For battery safety and performance, invest in a battery thermal management system
Battery thermal management allows batteries to safely and effectively provide power. At their heart, battery packs are conveniently-packaged electrochemical cells. When the battery is charged or discharged, a chemical reaction occurs. Like many chemical reactions, these reactions depend on temperature. Thermal management is all about maintaining batteries at temperatures that are safe and promote optimal battery performance.
Custom battery packs are an investment. The application of good thermal management techniques secures that investment by ensuring that batteries provide efficient power, have a long functional lifespan, and don’t become safety hazards.
In this article, we’ll explain the principles and techniques of thermal management. We’ll explore how the design of a thermal management system (TMS) has to balance different goals and costs. We’ll also dig into exactly how thermal management improves battery safety and performance.
Principles of thermal management
At the heart of all thermal management rests a simple equation: the rate of temperature change equals the rate of internal heat generation minus heat loss from convection, conduction, and radiation. Thus, thermal management depends on reducing excessive internal heat generation and promoting heat shedding to the environment.
Here, we’ll look at some of the core techniques used to shed heat. Additionally, we’ll discuss how an “ideal” TMS is limited by trade-offs.
Techniques of thermal management
Driven in large part by innovations in the electric vehicle industry, thermal management techniques continue to improve alongside battery technology. Many types of TMS exist. Each has advantages and disadvantages depending on the application. Let’s take a look at some of the most common types of thermal management systems.
Many batteries employ deliberate passive thermal management techniques, at least to some extent. Passive thermal management relies on heat transfer by convection, conduction, and radiation, without consuming battery power. Well-designed batteries shed excess heat to the environment utilizing various factors including:
- Cell selection
- Cell spacing
- Pack construction
- Thermal management materials
- Battery enclosure design
Passive thermal management works well for many applications, but can be overwhelmed quickly when ambient temperatures are high or batteries are charged or discharged aggressively. In these cases, active thermal management is needed. Active thermal management sheds heat efficiently but requires additional power, typically from the battery, to do so.
Two main types of active thermal management exist: air cooling and liquid cooling. Air cooling with fans is a very common technique and can be comparatively lightweight. However, air is not a good thermal conductor. It gets the job done for small electronics, but it might be insufficient for larger and more power-intensive applications. Additionally, unidirectional fans can lead to uneven cooling.
Temperature distribution is an important consideration for cooling systems that depend on circulation of air or liquids. These systems are often highly effective, but they can also lead to temperature heterogeneity. Image source: National Renewable Energy Laboratory
Liquid cooling uses the flow of substances with high specific heat capacities (the ability to absorb thermal energy without changing temperature) such as water, oil, glycerol, and mineral spirits. Thus, it is often more efficient than air cooling. However, liquid cooling systems add complexity, cost, and weight.
Thermal management ideals and tradeoffs
What does the ideal thermal management system look like? A perfect system would be:
- Cheap to manufacture
- Low-maintenance and easy to maintain
- Versatile (in terms of climate and use conditions)
Many of these features are mutually exclusive. There are design tradeoffs. Good thermal management systems find a balance between effectiveness, size, weight, cost, and energy usage.
Everyone wants a compact and lightweight battery pack, but reduced size and weight often comes at the cost of highly efficient thermal management measures. You can’t fit a liquid cooling system in a smartphone, for instance. Likewise, no one wants to use battery power to run a thermal management system. Minimizing parasitic power usage is a common goal which, again, comes at the expense of efficiency. Air and liquid cooling systems can be highly effective, but they consume battery power.
Finally, versatility is a useful feature. Some batteries need to work well in a wide range of temperatures. For example, batteries for an electric vehicle need to function properly in extremely hot and extremely cold conditions.
Thermal management helps batteries work as intended
A good TMS allows the battery to work as intended for as long as possible. By applying good thermal management techniques, designers and engineers ensure that their battery packs:
- Have a long functional lifespan
- Don’t lose their capacity to unwanted self-discharge reactions
- Discharge at the desired voltage
Of course, temperature control is just one of many variables related to battery performance and lifespan. Battery packs are designed for specific use conditions. To get the most out of your batteries, make sure to use them within optimal conditions. Be mindful of factors like depth of discharge and rate of charge or discharge as well.
Cycle life and temperature
Good thermal management extends a battery’s useful life. We don’t usually refer to a specific unit of time when talking about battery life span. Instead, we refer to the battery’s cycle life, the number of charge-discharge cycles that a battery can undergo before its performance degrades. As a battery is used, chemical changes slowly occur in the electrochemical cells. As these changes accumulate, the capacity of the battery decreases.
