Measuring the internal resistance of the power supply can use: Ohm's law for the entire circuit.
The test circuit is as shown below:
Derivation: Internal resistance of power supply r=(E/I)-R.
In the formula: E, I, R - everything can be determined using a high impedance digital multimeter. The value of r? is easy to find.
You can choose an external resistor, such as 100 ohms, first open the circuit and measure the electromotive force of the battery, for example, get 10V, then connect the resistor to the lithium battery. , then use a multimeter to measure the voltage of the two electric shocks of the battery, for example 9V. Divide the resulting voltage 9V by 100 to get the total circuit current 9/100. is the voltage across the internal resistance of the battery. Divide the voltage 1 of the internal resistance by the total current (9/100) will give you the internal resistance de the battery = 100/9 ohms.
You can choose the resistance at will. If the measured value is too small, you can change the selected resistance.
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