Battery packed heat generation

Heat generation in a battery occurs during charge and discharge due to enthalpy changes, electrochemical polarization and resistive heating inside the cell. Temperature variation inside the batteries can lead to uneven temperature distribution, which creates uneven charge/discharge behaviour within the battery pack. So, thermal runaway may occur in a cell when the battery heat is not properly regulated, and there is a possibility for fire or explosion. For this reason, battery thermal management is vital for electric and hybrid vehicles to keep the vehicle at its optimum performance.

Now let us see how a battery catches fire.

Stage 1: Overheating

Stage 2: Heat accumulation and gas release process

Stage 3: Combustion and explosion

Stage 1: Overheating
The battery pack overheating leads to thermal runaway. Many factors like cell temperature, internal resistance, State of Charge, and ambient temperature due to different climatic changes can lead to overheating of the battery pack. In this stage, battery system operation changes from normal to abnormal because of the above-listed issues. So, it moves to stage 2.

Stage 2: Battery Heat accumulation and gas release process
The Solid Electrolyte Interface (SEI) layer decomposes due to overheating. The temperature rises quickly, and oxygen accumulates inside the battery. So, it moves to stage 3 for battery combustion.

Stage 3: Combustion and explosion
The oxygen and heat released from stage 2 lead to combustion. The combustion happens in a flammable electrolyte and leads the battery to fire. 

Stage 2 and stage 3 are shown in the graph below, the Accelerated Rate Calorimetry (ARC) curve during the thermal abuse test. In stage 2, the temperature rises, and an external heat source raises the battery temperature to the onset temperature. So, the Solid Electrolyte Interface (SEI) layer decomposes, and the separator melts. Then, the self-heating rate will increase and lead to thermal runaway. The combustion and explosion take place in stage 3.

Now, let us see heat generation calculations.

Heat Generation Calculation:

There are two heat sources for battery heat generation.

•   Joule heat
•   Entropy heat

Heat generated = Joule heat + Entropy heat

Joule heat:

From Ohm’s Law, V = IR

Heat dissipates in the resistor when a current is flowing through a resistance. This heat dissipation is called joule heating. Joule heating is also known as ohmic heating.

Power,  

Heat, 

Joule heat, 

Where, P = Power

V = Voltage

I = Current

R = Resistance

H = Heat

t = Time

The heat produced in a resistor is-

•   Directly proportional to the square of the current flowing through it.
•   Directly proportional to resistance.
•   Directly proportional to the time for which current flows through it.

Entropy heat:

The heat generation due to entropy change inside a battery occurs when electrochemical reactions are performed. The entropy heat is reversible heat resulting from changes in open circuit voltage with respect to temperature at two electrodes.

This is the table given to calculate the average heat generated and maximum heat generated.

The formula will be shown below as entropic heat is twice the joule heat.

Average heat generated = average joule heat + 2(average joule heat)

Maximum heat generated = maximum joule heat + 2(maximum joule heat)

(Average current, maximum current, and time should be taken from the table)