What is the battery arrangement and the effect of the battery cell structure on heat dissipation?

 

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In a common battery module composed of cylindrical batteries, several battery cells are generally connected in series and parallel to form a battery module, and then according to the power output requirements of electric vehicles, the battery modules are arranged in a predetermined arrangement to form a battery assembly box. There are three common battery arrangements: in-line arrangement, fork arrangement and trapezoidal arrangement (Figure 1).

Figure 1 - Battery arrangement

In-line arrangement is to arrange the batteries in the battery box in sequence, and the cooling airflow entering from the outside will pass through the gaps between the batteries unimpeded. The advantage is that the flow resistance is small; the disadvantage is that the airflow is not easily disturbed to generate turbulent vortices, the effective contact area with the battery body is small, and the convective heat transfer is small, so the cooling efficiency is not high, and it is generally not used.

The fork row arrangement is to arrange the battery modules of two adjacent layers in the battery box in a staggered manner along the air circulation direction. After the cooling gas entering from the outside passes through the gap between the cells, it will blow directly on the surface of the next layer of cells, then bypass the surface of the cell body and flow to the gaps on both sides of the cells. The advantage is to increase the disturbance of the airflow through the battery, improve the convective heat transfer coefficient on the surface of the battery, reduce the thermal resistance, and improve the heat dissipation effect; the disadvantage is that the flow resistance loss is large.

The trapezoidal arrangement reduces the number of cells along the airflow direction, gradually reduces the cross-sectional area of the channel along the airflow direction, and gradually increases the wind speed, thereby increasing the heat transfer coefficient. Adopting the trapezoidal arrangement, although the temperature of the gas flow gradually increases along the flow direction, the heat transfer coefficient increases due to the gradual increase of the wind speed, which balances the heat dissipation effect of the upstream and downstream, so that the temperature of the upstream and downstream of the battery pack can be basically controlled at a relatively uniform level.

With the increase of ambient temperature, the increase of battery size and the increase of battery power requirements, the conventional air-cooled cooling system can no longer meet the cooling requirements. For lead-acid batteries, Choi and Yao pointed out that relying on natural convection or forced convection is not an effective solution to their temperature rise. For Li-Po batteries, due to the small thermal conductivity of polymers, only relying on air-cooled heat dissipation systems cannot effectively solve the heat dissipation problem. When Nelson et al. studied Li-ion PNGV (partnership for a new generation of vehicles) batteries, they found that when the battery is in a warm environment, it is difficult to reduce the temperature of the 66℃ battery below 52℃ by using air-cooled heat dissipation. Through numerical simulation analysis, Chen et al. found that after the intensity of forced convection increased to a certain extent, the balance of temperature distribution of Li-ion batteries was not fundamentally improved. Harmel et al. also found that when the wind speed increased to a certain limit, the temperature change was not obvious when the wind speed continued to increase. Kim and Pesaran also pointed out the shortcomings of air-cooled cooling. With the increase of the load, it is generally necessary to increase the active components (such as evaporators, heating cores, electric heaters or fuel heaters, etc.), or increase the load of the refrigeration system, air conditioner, etc., thereby increasing the secondary energy consumption of the battery pack, which contradicts the improvement of efficiency. At the same time, the overall system structure of the air-cooled battery pack becomes more complex with the increase of load.

Therefore, when the power is low and the temperature environment is not harsh, the air-cooled heat dissipation is preferred to help reduce the cost of the whole vehicle. When the power system requires a high-power power battery pack, and the battery size is large and the ambient temperature is high or low, the air-cooled heat dissipation cannot achieve the ideal thermal management effect, and other thermal management methods need to be considered.

2. The influence of battery cell structure

The structure of the battery cell also has a very important influence on heat dissipation. For this factor, Zhang et al. constructed corresponding battery models for four 3.2V/50A·h square LiFePO4 power battery cells, as shown in Figure 2. The four types of batteries have the same volume and the same height, but different cross-sectional perimeters. The specific dimensions are shown in Table 1. They used a combination of experiments and numerical simulations to study the heat dissipation performance of the battery under different discharge rates and heat transfer coefficients, and obtained the optimal external dimensions and battery size through the optimization analysis of the experimental results. The size and surface heat transfer coefficient of the single battery have an important influence on the heat dissipation performance of the battery. When the external size of the 3.2V/50A·h square LiFePO4 power battery is 180mm×30mm×185mm, its heat dissipation performance is relatively best; when the size of the battery is 180mm×30mm×185mm, and the surface heat transfer coefficient is at least 20W·m-2·K-1, the maximum temperature of the single cell can be effectively controlled below the optimal operating temperature.

