Battery & Resistance: Why is There a Difference?

[ttsky]

Hi every one,

I placed a multimeter probe across the terminals of a 1.5 v alkaline battery and saw 2 M Ohms, I did the same across a mV power supply at 1.5 volts and got little below 1.5 K ohms. I don't understand what makes them have diffrent resistance, and exactly why?

I did this because I had an odd situation with a SIL 2 (safty integrity level) rated gas analyser, the analyser would only work with a real sensor or a battery at correct voltage, but not a simulator (e.g mV supply).The diffrence as it seems is the sensor has very high impedance. So i measured the resistance of the battery and the supply, and now i am confused why the battery shows such high resistance.

 

[berkeman]

Hi every one,

I placed a multimeter probe across the terminals of a 1.5 v alkaline battery and saw 2 M Ohms, I did the same across a mV power supply at 1.5 volts and got little below 1.5 K ohms. I don't understand what makes them have diffrent resistance, and exactly why?

I did this because I had an odd situation with a SIL 2 (safty integrity level) rated gas analyser, the analyser would only work with a real sensor or a battery at correct voltage, but not a simulator (e.g mV supply).The diffrence as it seems is the sensor has very high impedance. So i measured the resistance of the battery and the supply, and now i am confused why the battery shows such high resistance.

That's not how you measure the output resistance of a voltage source. The DMM resistance measurement is for passive devices, not for active sources of voltage or current.

Instead, make 2 output voltage and current measurements using 2 different resistors, and solve the 2 simultaneous equations for the source resistance of the voltage source. Are you familiar with that measurement technique?

 

[jim hardy]

ttsky: You might want to read up on how an ohmmeter works.
http://en.wikipedia.org/wiki/Ohmmeter

A more accurate type of ohmmeter has an electronic circuit that passes a constant current (I) through the resistance, and another circuit that measures the voltage (V) across the resistance. According to the following equation, derived from Ohm's Law, the value of the resistance (R) is given by: R= V/I