Thermal management in Lithium Ion Battery packs

Thermal management in Lithium Ion Battery packs :

Introduction :

Electric vehicles use large batteries to store energy. The energy flowing into the battery pack as it is charged either from regenerative braking or from the grid and discharged from the pack to power the vehicle and its accessories is measured by electrical current and voltage. The flow of current causes heating in the battery cells and their interconnection systems proportional to the square of the current flowing multiplied by the internal resistance of the cells and the interconnect systems. The higher the current flow the more the heating effect will be. At AVID we design and manufacture lithium battery systems you can find out more about that here.

The performance of Lithium-Ion battery cells is greatly impacted by their temperature, they suffer from the Goldilocks effect, they do not perform well when too cold or too hot, which can lead to permanent and extreme damage of the cells or accelerated degradation. So in addition to cooling, heating of the cells may also be required at lower ambient temperatures to prevent damage during fast charging when the cells are too cold; this is because the internal resistance of the cells rises when they are cold. Most lithium battery cells cannot be fast-charged when they are less than 5oC and cannot be charged at all when they are below 0oC. Lithium cells also begin to degrade quickly when their temperature is above 45oC.

In the past, the largest battery packs did not necessarily need any special cooling as the physical size of the packs was sufficient and the relative flow of current was not large compared to the overall capacity of the pack. As ever faster battery charging rates are demanded with recharge power of over 200kW to deliver times of 30 minutes or less, higher performance electric vehicles with a requirement for consistent performance and adequate durability in global markets has meant that special thermal management methods for the battery pack are now required.

There are 3 common battery thermal management methods used today:

1. Convection to air either passively or forced.
2. Cooling by flooding the battery with a dielectric oil which is then pumped out to a heat exchanger system.
3. Cooling by the circulation of water-based coolant through cooling passages within the battery structure.

Need of Battery Cooling :

From a thermal point of view, there are three main aspects to consider when using lithium-ion batteries in an EV:

1. At temperatures below 0°C (32°F), batteries lose charge due to slower chemical reactions taking place in the battery cells. The result is a significant loss in power, acceleration and driving range, and higher potential for battery damage during charging.

1. At temperatures above 30°C (86°F) the battery performance degrades, posing a real issue if a vehicle’s air conditioner is needed for passengers. The result is an impact on power density and reduced acceleration response.
2. Temperatures above 40°C (104°F) can lead to serious and irreversible damage in the battery. At even higher temperatures, e.g. 70-100°C, thermal runaway can occur. This is triggered when the runaway temperature is reached. The result is a self-heating chain reaction in a battery cell that causes its destruction while propagating to adjacent cells.

The ideal temperature range for an EV’s lithium-ion battery is akin to that preferred by human beings. To keep it in this range, the battery temperature must be monitored and adjusted. A battery thermal management system (BTMS) is necessary to prevent temperature extremes, ensure proper battery performance, and achieve the expected life cycle. An effective BTMS keeps cell temperatures within their allowed operating range.

As defined by engineers at the U.S. Department of Energy’s NREL (National Renewable Energy Laboratory), EV battery pack thermal management is needed for three basic reasons:

1. To ensure the pack operates in the desired temperature range for optimum performance and working life. A typical temperature range is 15-35°C.
2. To reduce uneven temperature distribution in the cells. Temperature differences should be less than 3-4C°.
3. To eliminate potential hazards related to uncontrolled temperature, e.g. thermal runaway.

Types of Cooling :

Air Cooling :

The lowest cost method for EV battery cooling is with air. A passive air-cooling system uses outside air and the movement of the vehicle to cool the battery. Active air-cooling systems enhance this natural air with fans and blowers. Air cooling eliminates the need for cooling loop and any concerns about liquids leaking into the electronics. The added weight from using liquids, pumps and tubing is also avoided.

The trade-off is that air cooling, even with high-powered blowers, does not transport the same level of heat as a liquid system can. This has led to problems for EV in hot climates, including more temperature variation in battery pack cells. Blower noise can also be an issue.

Liquid cooling :

Piped liquid cooling systems provide better battery thermal management because they are better at conducting heat away from batteries than air-cooling systems. One downside is the limited supply of liquid in the system compared with the essentially limitless amount of air that can flow through a battery.

Tesla’s thermal management system (as well as GM’s) uses liquid glycol as a coolant. Both the GM and Tesla systems transfer heat via a refrigeration cycle. Glycol coolant is distributed throughout the battery pack to cool the cells. Considering that Tesla has 7,000 cells to cool, this is a challenge.

The Tesla Model S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery pack and carries a flow of water-glycol coolant; thermal contact with the cells is through their sides by thermal transfer material.

Following is the actual disassembled section of Tesla battery pack, where we can see grey ribbons arranged between the cells which act as thermal conductive material.

Liquid Immersion cooling :

An emerging option for managing the battery temperature is dielectric liquid immersion, widely known as a leading cooling technology in the data center industry. GRC and Engineered Fluids are two leading suppliers of liquid immersion technology. The objective of the approach is to trim the extreme temperatures the battery sees with a high-specific heat fluid that is safe around electronic components. Two-phase immersion cooling is a popular option for battery thermal management, but because it needs to operate at the saturation temperature, two-phase immersion has a limited operating window for a given heat transfer rate. Single-phase immersion had a broader range of applicability, and drastically extends the amount of time the battery takes to soak to extreme ambient temperatures. The high heat capacity also enables smaller components, helping the mass challenge, and dielectric fluids are available in environmentally-friendly chemistries.

Advantages of such arrangement :

1. Direct cell to cell contact enhances heat transfer.
2. Helps improve pack thermal uniformity.
3. Helps protect against cell to cell thermal runaway.
4. Potentiall enables high charge/discharge rates.