Thermal Management of Electrical Vehicle Battery Sytems
Posted by Adelle Webber on
Why Do You Need BTMS for Electrical Vehicles?
The BTMS consists of more components than just the batteries. In most EVs there is an inverter for home charging that changes AC to DC. There is also an inverter for the output to the drive motor. There are temperature controls, heater, cooling devices, and fans. All of these components are part of a BTMS.
Battery thermal management systems for electrical vehicles is a new concept for automotive engineers and consumers. Little is understood about requirements for maintaining optimal temperatures for efficient charging and operation of large battery packs required for electrical vehicles.
The power consumption and discharge rates required for the heating and cooling systems can impact the performance of the vehicle if not properly designed and maintained.
Overheating of lithium ion batteries
Lithium ion batteries have caused fires in mobile devices, hoover boards, and cars while charging. General Motors recalled 141,000 Bolts due to fires caused while owners charged their cars overnight.
Performance of lithium ion batteries in cold weather
Tesla has a performance problem due to cold weather conditions. Read Forbes article. Consumers are beginning to learn that during winter months battery performance can be reduced to 40% which translates to much shorter driving distances.
Solutions for Thermal Management of Electrical Vehicle Battery Systems.
How can battery systems in vehicles be heated or cooled effectively?
The heating and cooling of the battery packs or systems are typically convection of air surrounding the battery compartment or conduction by transferring heat from through the frame and structure containing the battery elements. There are more complex systems using heat pipes, liquid cooling, and even the electrolytic inside the batteries. These systems can be expensive and add weight to the vehicle’s already heavy battery pack.
Heating systems for EV BTMS
PTC heating elements are commonly used for both convection air cooling and conduction or direct contact of battery packs. PTC, positive temperature coefficient heaters are ideal due to their small size, high watt density, and self-regulating temperature control for safety. These devices have been used for years to heat up industry in cold climates.
Flexible etched wire heaters are another option due to their versatility in custom sizing, even heating, and very thin thicknesses. These heating elements can be sandwiched between battery modules for more uniform heating. Flexible silicone encased heaters can be placed under the battery modules for direct conductive heat transfer.
Heat pumps are another way of heating both the battery and passenger compartments. Heat pumps aren’t effective for heating when the outside temperature is below 32 degrees Fahrenheit. Manufacturers of EVs are learning to use the heat pump to capture excess heat of the batteries in winter to warm the interior passenger compartment
Tesla uses both resistive heating and heat pumps in their vehicles.
Cooling BTMS for vehicles
Keeping EV batteries cool is foremost in manufacturers minds due to the rash of fires and recalls for General Motors. Circulating air and ventilation of the battery compartment is sufficient in most cases.
Some manufacturers are taking additional precautions using heat pumps for cooling and heating. Others are using thermoelectric cooling devices such as Peltier coolers.
The Peltier effect coolers are a solid-state device not requiring refrigerant or other liquids to operate. These coolers take advantage of the Peltier Coefficients of different materials to transfer heat and cold cooling the air. Using air flow enhances the effect supporting more efficient cooling. These coolers come in standard sizes from 60 to 200 watts.
Ideal Operating Temperatures for EV Batteries
Consumers and engineers are familiar with radiators and oil circulations systems in combustible engines that have been around for 100 years. The circulating coolant and oil maintain optimal operating temperatures for the engine and occupants of the vehicles. EVs or electrical vehicles require a much different approach. Design and construction of the batteries drivetrain, and BTMS requires an electrical or electronics engineer.
Battery packs used in electric vehicles have a longer life with less maintenance than combustion engines but still, require careful attention to operational temperatures to ensure their longevity and safety.
Lithium-ion batteries are the most common battery packs used in today’s vehicles due to their weight, storage capacity, and output. These commonly used batteries lose 40% or more of their output at cold temperatures and risk rapid deterioration and potential thermal runaway at high temperatures.
Optimum operating conditions are at temperatures between 59 - 95 degrees Fahrenheit. Most everyone understands that cars, trucks, and other vehicles operate at a much wider range of ambient temperatures than the optimum conditions. Outdoor extremes range from sub-zero to plus 100 degrees Fahrenheit.
A typical vehicle can easily be exposed to both ends of the temperature range in the United States. Internal temperatures of a vehicle in the summer can range up to 150 degrees F and higher if parked in the sun. Hence the need for thermal management of the battery system.
