Does different loads produced different internal resistance ?

[jak9]

hi,
if suppose a battery is 100% charged and is capable of delivering 12V. Now if i connect a bulb of 40watts across it then their will be a voltage drop and the internal resistance can be calculated. If the same battery with 100% charge is used with a 80watts bulb then again their will be a voltage drop and internal resistance can be calculated. So the point is does the internal resistance of a battery will be the same in both cases?

 

[sophiecentaur]

The concept of Internal Resistance is just a broad method of describing how a battery (and any other source) responds to load. It's just 'an equivalent series resistance' value which accounts for the voltage drop under load. In as battery, the Internal Resistance will depend upon how 'easily' the chemical processes inside it can cause current to flow. The volts you measure for different loads and at different states of charge (and temperature) won't necessarily correspond to what you'd expect from one measured value of internal resistance. The 'resistance' is only the ratio of volts lost over the current supplied. This is something that applies to Resistance in general; it's just a ratio of volts to current and nothing more.
Generators and other sources of current can also be assigned an equivalent source resistance. For a generator, in addition to the resistance of the windings, the actual revs (and hence the volts) may drop under load as the steam pressure at the input to the turbine starts to drop or the fuel flow to the engine becomes limited. These effects will also cause a voltage drop which will 'look like' an additional source resistance. But it won't necessarily look the same value for all loads. It may even vary dynamically, as the load changes.

 

[jak9]

thanks but one way to calculate the internal resistance is dc load method that is
Ri=V1-V2/I1-I2...so now that means that if you apply different loads for the same state of charge of a battery the internal resistance will be different
for example if we consider a car battery and when we start the car their would be a heavy drop in the voltage because of a starter motor and if we use the same battery and apply a bulb across it and then their will be again a slight drop in the voltage...so that means internal resistance will change as per the load if you apply different loads at same state of charge...is it right?

 

[sophiecentaur]

No it isn't right. The volts dropped / lost across an 'ideal' series, internal resistance will be proportional to the current drawn - which is just what you'd expect. After all, we're dealing with the good old potential divider circuit, aren't we? The available volts are in proportion to the ratio of load resistor and total resistance (load +r).