Custom battery packs charge and discharge at different rates. During these processes, they can generate heat. While a small amount of heat is expected, elevated levels can lead to battery problems. Battery pack operation can also be influenced by environmental temperatures. In addition to being negatively impacted by high temperatures, low temperatures also cause lower battery performance.
When designing a custom battery pack, thermal management allows batteries to operate at the right temperature based on the changing factors of the battery and its environment. It prevents the batteries from becoming too hot or too cold during operation and charging.
What Happens to Batteries?
Batteries experience temperature changes due to the resistance experienced by the electrical current. When the electrical current passes through components such as connections, electrodes, and busbars, and/or passes through the electrolytes, some of the current will experience this resistance and turn into heat. The more resistance that occurs, the more heat that is generated until this heat impacts the components. Excessive heat can cause rapid aging and degrade the materials as the battery can lose capacity. High temperatures can also lead to thermal runaway.
Cold temperatures can cause the conductivity to lower in the pack. So, the electrons don’t move as easily while the materials increase in resistance. This problem leads to a lack of efficient performance as the battery pack struggles to operate. For the chemistry of some batteries, they experience charging failures in cold temperatures.
What is Thermal Management?
Thermal management systems are designed to regulate temperatures for the battery and the enclosure. It relies on heat transfer to either warm up the cold conditions around the battery or dissipate heat buildup inside the enclosure. Thermal management can be performed using several methods, such as active heat transfer, passive heat transfer, or a hybrid of the two. These methods will employ either air cooling or liquid cooling techniques.
Custom battery pack designed with space in the enclosure to help dissipate heat.
Active Heat Transfer
Active heat transfer uses external devices that actively target the heat to dissipate it or increase the battery's temperature when in cold environments. This method may involve the use of fans to pull heat away, or pumps that push water, air, or other liquids to regulate the temperatures. Active methods are typically more expensive since they require energy to operate and have complex components that require extra maintenance. Yet they offer greater levels of thermal monitoring and control.
Passive Heat Transfer
Passive heat transfer has materials that are thermal conductive that are piped around the battery. The thermal conductive material transfers the heat away. In other designs, a heat sink is used. In both instances, there are not any actively operating components used to perform the method. Instead, the heat passes through by itself. Passive heat transfer systems use less energy to operate. This makes them less expensive and easier to maintain. However, they are less efficient.
Hybrid Heat Transfer
Hybrid heat transfer uses both active and passive transfer methods. There may be an active pump or fan that pushes the heat toward a passive conductive material. Depending on the design, a hybrid design combines the best aspects of passive and thermal methods for efficiency and control.
Most thermal management systems will use either air or liquid as the heat transfer medium. Devices for air heat transfer are typically more affordable than liquid heat transfer applications. On the other hand, liquid can absorb heat faster and at larger quantities, making this medium more effective.
Thermal Management for Battery Chemistries
Every type of battery chemistry has a temperature parameter where it operates at the most efficient level. While battery chemistry can vary due to manufacturing methods, the charge and discharge temperatures are as follows:
- Lithium-ion: 0°C (32°F) to 45°C (113°F) for charging; -20°C (-4°F) to 60°C (140°F) for discharging
- NiMH/NiCAD: 0°C (32°F) to 45°C (113°F) for charging; -20°C (-4°F) to 65°C (149°F) for discharging
- Lead Acid: -20°C (-4°F) to 50°C (122°F) for charging; -20°C (-4°F) to 50°C (122°F) for discharging
Battery manufacturers will use existing pack technical information and build computer models based on the customer's specifications. They can use these models to simulate use with different applications and how the batteries will undergo temperature fluctuations. With the use of models, the designer can understand the thermal limits for the batteries as well as design the appropriate thermal management system that best suits the specific battery pack. Then they can run the models to determine if the thermal management system will be sufficient during a wide range of scenarios for battery pack applications.
Summary
Thermal runaway and lower operational productivity are two thermal management issues that can impact batteries. Many organizations require that battery packs pass thermal testing before receiving certification. Understanding what is required for your application and industry allows you to design a battery pack that can meet these standards to show that the pack can operate safely.
