A battery, in general, has an electrolyte solution in between its two terminals. This electrolyte is a chemical solution through which electrons flow from one terminal to another and in turn, generate power. The electrolyte solution reacts with metals and separates them into their constituent ions.
One terminal is an anode and it has a majority of negatively charged ions or metallic ions and is excess of electrons. The other terminal is cathode which comprises of positively charged ions or metallic ions and lacks in electrons.
The electrolyte solution oxidizes the cathode which forces it to lose electrons. These electrons travel through the circuit and in turn reduce the anode. This is called a redox reaction. This system continues to work till no reactants are left to undergo a redox reaction. At this point, the dead batteries can no longer work. To make it work again the chemical energy of the battery is refilled through an external source. After knowing the basic working principle of a battery, let’s have a deeper insight of a battery.
Why Does a Battery Have an Internal Resistance?
Resistance is a hindrance to the electrons while passing through a conductor. The resistors are made from insulating materials like carbon or plastic. These materials have variable structural properties based on which they offer variable resistance. Conductors have very few electrons affixed to voids and mostly the electrons are free to move around within their crystalline structure.
The electrons which can move, pass on the current through the conductor. Whereas, insulators have crystalline structures that have all its electrons fixed to a location. Therefore, there is no free electron to move and carry the current.
In the same manner, batteries are made from a material having non-zero resistivity. The internal resistance is produced due to the negligible resistance of the material of which battery is made of. Metals also offer some negligible resistance because of impurities present in them or bulk collision of electrons on randomized heating. This brings us to a conclusion that there is no practical workable battery with 100% voltage source.
This voltage, also known as the electromotive force, is represented by ε in series with a small internal resistance of r. On connecting an external resistance “R” in the circuit series to complete the circuit, we obtain the following through Ohm’s Law:
Here, V is the voltage drop across the load (resistance) “R”. Additionally, there is a reduction in the amount of maximum current that can be drawn from a battery due to its internal resistance.
If the value of I > I(0), then the value of V becomes negative. This implies a negative R which is practically not possible. Therefore, one can conclude that if the circuit is short-circuited by directly connecting the two terminals with a wire, the maximum current that will be drawn is now limited to I(0).
The next thing is ‘power dissipation from a battery’. The presence of intrinsic resistance hints that a battery is not fully efficient at transferring its electrical power to an external circuit. A small amount of power is always dissipated in the form of heat and gets wasted in the resistor present inside the battery.
This gives resemblance to the laws of thermodynamics which states – “no mechanical machine can use all input energy to do work without expending some energy on itself. This energy is the heat energy that increases the temperature of the machine.”