The effect of temperature on reaction speed in lead-acid batteries

The effect of temperature on reaction speed in lead-acid batteries

The Arrhenius equation in the theory of physical chemistry expresses the correlation between the activation energy and the reaction rate. The temperature dependence of kinetic parameters can usually be linearized, called the Arrhenius line. Under conditions close to room temperature, approximately, the reaction rate doubles when the temperature increases by 10°C. In an electrochemical reaction, this means that the current is doubled. When the temperature increases by 20℃, the current increases by 4 times; when the temperature increases by 30℃, the current increases by 8 times.

For the reversible heat of reaction of lead-acid batteries, the difference between the reaction enthalpy △H and the reaction free energy △G can be used to calculate the reversible heating effect, and the result of the calculation is

Qr=△H-△G=T△S=﹣359.4kJ-(﹣372.6)kJ= 13.2kJ

It can be seen that the reversible heat is only 3.54% compared with the maximum amount of electricity △G that can be released, indicating that the reversible heat is relatively small.

The reversible heat is greater than zero, which means that the lead-acid battery obtains extra electric energy during the discharge. This part of the heat absorbs heat from the environment; when the lead-acid battery is charged, the reversible heating effect causes the battery to release heat to the environment.

Assuming that all the reversible heat is converted into electric energy, the voltage generated is Qr/2F=0.068V, and the heating value voltage of the lead-acid battery is

Ecal=E0-0.068

This means that the calorific value voltage and thus the thermal effect is equivalent to a voltage slightly lower than the equilibrium voltage.

The reversible reaction heat is relatively small compared to the Joule heat generated by the ohmic resistance, so this heat is generally covered by the Joule heat.

When current flows through the electrodes or components of the battery, heat will be generated due to the existence of resistance, and the heat generated conforms to the laws of physics:

Q=I²Rt

Where
Q-Joule heat generated when current passes;
I- the current passing through;
R- the resistance of the passing material;
t-power-on time.
This shows that the greater the current passed, the more heat is generated; the greater the resistance, the more heat. This is one of the reasons why a large amount of heat is generated and the temperature rises when the battery is charged or the electrode plate is charged.

The heat capacity is the amount of heat required to raise the temperature of 1 mol object by 1°C.

In 1833, British scientist Faraday elaborated on the relationship between the amount of electricity passing through the electrode and the amount of reactants during the electrolysis process, which is called Faraday’s law, which is the most widely used law of electrochemistry. It means that when current passes through the electrolyte solution, the amount of chemically reacted substances on the electrode is proportional to the amount of electricity passed. Can be written as:

m=kQ

Where m-the mass of the reacted substance on the electrode (g);
Q-The amount of electricity passed (A·h);
k-proportional constant, called electrochemical equivalent.

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