Battery Thermal Management

Thermal Management

All batteries depend for their action on an electrochemical process whether charging or discharging and we know that these chemical reactions are in some way dependent on temperature. Nominal battery performance is usually specified for working temperatures somewhere in the + 20°C to +30°C range however the actual performance can deviate substantially from this if the battery is operated at higher or lower temperatures.

Arrhenius Law tells us that the rate at which a chemical reaction proceeds, increases exponentially as temperature. This allows more instantaneous power to be extracted from the battery at higher temperatures. At the same time, higher temperatures improve electron or ion mobility reducing the cell's internal impedance and increasing its capacity.

At the upper end of the scale, the high temperatures may also initiate unwanted or irreversible chemical reactions and loss of electrolyte which can cause permanent damage or complete failure of the battery. This in turn sets an upper temperature operating limit for the battery.

At the lower end of the scale, the electrolyte may freeze, setting a limit to low-temperature performance. But well above the freezing point of the electrolyte, battery performance starts to deteriorate as the rate of the chemical reaction is reduced. Even though a battery may be specified to work down to -20°C or -30°C the performance at 0°C and below may be seriously impaired.

Note also that the lower temperature working limit of a battery may be dependent on its State of Charge. In a Lead Acid battery for instance, as the battery is discharged the Sulphuric Acid electrolyte becomes increasingly diluted with water and its freezing point increases accordingly. Thus the battery must be kept within a limited operating temperature range so that both charge capacity and cycle life can be optimised. A practical system may therefore need both heating and cooling to keep it not just within the battery manufacturer's specified working limits, but within a more limited range to achieve optimal performance.

Thermal management is not just about keeping within these limits. The battery is subject to several simultaneous internal and external thermal effects which must be kept within control.

Heat Sources and Sinks

1. Electrical Heating (Joule Heating)
The operation of any battery generates heat due to the I2R losses as current flows through the internal resistance of the battery whether it is being charged or discharged. This is also known as Joule heating. In the case of discharging, the total energy within the system is fixed and the temperature rise will be limited by the available energy. However, this can still cause very high localised temperatures even in low power batteries. No such automatic limit applies while charging as there is nothing to stop the user continuing to pump electrical energy into the battery after it has become fully charged. This can be a very risky situation. 

Battery designers strive to keep the internal resistance of the cells as low as possible to minimise the heat losses or heat generation within the battery but even with cell resistances as low as 1milli Ohm, the heating can be substantial.

2. Thermochemical Heating and Cooling

The chemical reactions which take place in the cells may be exothermic, adding to the heat generated or they may be endothermic, absorbing heat during the process of the chemical action. Overheating is, therefore, more likely to be a problem with exothermic reactions in which the chemical reaction reinforces the heat generated by the current flow rather than with endothermic reactions where the chemical action counteracts it. In secondary batteries, because the chemical reactions are reversible, chemistries which are exothermic during charging will be endothermic during discharging and vice versa. So there's no escaping the problem. In most situations, the Joule heating will exceed the endothermic cooling effect so precautions still need to be taken.

Lead-acid batteries are exothermic during charging and VRLA batteries are prone to thermal runaway (See below). NiMH cells are also exothermic during charging and as they approach full charge, the cell temperature can rise dramatically. Consequently, chargers for NiMH cells must be designed to sense this temperature rise and cut off the charger to prevent damage to the cells. By contrast, Nickel based batteries with alkaline electrolytes (NiCads) and Lithium batteries are endothermic during charging. Nevertheless, thermal runaway is still possible during charging with these batteries if they are subject to overcharging.

The thermochemistry of Lithium cells is slightly more complex, depending on the state of intercalation of the Lithium ions into the crystal lattice. During charging the reaction is initially endothermic then moving to slightly exothermic during most of the charging cycle. During discharge, the reaction is the reverse, initially exothermic then moving to slightly endothermic for most of the discharge cycle. In common with the other chemistries, the Joule heating effect is greater than the thermochemical effect so long as the cells remain within their design limits.

3. External Thermal Effects
The thermal condition of the battery is also dependent on its environment. If its temperature is above the ambient temperature it will lose heat through conduction, convection and radiation. If the ambient temperature is higher, the battery will gain heat from its surroundings. When the ambient temperature is very high the thermal management system has to work very hard to keep the temperature under control. A single cell may work very well at room temperature on its own, but if it is part of a battery pack surrounded by similar cells all generating heat, even if it is carrying the same load, it could well exceed its temperature limits.

