What determines the internal resistance of batteries

Hello dear guests and subscribers of my channel. Today I want to talk about such a phenomenon as the internal resistance of batteries and what this parameter depends on. So let's get started.

Let's take a lithium-ion battery, for example, the most common form factor 18650 with a nominal capacity of 2500 mAh, and charge it to an operating voltage of 3.7 volts.

Now let's connect a load in the form of a 1 ohm resistor, rated for 10 watts. What do you think the current will flow in such a system at first?

We can easily calculate this current according to Ohm's law

But if we connect an ammeter, then the real current will differ from the calculated one and will be equal to I = 3.6 A. And the reason is as follows.

Internal resistance

So, the reason for this deviation lies in the fact that inside absolutely any storage battery there is its own internal resistance. And in our mini-circuit, in addition to the 1 Ohm resistor, there will be one more resistance.

Let's imagine our battery in the form of a real two-pole.

So, according to the above diagram, the voltage is 3.7 Volts - this will be nothing more than the EMF of the source.

r is the internal resistance of the source, which in this particular example will be approximately equal to 0.028 Ohm.

If in reality you measure the voltage across the connected resistor, then it will be 3.6 Volts, which means that the voltage drop across the internal resistance of the battery was 0.1 Volts.

It turns out that according to the same Ohm's law, with a voltage of 3.6 Volts, and a resistance of 1 Ohm, the current will be 3.6 Amperes.

And since our circuit is sequential, a similar current will flow through the internal resistance, which means that through simple calculations we get that the internal resistance is equal to:

Now let's find out what parameters this internal resistance depends on and whether its value is a constant.

What parameters determine the internal resistance of the source

So, in reality, the internal resistance of different types of batteries has completely different meanings. It is actively changing, and these changes depend on the following parameters:

1. Amount of current.
2. Battery capacity.
3. From the full charge of the battery.
4. Battery electrolyte temperature.

So there is such a pattern: the greater the load current, the lower the internal resistance. This is due to the process of charge redistribution within the electrolyte.

Since the current strength is high, it means that the rate of transfer of charges by ions from electrode to electrode is high, and this is possible with low resistance.

The current strength is less - ions do not transfer charge as actively. This means that the internal resistance will be great.

Large-capacity batteries have significantly more electrodes, and this, in turn, indicates that the process of interaction of electrodes with electrolyte is more extensive. This means that a significantly larger number of ions simultaneously enter into the charge transfer process.

This increases the current strength and decreases the internal resistance.

Now let's talk about the next important factor - temperature.

A few words about the temperature regime and battery charge

Each battery is designed for a specific operating temperature range. At the same time, the temperature is different for different manufacturers.

But at the same time, the following pattern works: the higher the temperature of the electrolyte, the higher the rate of the reaction in it, and therefore the lower the internal resistance.

Modern batteries have an almost linear dependence of internal resistance on temperature.

But at the same time, the temperature cannot rise indefinitely and without consequences. If the reaction proceeds too violently, then the active evolution of oxygen in the electrolyte (as a result of the disintegration of the anode) can lead to fire.

For this reason, all modern batteries have overheating protection.

In the process of giving off the battery charge, its capacity begins to decrease as a result of the fact that less and less charged ions remain involved in the charge redistribution reaction.

Consequently, the current decreases, while the internal resistance, on the contrary, increases. Therefore, the following is true: the more charged the battery, the lower its internal resistance.

That's all I wanted to say about the internal resistance of batteries and the factors that affect it.

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