battery internal resistance

[Lalit]

Hi everybody,
Is it advisable to measure the internal resistance of a battery using a multimeter ?
Lalit

 

[Roff]

You will blow the fuse on your multimeter if you try to use the ohmmeter function. You can get a rough idea of internal resistance by connecting the meter probes, in DC volts mode, directly across the battery and recording the unloaded voltage - call this reading Voc. Now briefly connect a 100 ohm resistor (Rload) across the battery until your voltmeter reading settles, and record the new reading. Disconnect the resistor. Call this reading Vld.

Rint=Rload*(Voc-Vld)/Vld

If you only get a few millivolts change, your result will be inaccurate. You can use smaller resistance loads for more accuracy, but remember to use a resistor of appropriate wattage, or you will burn your resistor and/or your fingers. You will also discharge your battery more rapidly.

I'm not an expert on battery impedance measurements, but I think I would use an electronically pulsed load with a low duty cycle, measure the resulting voltage waveform across the battery using an oscilloscope, and calculate the resistance in a similar fashion.

Ron

 

[abhishek singh]

hi,

This is my first post in this forum so mistakes are forgivalbe. to measure the internal resistance you may take a high watta wire wound potentiometer and connect it to batteries terminals. now turn the knob so that the voltage between both the terminals is just half of the initial unloaded battry voltage, dont keep the potentiometer always connected just measure and disconnect. turn the knob again and measure again untill you get half point. measure the resistance of potentio meter which will be equal to battries internal resistance. dont use this method with low resistance battries.


abhishek

 

[bogdanfirst]

here is a better way to mesure the internal resistance of a battery.....
but you don't only need the multimeter......you also need a resistor....
for normal batteries, liow voltage , use a resistor like 10-500 Ohms. for 1.5V bats i recommend around 150R.....or 100R for your accurate mesuring use 5% or even 1%. also it is better to use 0.5W or 1W . ..so you can be sure not to burn them....
anyway the value will be considered R
the multimeter will be used as a voltmeter
first it is important what kind of multimeter you are using.....if you are using a digital one it should have an impedance of 1Mohm...so this quite larger compared to ther resistor you are using.
ok....here is how to do it....
first you connect the multimeter and mesure the voltage at the battery without any load on it. this will be noted E.
now connect the resistor to ther battery and mesure the voltage across it. note that as U.
now you will find the internal resistance of the battry as being r=R(U/E-1)
understood?
do you need the calculations.....and the explenation of where the formula came from?

 

[Roff]

hi,

This is my first post in this forum so mistakes are forgivalbe. to measure the internal resistance you may take a high watta wire wound potentiometer and connect it to batteries terminals. now turn the knob so that the voltage between both the terminals is just half of the initial unloaded battry voltage, dont keep the potentiometer always connected just measure and disconnect. turn the knob again and measure again untill you get half point. measure the resistance of potentio meter which will be equal to battries internal resistance. dont use this method with low resistance battries.


abhishek

Good idea in theory, but most good batteries have internal resistances of 50 milliohms to 500 milliohms, depending on open circuit voltage. Using this method could damage the battery, the rheostat (potentiometer), the interconnecting wires, or yourself. Abhishek warned against using this method on low resistance batteries, but how do you know in advance whether the internal resistance is low or high?

 

[abhishek singh]

using a wheatstones bridge (meter bridge)

hi all,

one another setup can be build around a wheatstone bridge i dont know how to draw the setup here but little bit of 'googling' does it.

abhishek

 

[Roff]

here is a better way to mesure the internal resistance of a battery.....
but you don't only need the multimeter......you also need a resistor....
for normal batteries, liow voltage , use a resistor like 10-500 Ohms. for 1.5V bats i recommend around 150R.....or 100R for your accurate mesuring use 5% or even 1%. also it is better to use 0.5W or 1W . ..so you can be sure not to burn them....
anyway the value will be considered R
the multimeter will be used as a voltmeter
first it is important what kind of multimeter you are using.....if you are using a digital one it should have an impedance of 1Mohm...so this quite larger compared to ther resistor you are using.
ok....here is how to do it....
first you connect the multimeter and mesure the voltage at the battery without any load on it. this will be noted E.
now connect the resistor to ther battery and mesure the voltage across it. note that as U.
now you will find the internal resistance of the battry as being r=R(U/E-1)
understood?
do you need the calculations.....and the explenation of where the formula came from?

You have transposed U and E. The equation should be

r=R((E/U)-1) (extra parentheses added for clarity)

Which is the same equation I posted above in a different form.

Ron

 

[bogdanfirst]

you know........i didn't see your post......
sorry for posting the same thing twice........ :cry: