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Measurement of Internal Resistance of Battery

A

Animesh Maurya

Jan 1, 1970
0
Can I measure the internal resistance of a freshly charged battery by
directly using a Multimeter. Or it will damage my Multimeter.

Thanks
 
E

Eric Immel

Jan 1, 1970
0
Animesh said:
Can I measure the internal resistance of a freshly charged battery by
directly using a Multimeter. Or it will damage my Multimeter.

Thanks

This will wipe out an analog meter in a big hurry. A digital multimeter
is probably protected against such an event, but it will not give you an
accurate measurement.

If you have the continuous current draw specification for your battery,
divide the measured voltage by that value to get a comfortable design value.

The internal resistance of a battery can be found experimentally by
measuring its unloaded voltage (Vu), then loading it with a resistor
that is small enough to cause the battery voltage (Vl) to sag and and
dividing the difference between them by the load current(Il). (Vu-Vl)/Il
To get good resolution the load resistance should be about three times
the estimated internal resistance. The manufacturer's web page or data
sheet or someone with experience with the type of battery you have can
probably tell you how much current it can deliver. From that you can
approximate the internal resistance and pick your load resistor.
Remember that unless your battery is very small, the load resistor will
dissipate a great deal of power, so select one with an appropriate rating.

This method is dangerous for large batteries (like car batteries)
because the amount of current needed to cause the voltage to sag can be
enormous (hundreds of amps). The battery may also be damaged if the load
is connected for more than a few seconds.

I have done this many times with bench power supplies. I have never
tried it with a battery. You should definitely get a second opinion WRT
safety and potential battery damage before you try it.

EI
 
A

Animesh Maurya

Jan 1, 1970
0
Eric Immel said:
This will wipe out an analog meter in a big hurry. A digital multimeter
is probably protected against such an event, but it will not give you an
accurate measurement.

You said that internal resistance can be measured with Digital
Multimeter with some lack of accuracy, so what will be the percentage
error in this measurement.

Thanks
 
E

Eric Immel

Jan 1, 1970
0
Animesh said:
You said that internal resistance can be measured with Digital
Multimeter with some lack of accuracy, so what will be the percentage
error in this measurement.

Thanks

Wildly inaccurate. Don't do it. Use an indirect method or the
manufacturer's spec. How big is the battery? What type of battery is it?
What kind of load do you hope to drive?

EI
 
A

Animesh Maurya

Jan 1, 1970
0
Eric Immel said:
Wildly inaccurate. Don't do it. Use an indirect method or the
manufacturer's spec. How big is the battery? What type of battery is it?
What kind of load do you hope to drive?

EI

Iam using a 1.2V NiCd battery and the indirect method is working fine.

Animesh Maurya
 
A

Animesh Maurya

Jan 1, 1970
0
Eric Immel said:
Cool. What resistance values are you finding?
EI


Resistance calculated by indirect method yields 1.17 Ohms, and that
measured by DM gives 798 Ohms ! Really Wild. But now a new problem
arised in front of me that when I connected the +ve terminal of DM
with -ve terminal of Battery it displays 798 Ohms, but in other case
when I connected +ve terminal of DM with +ve terminal of Battery it
always tends to exceed the range until I reached the maximum limit of
2M Ohms. Why is this happening so ?

Animesh Maurya
 
R

Rich Grise

Jan 1, 1970
0
[email protected] (Animesh Maurya) wrote in message
Resistance calculated by indirect method yields 1.17 Ohms, and that
measured by DM gives 798 Ohms ! Really Wild. But now a new problem
arised in front of me that when I connected the +ve terminal of DM
with -ve terminal of Battery it displays 798 Ohms, but in other case
when I connected +ve terminal of DM with +ve terminal of Battery it
always tends to exceed the range until I reached the maximum limit of
2M Ohms. Why is this happening so ?

Animesh Maurya

You are very lucky to have not already destroyed your meter.

Rule one of using an ohmmeter is NEVER use it on any powered circuit,
and a battery is about as powered as a circuit can get! Even if it
didn't destroy the meter, the reading would be meaningless because
of the 1.2V. With the "right" polarity, the battery is essentially
in series-aiding with the internal battery, so the ohmmeter thinks
that the resistance is whatever value gives a reading of 1.2V at the
probes, evidently 798 ohms (meaning the meter tries to apply a test
current of whatever 1.2/780 is). When you connect it in reverse,
the batteries oppose each other and the meter doesn't know WHAT
to do.

But yes, you can deduce the internal resistance with some load and
a few meters.

But the question that's really at the top of my mind is, Why? In
what context, besides the classroom [we get uppity about homework
questions], does anyone ever need to know the internal resistance
of a battery? The battery maker will tell you its amp-hr capacity
and maximum recommended discharge current; what else do you need?

Good Luck!
Rich
 
E

Eric Immel

Jan 1, 1970
0
Rich said:
[email protected] (Animesh Maurya) wrote in message
Resistance calculated by indirect method yields 1.17 Ohms, and that
measured by DM gives 798 Ohms ! Really Wild. But now a new problem [Excision]
Animesh Maurya


[Rich's wise warning and succinct explaination excised]
But the question that's really at the top of my mind is, Why? In
what context, besides the classroom [we get uppity about homework
questions], does anyone ever need to know the internal resistance
of a battery? The battery maker will tell you its amp-hr capacity
and maximum recommended discharge current; what else do you need?

Good Luck!
Rich

A few possibilties as to why (pending Animesh's actual reason):

To calculate power dissipated within the battery. This would be useful
in derating discharge rate based on an enclosure's thermal behavior.
What is the maximum operating temperature for NiCad?

If the battery maker is unknown, the internal resistance and a power
calculation would provide guidance for a maximum discharge current. I'd
bet that someone experienced with batteries can probably look at the
physical size and make an adequate guess.

And, my favorite reason: curiosity. The quest for information for its
own sake.

EI
 
E

Eric Immel

Jan 1, 1970
0
Eric said:
The amount of current flow is directly proportional to resistance.

But was wrong to write that, so to right that: Current flow is
*INVERSELY* proportional to resistance. Resistance big, current small.
Resistance small, current big.

EI
 
R

Rich Grise

Jan 1, 1970
0
Eric Immel said:
Rich Grise wrote: ....
[Rich's wise warning and succinct explaination excised]

Remind me not to dislocate my shoulder. ;oD
But the question that's really at the top of my mind is, Why? In
what context, besides the classroom [we get uppity about homework
questions], does anyone ever need to know the internal resistance
of a battery? The battery maker will tell you its amp-hr capacity
and maximum recommended discharge current; what else do you need?

Good Luck!
Rich

A few possibilties as to why (pending Animesh's actual reason):

To calculate power dissipated within the battery. This would be useful
in derating discharge rate based on an enclosure's thermal behavior.
What is the maximum operating temperature for NiCad?

If the battery maker is unknown, the internal resistance and a power
calculation would provide guidance for a maximum discharge current. I'd
bet that someone experienced with batteries can probably look at the
physical size and make an adequate guess.

And, my favorite reason: curiosity. The quest for information for its
own sake.

EI

Thanks! That makes a lot of sense!

Cheers!
Rich
 
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