Figure 2 - Single cells with four structures

Table 1 - Outline dimensions of the four batteries

Batteries in series and parallel

In the Primary Connections Year 6 unit Circuits and Switches, students learn about how electrical energy is transferred, transformed and generated. Download the Circuits and Switches unit now for hands-on investigations into electrical circuits.

How can batteries be connected in circuits?
It is possible to vary total voltage and current from a number of batteries by connecting them in different ways in the circuit. It does not matter where in the circuit the batteries are placed, it is how they are placed with respect to each other that is important.

Basically, they can be connected in series or in parallel. The resultant voltage and current can be calculated by using a few simple rules.

Battery terminals
In this picture of a battery, the protruding bit on top is the positive terminal, and the flat bit on the base is the negative terminal. Electrons flow from the negative terminal to the positive terminal as they move through an electrical circuit.

A standard single dry cell battery produces a voltage of 1.5 Volt, with its current dependent on the size of the cell. The bigger the cell, the bigger the current.

Note:- 9 Volt batteries used in larger flash lights are really a series of 6 cells or batteries in a single case.

 

Connecting batteries in series
The word series means "following on from the previous one", like a TV series for example.

It's important to connect the batteries with their terminals in the correct order. Batteries in series need to be connected with the positive end of one battery to the negative end of the next battery.

If they are incorrectly connected, the batteries will cancel out each other's energy and quickly flatten each other.

Batteries correctly placed in series, positive to negative, will add their output voltages, producing a greater voltage.

Voltage and current produced by batteries in series
If two 1.5 volt batteries are connected head to tail, the total voltage is 3.0 volt. This is because batteries in series produce a voltage equal to the number of batteries multiplied by the voltage of each individual battery.

Batteries with voltages greater than 1.5 volts are made up of cells connected in series inside a single case. In the 9 volt battery above, there are six cells connected in series. The calculation is 6 × 1.5 Volt = 9 Volt.

When batteries are connected in series the flow of electrons, as measured by the current, is the same anywhere in the circuit.

A 9 Volt battery will produce a voltage 6 times larger than a single 1.5 Volt battery in the same circuit, but the current in each circuit will be the same no matter where the current is measured.

This happens because the batteries are arranged in a line, and like water flowing through different hoses connected in a line, what goes in one end must come out the other. The same electrons must flow through all the batteries at the same rate, so the current must be the same in each battery and in each part of the circuit.

Batteries in parallel
The word parallel means "alongside each other". When batteries are placed in parallel all the positive terminals are joined together with a single wire to one part of the circuit, and all the negative terminals are joined with a single wire to the rest of the circuit.

Remember the voltage increases when batteries are in series, but with batteries in parallel this is not the case. When two or more batteries are placed in parallel, the voltage in the circuit is the same as each individual battery. That is two, three, four or more 1.5 volt batteries in parallel will produce a voltage of 1.5 Volts!

In a parallel circuit, individual electrons can only pass through one of the alternative paths and batteries at a time, thus each electron can only gain energy from one of the batteries in the circuit. As voltage is a measure of the energy carried by the electrons in the circuit, the increase in voltage for each electron in the circuit is the same as if they had passed through only one battery.

What is the purpose of batteries in parallel?
When batteries are connected in parallel, the current flowing through the circuit increases with the number of batteries in the circuit. Each battery can pump a set number of electrons per second, for a given circuit, so if two or more batteries are connected in parallel the number of electrons they push out each second and energy supplied is added, hence the total current in the circuit is increased.

A summary of batteries in series
When batteries are connected in series, the voltage increases.

A summary of batteries in parallel
When batteries are connected in parallel, the voltage remains the same, but the current that can flow in the circuit increases.  
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