 # Nominal voltage and open circuit voltage of lead acid batteries

This chapter introduces the definition of the nominal voltage of lead-acid batteries and the relationship between open circuit voltage and capacity

1.1 Nominal voltage
The fully charged positive plate is PbO2, the negative plate is Pb, and the middle is H2SO4, solution. Since two different substances exhibit different abilities to gain and lose electrons in the electrolyte, there is a tendency for electrons to transfer from one substance to another. In lead-acid batteries, the negative electrode is easy to lose electrons, and the positive electrode is easy to get electrons, so electrons have a tendency to flow from the negative electrode to the positive electrode through the electronic conductive material. This trend is the electromotive force, and its unit of measurement is volts (V).
The electromotive force can be roughly understood as the open-circuit terminal voltage. The magnitude of this voltage value is only related to certain physical and chemical properties of the substance, and has nothing to do with the quantity of the substance, the geometric shape of the substance, the microstructure of the substance pellet, and the temperature of the working environment. Therefore, as long as it is a lead-acid battery, the terminal voltage is about 2V in spring, summer, autumn, and winter regardless of the size of the body, the shape of the outer shape, and the spring, summer, autumn, and winter.
Some lead-acid batteries used in industry are as tall as humans; some batteries used in meters are as large as those used in the set, as long as the electrolyte is not frozen, and the measured clear voltage is about 2V. The reaction The principle is the same, and the difference lies in the capacity and size.

The voltage of the battery, whether in the charged state or in the discharged state, is constantly changing. In order to have a unified court law, there is the concept of “nominal voltage”. The value of the nominal voltage is usually expressed in terms of the general working voltage. Lead-acid batteries are 2V, ternary lithium-ion batteries are 3.6V, lithium iron phosphate batteries are 3V, and nickel-hydrogen batteries are 1V.

1.2 The relationship between open circuit voltage and capacity
The open-circuit terminal voltage of the battery cannot express the capacity number, only the flow factor.

PbO2+2H2SO4+Pb=PbSO4+2H2O+PbSO4

It can be seen from the reaction formula that as long as there is PbO2 on the positive plate, Pb on the negative plate, and H2SO4 in the electrolyte, this battery has a potential difference that transfers electrons from the negative column to the positive column, which is usually called electromotive force. Think of it as the open-circuit terminal voltage. Obviously, this value has nothing to do with the number of active materials on the plates, whether the positive and negative active materials are matched in the correct ratio, and has nothing to do with the activation state of the plates. The magnitude of the flat voltage depends only on the density of the electrolyte. Therefore, we cannot use the method of measuring the battery terminal voltage to determine the size of the electric ground capacity, the amount of power charged, the strength of the discharge capacity, and whether the use is normal.

When the original state of the lead-acid battery (model, electro-hydraulic density) is determined, there is a definite relationship between the open-circuit terminal voltage and the capacity, and the relationship between the state of charge and the terminal voltage of the battery is demonstrated in detail in the theory.
And under the experimental conditions, conclusions with reference value are drawn.
If the state of charge S of the battery is used as an independent variable to describe the change of electromotive force, it is very close to a straight line. The slope of the straight line is approximately: every time the state of charge decreases by 0.1C (capacity 10%), the electromotive force decreases by 0.16V, regardless of the calculated value and the experimental value. The main reason for this phenomenon is the inherent inhomogeneity of the electrolyte of the battery. When measuring the electrolyte, only the upper electrolyte can be extracted, and the density of this part is the smallest.