Temperature - The Accelerator

The net result of the thermo-electrical and thermo-chemical effects possibly augmented by the environmental conditions is usually a rise in temperature and as we noted above this will cause an exponential increase in the rate at which a chemical reaction proceeds. We also know that if the temperature rise is excessive a lot of nasty things can happen

• The active chemicals expand causing the cell to swell
• Mechanical distortion of the cell components may result in short circuits or open circuits
• Irreversible chemical reactions can occur which cause a permanent reduction in the active chemicals and hence the capacity of the cell
• Prolonged operation at a high temperature can cause cracking in plastic parts of the cell
• The temperature rise causes the chemical reaction to speed up increasing the temperature even more and could lead to thermal runaway
• Gases may be given off
• Pressure builds up inside the cell
• The cell may eventually rupture or explode
• Toxic or inflammable chemicals may be released
• Law suits will follow

Thermal Consideration with Vehicle Application

The EV battery is large with good heat dissipation possibilities by convection and conduction and subject to a low-temperature rise due to its high thermal capacity. On the other hand, the HEV battery with fewer cells, but each carrying higher currents, must handle the same power as the EV battery in less than one-tenth of the size. With a lower thermal capacity and lower heat dissipation properties, this means that the HEV battery will be subject to a much higher temperature rise.

Taking into account the need to keep the cells operating within their allowable temperature range the EV battery is more likely to encounter problems to keep it warm at the low end of the temperature range while the HEV battery is more likely to have overheating problems in high-temperature environments even though they both dissipate the same amount of heat.

In the case of the EV, at very low ambient temperatures, self-heating (I²R heating) by the current flow during operation will most likely be insufficient to raise the temperature to the desired operating levels because of the battery's bulk and external heaters may be required to raise the temperature. This could be provided by diverting some of the battery capacity for heating purposes. On the other hand, the same I²R heat generation in the HEV battery working in high-temperature environments could send it into thermal runaway and forced cooling must be provided.

Thermal Runaway

The operating temperature which is reached in a battery is the result of the ambient temperature augmented by the heat generated by the battery. If a battery is subject to excessive currents the possibility of thermal runaway arises resulting in the catastrophic destruction of the battery. This occurs when the rate of heat generation within the battery exceeds its heat dissipation capacity.
There are several conditions which can bring this about:

• Initially, the thermal I2R losses of the charging current flowing through the cell heat up the electrolyte, but the resistance of the electrolyte decreases with temperature, so this will, in turn, result in a higher current driving the temperature still higher, reinforcing the reaction till a runaway condition is reached.
• During charging the charging current induces an exothermic chemical reaction of the chemicals in the cell which reinforces the heat generated by the charging current.
• During discharging the heat produced by the exothermic chemical action generating the current reinforces the resistive heating due to the current flow within the cell.
• The ambient temperature is excessive.
• Inadequate cooling

Unless some protective measures are in place the consequences of the thermal runaway could be the meltdown of the cell or a build-up of pressure resulting in an explosion or fire depending on the cell chemistry and construction.
The thermal management system must keep all of these factors under control.

Thermal management Techniques used in Battery Packs

Battery thermal management systems can be either passive or active, and the cooling medium can either be air, liquid, or some form of phase change. Air cooling is advantageous in its simplicity. Such systems can be passive, relying only on the convection of the surrounding air, or active, utilizing fans for airflow.

1) Passive Colling

The passive cooling strategy is the most straight forward approach, taking advantage of conduction through mounts and brackets, as well as natural convection with the air in the pack, to transfer the heat generated inside the pack to the environment with no additional hardware for increasing heat transfer.  Passive cooling is low cost and "energy-efficient," as it requires no energy from the car. However, even though it is the most prevalent cooling method seen today, it is not capable of keeping a battery pack within optimal cooling temperatures for high-performance applications and long-distance driving with multiple fast charges.  This method is falling out of favour as companies seek thermal management strategies that keep warranty claims to a minimum and extend usability.

