Why does the internal resistance increase over time?

Voltage source

A voltage generator always consists of a voltage source and a resistor connected in series. The voltage source is called the source voltage Uq designated. The resistance is called the internal resistance Ri designated.
The circuit consisting of the voltage source and internal resistance is the equivalent circuit of a voltage generator.

Open circuit voltage U0 / Source voltage Uq

The open circuit voltage U0 or source voltage Uq called the output voltage of a voltage source in the unloaded state.
The measurement of the open circuit voltage U0 can only be done with an extremely high-resistance voltage measuring device, the measuring current of which has a negligible voltage drop at the internal resistance Ri generated.

If there is no current flowing between the terminals, the measured terminal voltage is equal to U.q. However, every voltage source has a physical or chemical internal resistance R.i. It influences the voltage value which can actually be taken from the voltage source. In the case of primary elements, it is essentially formed from the resistance that opposes the movement of the positive ions through the electrolyte. The internal resistance is therefore mainly dependent on the condition of the electrolyte. If the concentration decreases in the course of the operating time (with increasing discharge), the internal resistance increases.

Calculation of the internal resistance

The internal resistance is often also referred to as the output resistance or source resistance.

Source voltage Uq and short circuit current I.K
Current and voltage difference

Terminal voltage UKL

When a voltage source is loaded with a consumer RKl, there is a lower output voltage UKL on than the open circuit voltage U0 or source voltage Uq.
The reason is that the consumer RKl the internal resistance Ri from the point of view of the current I is connected in series. The current I generates a voltage drop U at the internal resistanceRi.
The source voltage Uq is reduced by the load on the terminal resistance / load resistance RKl on the terminal voltage UKL.

The source voltage is divided between the two resistors. If the terminal voltage is to match the source voltage as closely as possible, the internal resistance of the voltage source must be as low as possible.

The current through which the source voltage Uq caused, flows through the internal resistance Ri and the external resistance RKl.

Why does the voltage arrow on the internal resistance point in the other direction than the arrow on the source voltage?
If you look closely at the circuit, then the internal resistance in this circuit is not in series with the source voltage, but in series with the terminal resistance / load resistance. The source voltage is divided between the internal resistance and the terminal resistance. Therefore the voltage arrow points from the positive pole to the negative pole.

Short circuit current I.K

A very large short-circuit current I develops due to the usually very small internal resistance of a voltage sourceKthat can destroy the voltage source.

Circuit symbols for voltage sources

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