# Internal resistance of a battery

Discussion in 'Power Electronics' started by jak9, Nov 15, 2010.

1. ### jak9

1
0
Nov 15, 2010
hello friends,
i am calculating internal resistance of a car battery during the charging phase. When the car is moving the alternator keeps on charging the battery if the output voltage of the alternator is greater than the battery voltage. Basically to proceed with this i have input of continuous voltage and current which is measured at the battery terminals. These signals are in phase and that means its a pure resistive circuit. So the voltage divide by the current has to give the internal resistance value. If we consider an open circuit in which the internal resistance would be calculated as,
Ri=Voc-Vterminal/Ibatt
but when i have to calculate the battery internal resistance during its charging phase then how i amply this formula because in this case i have voltage and current signal which are measured at its terminal and its going to be a closed circuit. So should i directly divide Vbatt/Ibatt to get internal resistance or something else?
The other thing is the voltage and current has ac component that is some ripples so how to consider these ripples to get internal resistance?
thanks

2. ### barathbushan

223
0
Sep 26, 2009
why such complications , cant you just place a multimeter across the battery terminals and
measure the battery resistance

3. ### Resqueline

2,848
2
Jul 31, 2009
jak9, I think you have some misconceptions about what internal resistance really is and how to measure it. What are you trying to achieve and why, what's the goal?

barathbushan, can you provide a link to a multimeter that can actually do this? Granted there are special purpose battery ESR meters, but..

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,404
2,777
Jan 21, 2010
One simple way is to measure the battery voltage off load, then place a known (current) load on the battery and measure the voltage across the load (or use an ammeter and a volt meter to measure voltage and current simultaneously).

The internal resistance acts as if it were a resistance in series with the battery, so the calculation of the internal resistance is possible with these figures.

5. ### barathbushan

223
0
Sep 26, 2009

sorry I was being stupid , for a moment i forgot that a regular multimeter can not be used to
find resistance of powered or active circuit , in this case a battery , the readings resulting in doing so are erraneous , also i dont know of any special multimeter for the job .