2) Active Air Cooling

The active air-cooling strategy uses a fan with forced air passed over the batteries to remove heat. This strategy extends the lifetime of the battery pack compared to passive cooling as it keeps the batteries at a more consistent operating temperature. It is cheaper and lighter than liquid cooling and is more manageable to design because it does not have to interface with the other cooling networks in the vehicle. However, it can be challenging to develop battery packaging and mounting for airflow. Bracing and mounts can get in the way of airflow, and it can be challenging to distribute the airflow to maintain a uniform temperature. Using modeling techniques such as Computational Fluid Dynamics (CFD) or fluid nodal networks helps engineers solve these problems, but they can be tricky to overcome given the size and weight restrictions associated with designing a vehicle. Even though active air cooling is better than passive cooling at maintaining optimal operating temperatures, the most effective way to keep a uniform temperature within a pack and meet warranty requirements is liquid cooling. 

3) Liquid Cooling

Liquid cooling is the most effective way to remove heat from the battery pack. It is also better than active air cooling at keeping the battery pack within optimal operating temperatures. Designing a system that cools all of the batteries uniformly leads to better battery performance and lifetime. Liquid cooling also allows the battery pack to be operated with higher peak power loads because it dissipates more heat than other cooling methods.

There are three main approaches to liquid cooling: serpentine ribbon-shaped cooling tubes, cooling plates with cooling channels inside them, and direct/immersive cooling.  The cooling tube approach is the most effective at maintaining uniform cell temperatures but is more challenging to manufacture and can result in higher pressure drops.  The cooling plate approach is reasonably simple to implement but can lead to large temperature gradients across individual batteries.  The direct cooling approach may present the most effective means of heat removal but is relatively new and requires expensive dielectric coolants instead of conventional cooling fluids.

It can be more challenging to design a system with liquid cooling because it has to be integrated with other electrical and fluid systems in the vehicle. The potential for a fluid leak must also be considered because it can cause an electrical short. Liquid cooling systems are generally heavier, more expensive, and more complicated to repair. However, the trade-offs can be worth it because liquid cooling systems provide extended lifetime and higher performance compared to air-cooled and passively cooled packs of the same size.

4) Phase Change Material

Phase change materials (abbreviated to PCM) have high fusion heat, which stores and releases the amount of heat during melting and solidifying at a fixed point. When the temperature is lower than the melting point, PCM is solid and heat is absorbed as sensible heat with the temperature rise. When the temperature reaches the melting point, heat is absorbed and stored as latent heat until the latent heat is up to the maximum without the temperature increasing. At the same time, PCM changes its phase from solid to liquid. After that, PCM becomes liquid, and heat is absorbed by PCM and stored as sensible heat. The melting temperature of PCM is variable and can be chosen according to requirements.

The working mechanism of PCM 

The relationship of temperature and energy storage of Phase Change Material 

 5) Thermo-electric Module

The thermoelectric module can convert the electric voltage to temperature difference and vice versa. Here the former effect is adopted. That means it transfers heat through the module by consuming electricity directly. To combine a passive air system with the thermoelectric module, the combined system is able to cool down the battery even lower than the intake air temperature, but the power is still limited to around some hundreds of watts and less than one kW. It is easy to switch between cooling and heating operations. To achieve that, the poles of electrodes need to be reversed.

6) Heat Pipe

A heat pipe is another way to upgrade passive air systems. The structure of a heat pipe is shown in The flat copper envelope of the heat pipe was under partial vacuum. The capillary structure is made of sintered copper powder. The heat pipe uses water as the working fluid. Water on the evaporator side will absorb heat and become vapor lower 100°C due to low pressure inside. Water on the condenser will dissipate heat to the surrounding and become liquid again. This cycle repeats repeatedly.

Heat Pipe Structure 

Note that, in comparison to thermo-electric, a heat pipe is more reliable, because there are no moving parts and no energy consumption. However, a heat pipe is unable to heat the battery due to its fixed structural layout.

7)PTC Heater

PTC heaters utilize Positive Temperature Coefficient materials. PTC thermistors have many self-heated applications by utilizing their own voltage-current or current-time characteristics. One of the applications is a self-regulating heater known as a PTC heater. The temperature of a PTC heater can be kept at a fixed point by adjusting the resistance of the PTC heater automatically.

References

1. https://www.mpoweruk.com/thermal.htm

2. http://publications.lib.chalmers.se/records/fulltext/200046/200046.pdf

3. https://res.mdpi.com/d_attachment/energies/energies-13-05695/article_deploy/energies-13-05695-v2.pdf

4. http://blog.thermoanalytics.com/blog/3-strategies-for-battery-packaging-cooling-and-system-integration-for-electric-vehicles