Calculate internal resistance

Projects General Guidance
1

Hello there.

I planned to measure the internal resistance of a 4.2V lithium ion battery, then a load such as LED will connected in series to the battery. So I wonder how to measure the actual open circuit voltage but still it is connecting to the load. Is there any fact can apply on this experiment? Because I want to use the formula: (Voc-Vload)/current to get the internal resistance. And this formula can be applied?

I'm a newbie in electronics field, so I hope someone can help and answer me on this.

Thank you.

2

Voc is the voltage in the open circuit, no load, when no current flows.

3

As @Railroader pointed out, the definition of Voc implies NO load connected.
So it's not possible with a load conneced.

4

Use a MOSFET to switch on and off the path to your load.

With the MOSFET open, the voltage will be essential the open circuit voltage.

Then switch on the MOSFET to measure voltage under your desired load.

You can google about the loads and the timing of that as under load the voltage will be decreasing and when released from the load the voltage will have some recovery from what you might measure at that very instant.

a7

5

thanks a lot. I think this will be a great solution :saluting_face:

6

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