 1% difference). When you get to 10M the meter will read 50% low.

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