The calculation of heat generation of lithium batteries is an important part of battery thermal management, involving multiple heat sources. The following are the detailed calculation methods and steps:

1. Main source of heat
The heat generation of lithium batteries mainly comes from the following parts:
Joule heat (Qj) : The heat generated when current passes through the internal resistance of a battery.
2. Heat of Reaction (Qr) : The heat generated by an electrochemical reaction.
3. Polarization heat (Qp) : The heat caused by electrode polarization.
Heat of side reaction (Qs) : The heat generated by side reactions (such as electrolyte decomposition).

2. Joule Heat (Qj
Joule heat is the main source of heat generation in lithium batteries. The calculation formula is:
Qj=I²⋅R⋅t
Qj: Joule Heat (J
I: Current (A)
R: Battery internal resistance (Ω)
t: Time (s)

3. Heat of Reaction (Qr)
Reaction heat is the heat generated by an electrochemical reaction, and its calculation formula is:
Qr =n⋅ΔH
Qr: Heat of Reaction (J)
n: Molar number (mol) of the reactive substance
ΔH: Enthalpy change of reaction (J/mol)

4. Polarization heat (Qp)
Polarization heat is caused by electrode polarization, and the calculation formula is:
Qp =I⋅η⋅t
Qp: Polarization heat (J)
I: Current (A)
η : Overpotential (V)
t: Time (s)

5. Heat of Side Reaction (Qs)
The heat of the secondary reaction is usually small, but it cannot be ignored under high-temperature or overcharge/overdischarge conditions. Its calculation needs to be carried out based on the thermodynamic parameters of the specific side reactions.

6. Total calorific value
The total calorific value is the sum of the heat of the above-mentioned parts:
Qtotal=Qj+Qr+Qp+Qs

7. Sample calculation
Hypothesis:
The current I=10A
The internal resistance of the battery R=0.01Ω
The discharge time t=3600 seconds
The molar number of the reactive substance n=0.05mol
The enthalpy change of the reaction ΔH= -100 kJ/mol
Overpotential η=0.1V
(1) Calculate the Joule heat
Qj =I²⋅R⋅t=(10)²⋅0.01⋅3600=3600J
(2) Calculate the heat of reaction
Qr =n⋅ΔH=0.05⋅(-100 ×1000)= -5000 J
(A negative value indicates heat absorption)
(3) Calculate the polarization heat
Qp =I⋅η⋅t=10⋅0.1⋅3600=3600J
(4) Total calorific value
Suppose the heat of the side reaction Qs =0 (ignored) :
Qtotal=Qj+Qr+Qp=3600+(−5000)+3600=2200J

8. Unit conversion
1 J = 0.239 cal
1 cal = 4.184 J
For example, convert the total calorific value into calories:
Qtotal =2200 × 0.239=525.8cal

9. Precautions
Internal resistance variation: The internal resistance of the battery varies with factors such as temperature and aging, and needs to be adjusted according to the actual situation.
2. Heat dissipation conditions: The actual heat generation is affected by heat dissipation conditions and needs to be analyzed in combination with the thermal management system.
3. Secondary reaction heat: Under high-temperature or extreme working conditions, the secondary reaction heat may increase significantly.

Summary
The heat generation calculation of lithium batteries involves Joule heat, reaction heat, polarization heat and secondary reaction heat. Through the above formulas and steps, the heat generated by the battery during the charging and discharging process can be estimated, providing a basis for thermal management design.