EV Batteries and Cold Weather
Thermal management of electric vehicle battery systems is an understated problem by vehicle manufacturers. What happens to the performance of EV batteries at low temperatures is the elephant in the room. You won’t see it in the car manufacturers marketing brochures yet performance can drop to 40% of what is advertised in cold weather.
Improper thermal management of the battery systems can decrease battery life and reduce the efficiency of the battery packs. Batteries used for starting internal combustion engines exhibit the same issues of low cranking power in winter months and difficulty in maintaining a charge.
Electrical vehicles with much larger battery systems require heating management in the winter climates to charge efficiently and maintain driving range.
Why is cold weather a problem for EV battery systems?
Preheating
Electrical vehicle manufacturers understand this issue and some have BTMS that will turn on the battery preheating system when the vehicle senses the owner is looking for a recharging station. When a battery is cold the cells will not fully charge resulting in less than 100 % capacity and resulting range for the vehicle. This is especially true when rapid charging.
This is true of homeowners’ garage-type 1 or 2 battery chargers. To get a full charge on the batteries overnight the battery needs to be preheated prior to charging. It is possible with a trickle charge over eight hours that internal resistance in the battery cells will heat the battery sufficiently for a full charge.
Maintaining ideal battery operating conditions for the battery system.
The temperature of the battery varies during charging, operation, and storage - a parked vehicle. Li-ion batteries operate best within the temperature range of 59-95 degrees F. When temperatures of the battery pack get to 32 degrees the charging and output efficiencies decrease. Temperatures below -4 degrees can result in operational ranges of 40 percent of normal.
If your vehicle’s range is rated at 320 miles on a fully charged battery this means you may only go 128 miles before recharging. If you are charging the battery at that temperature it may only take 40% of the normal charge. That reduces your range to 51 miles! That is just a hypothetical situation but you can see how your road trip might take a lot longer than you planned.
While we take the operation of our lap-top, phones, and other mobile devices using the batteries for granted. Most of our devices aren’t left out in the cold so we don’t realize the same difference as a car battery in performance. However, if you operate your laptop on your “lap” you might feel how warm the battery gets. A car battery has 1,000 times more power than your laptop and creates much more heat during operation.
There is a reason airlines don’t want these batteries in the luggage compartment of a plane. It is also why you see any batteries shipped through the mail with a warning on the package.
Why is cooling so important for an electric vehicle battery?
EV battery systems are made up of hundreds of individual batteries connected together to produce the 100-400 volts required to run the vehicle’s electric motor. The DC output is converted to AC to run the primary AC motor for the drive train.
Each of the small 3-5 volt batteries and their connections create resistance to current when being charged. Current flowing through a resistor creates heat. The high ambient temperature of the battery or too high of a charging current creates enough heat to vaporize the fluids inside the batteries creating gas. It is the expanding gas that causes the batteries to explode and the battery fluids and gases produced to catch fire.
To prevent this from occurring the battery packs have venting systems to release the gas if it builds up and controls that restrict current during charging. Even then the high heat and current can cause plating of the electrodes inside the batteries which deteriorates the life and power of the battery over time.
The solution is a BTMS that cools the batteries or the battery compartment to maintain safe and efficient operating temperatures.
There have been major recalls of EV battery systems due to overheating while charging.
Outdoor EV charging stations also need thermal management.
Car owners expect outdoor gas pumps, ATMs and now electric chargers to work no matter what the weather conditions. All of these outdoor fixtures need climate control for their internal electronics to keep operating properly.
Electrical vehicle chargers run on AC, alternating current that is converted to DC, direct current for charging the batteries. It requires an electronic converter to change the current from AC to DC for fast charging of the batteries. These devices and components that control them don’t operate well in cold, heat, or humidity.
The outdoor cabinets use heating, cooling, and humidity control devices to maintain the proper operating temperature range inside the cabinet. The same type of electronic heating and cooling devices that are used for battery thermal management are used in outdoor electrical cabinets. In addition electric condensate evaporators can be used to reduce the humidity. Read more about controlling the climate inside outdoor electrical control cabinets.
We are Thermal Management Experts at DBK USA
Help for solving you battery thermal management systems issues can be found by calling our thermal management experts at 864-599-1600