Temperature has a strong effect on the cycle life of a battery. High temperatures speed up the rate of chemical reactions, including undesirable reactions. Using a battery at high temperatures causes the electrochemical cell to degrade quickly, reducing the cycle life. Good thermal management provides a long cycle life.
Self-discharge, shelf life, and temperature
All batteries experience self-discharge, the spontaneous loss of stored capacity due to unwanted chemical reactions in the cells. Self-discharge does not require a connection between the battery’s terminals.
Self-discharge reduces the capacity of batteries over time. This means that primary (non-rechargeable) batteries have finite shelf lives. You can’t keep primary cells in storage forever because self-discharge will slowly but steadily deplete their capacity. This also means that secondary (rechargeable) battery packs lose capacity over time.
As mentioned, unwanted chemical reactions cause self-discharge. These reactions occur more rapidly at higher temperatures. Whether in storage or in use, maintaining batteries within an optimum temperature range minimizes self-discharge, preventing loss of capacity and extending their shelf-life.
Voltage efficiency and temperature
To function as designed, battery packs need to provide a specific voltage between their terminals. Though it might be advertised as a nominal voltage—say, 12 volts—battery packs actually produce a range of voltages throughout their discharge cycle.
As shown here, low temperatures decrease voltage. The curves represent discharge at, from top to bottom, 45°C, 34°C, 23°C, 10°C, 0°C, -10°C, and -20°C. Low temperatures decrease voltage. High temperatures, while they increase voltage, promote self-discharge and reduce cycle life. Image source: Lijun Gao
Temperature is one important part of a battery’s voltage efficiency. High temperatures increase the actual voltage of a battery. This might not create a performance problem, but it can harm longevity. Recall that high temperatures can degrade the electrochemical cell and decrease the lifespan of the battery. Conversely, low temperatures can significantly decrease the voltage efficiency of a battery pack.
Thermal management systems keep batteries and people safe
Thermal management is paramount to battery safety. Fires related to the lithium-ion batteries in Samsung’s Galaxy Note 7 phones highlight the importance of good design and temperature regulation. Lack of thermal management was not the cause of the Note 7’s problems, per se—according to Samsung, battery defects caused short-circuiting and overheating—but thermal management definitely reduces the risk of these events.
Battery designers intend for their batteries to be used within certain operating conditions. But they plan for abuse conditions. The real world is unpredictable and complicated. Intentionally or unintentionally, batteries are subjected to abuse. Abuse conditions refer to any situation where batteries sustain damage or operate outside of design conditions.
Abuse conditions related to temperature include short-circuiting (such as that caused by mechanical damage or misuse), rapid charge or discharge, and overcharge or overvoltage. An effective thermal management system prevents fires, explosions, and other dangers by controlling the temperature of the battery pack. A TMS might not be able to maintain optimal operating temperature under abuse conditions, but it can forestall or prevent thermal runaway.
When a battery produces heat more rapidly than it can shed it, it is in thermal runaway. Thermal runaway is among the foremost concerns in battery engineering research. During thermal runaway, multiple failures occur in a destructive positive feedback loop. Cells rupture, chemicals vaporize, and the battery catches fire. A TMS prevents this from happening.
This video from CNET explains how flaws in the design of the Note 7’s battery meant that they frequently operated in abuse conditions, experiencing short circuits and sometime thermal runaway.
For safety and performance, invest in effective battery thermal management
Batteries store and release energy through controlled electrochemical reactions. The speed and efficiency of these reactions depend on the ambient temperature. The discharge of electricity also releases heat. An effective thermal management system (TMS) is essential for battery safety and performance.
A TMS regulates battery temperature passively or actively. Passive thermal management simply facilitates heat exchange between the battery and the environment. This is low-cost and consumes no battery power, but often can’t handle abuse conditions. Active thermal management consumes power to maintain battery temperature, usually with air or fluid flow. These types of TMS can be very efficient at managing temperature but are also often bulkier, heavier, and more expensive.
Thermal management allows batteries to operate at peak efficiency and with the longest possible lifespan. Battery voltage efficiency, cycle life, self-discharge, and shelf life are all influenced by temperature. A TMS maintains optimal temperature in order to maintain optimal performance.
Finally, thermal management ensures safety. By keeping batteries from overheating, a TMS prevents fires, explosions, the release of toxic gasses, and other hazards.
Whatever your application, Aved can design, test, and manufacture battery packs with a thermal management system that ensures safe and efficient operation. Contact us to discuss your specific project or request a quote to get started on a custom battery pack solution.