Battery Preheating Technology
1. What is Battery Thermal Management?
Batteries can’t handle excessive heat or extreme cold. Their optimal operating temperature range is between 15-40°C.
However, vehicles operate in a wide range of conditions, from -20°C to 55°C, which creates a temperature challenge.
How is this issue typically addressed? The solution is to equip the battery with an air conditioning system to achieve three functions of thermal management:
When the temperature becomes too high, the battery’s lifespan is reduced (capacity degradation) . What’s more, there’s an increased risk of overheating (thermal runaway). Therefore, cooling is essential when the temperature rises.
When the temperature is too low, the battery’s lifespan is reduced (capacity degradation) and its performance weakens. Charging the battery in such low temperatures can also pose a risk of thermal runaway due to lithium plating-induced internal short circuits. Therefore, heating (or insulation) is required when the temperature is too low.
(3) Temperature Consistency
Do you remember the old air conditioners that would start with a blast of cold air and then take a break? Nowadays, most air conditioners have variable frequency and surround air distribution functions, all designed to maintain temperature consistency in both time and space dimensions. Similarly, electric vehicle batteries also need to minimize temperature differences in space as much as possible. (Smaller temperature differences between battery cells are better.)
2. Effects of Low-Temperature Environments on Vehicles and Batteries
As we all know, the power battery is one of the most critical components of an electric vehicle, influencing the vehicle’s performance in various aspects: how far it can go, its maximum acceleration, and its overall lifespan.
Of course, safety performance is also essential. And many of these factors are heavily influenced by the power battery.
The most significant factor affecting the performance of the power battery is temperature. Many early electric vehicles could only travel 70% of their usual range in winter. Some people were reluctant to turn on the heating to avoid affecting their driving range.
In fact, low temperatures also lead to a decrease in the battery’s discharge capacity.
Lower battery temperatures completely suppress the battery’s discharge capacity, affecting not only the driving range but also the vehicle’s performance, energy recovery, and more.
Using common lithium-ion batteries as an example:
The working principle of lithium-ion batteries is essentially the redox reaction between the positive and negative electrodes and the electrolyte.
At low temperatures, the reaction rate of lithium intercalation into the surface-active materials of the electrodes slows down, and the internal lithium ion concentration in these materials decreases. This leads to a reduction in the battery’s equilibrium potential, an increase in internal resistance, a decrease in discharge capacity.
What’s worse, in extreme cold, phenomena like electrolyte freezing and the battery’s inability to discharge may occur.
This significantly impacts the low-temperature performance of battery systems, resulting in reduced power output and decreased driving range for electric vehicles.
Furthermore, at low temperatures, charging can lead to the deposition of lithium on the surface of the negative electrode. Accumulation of metallic lithium on the surface of the negative electrode can puncture the battery separator, causing a short circuit between the positive and negative electrodes, posing a safety threat to the battery.
The safety issues associated with low-temperature charging of electric vehicle battery systems greatly hinder the widespread adoption of electric vehicles in cold regions.
Schematic Diagram of Internal Reactions in Lithium-ion Batteries
3. Is there a technology that can alleviate the above problems?
From the information above, we can see how much impact a lack of battery thermal management or poor thermal management can have on the performance of electric vehicles.
Of course, with technological advancements, most modern electric vehicles are equipped with battery thermal management systems. The ultimate goal of battery thermal management systems is, simply put, to maintain the battery’s temperature as close as possible to its ideal operating temperature.
The necessity of battery thermal management depends on the choice of different types of batteries in vehicles, as well as the heat generation rate, energy efficiency, and temperature sensitivity of these batteries.
Thermal management includes both heating and cooling, and both are equally important.
Battery preheating technology is a vital component of battery thermal management. It is a technology designed to rapidly raise the battery’s temperature to its optimal operating range when the temperature is low. Typically, there are several mainstream methods for battery heating, including the following:
(1) Battery Self-Heating
This method utilizes the heat generated during the battery’s operation, either from discharging or charging, to raise the battery’s temperature.
This heating method is relatively slow, and sometimes the vehicle may have completed its journey before the battery temperature reaches the desired level. It is primarily found in some early car models and low-cost vehicles.
(2) Air Blower Heating
To be honest, air-cooled battery packs are not very common in the market. It is said that BYD has developed an air-cooled battery pack. This technology involves using external air conditioning to blow hot or cold air into the battery pack to control its temperature.
However, this technique requires a meticulous design of the airflow within the battery pack, and the temperature rise effect is relatively slow.
Moreover, if the design is inadequate, it can lead to localized overheating.
(3) Internal Battery Heating Device
The heating system mainly consists of heating elements and circuits, with the heating element being the most crucial part.
Common heating elements include variable resistance heating elements and constant resistance heating elements.
The former is often referred to as PTC (positive temperature coefficient). While the latter typically consists of a heating film made of metal heating wires, such as silicone heating films and flexible electric heating films.
In the case of PTC or heating films, the heating effect is generally good and fast. However, there can also be instances of uneven temperature rise within the battery, with cells closer to the heat source experiencing a significantly higher temperature rise than those farther away. This is especially true for heating films, which are in direct contact with the surface of the battery module for heating.
Therefore, there are certain requirements for the heat dissipation structure inside the battery pack.
Common PTC and Heating Films
PTC is widely used due to its safety, high heat conversion efficiency, rapid heating, no open flame, and automatic temperature regulation.
It has a lower cost, which is advantageous for today’s relatively expensive power batteries.
However, PTC heating elements have a larger volume, which can occupy a significant amount of space inside the battery system.
Insulated flexible electric heating films are another type of heater. They can bend to match the shape of the workpiece, ensuring close contact with the workpiece and maximizing heat transfer.
Silicone heating films are thin, flexible, and have good heat distribution properties, but they require close contact with the object being heated, and their safety is not as good as PTC.
(4) Liquid Circulation Heating
Liquid-cooled battery packs currently dominate due to their effective heating, even heat dissipation, and safety features.
In the battery pack structure, there are usually designed channels for efficient heat dissipation, ensuring uniform heat distribution throughout the battery pack and achieving even temperature rise.
Schematic Diagrams of Two Common Battery Liquid Cooling Systems
In terms of control principles, taking the Xiaopeng G3 as an example, which is already on the market, it employs a more advanced liquid cooling control method for thermal management.
The G3 uses a more integrated HVAC controller, which makes it more sensitive to battery temperature control.
As shown in the diagram above, for battery heating, a separate internal PTC heating liquid is used within the battery pack for circulation heating. This can make battery heating faster and more uniform.
At the beginning of the charging process when the user starts the vehicle (whether it’s fast charging or slow charging), the vehicle controller collects temperature signals from the battery. When the battery temperature is low and heating is required, the vehicle controller controls the coolant to enter the heating circuit.
Typically, there is a heat source (PTC2 heater in the diagram) that heats the circulating liquid, which then flows through the power battery, heating the battery. This is the principle of battery heating.
4. Battery Preheating Usage Scenarios and Characteristics
The primary usage scenarios for battery preheating are mostly concentrated in northern cities during the winter. The main usage scenarios include both discharge and charging scenarios.
When a vehicle has been parked in a cold environment for some time and is then started, the battery temperature is low, which significantly affects the driving experience and performance. Charging efficiency is also seriously impacted when charging at this point. Therefore, for the start and stop strategies of battery preheating, detailed temperature calibration is necessary to achieve better usability without wasting resources, while meeting customer usage scenarios. This is also a test of an automaker’s integration and matching capabilities.
The “barrel effect” of the power battery (the performance and reliability of the battery system depend on the weakest individual cell, and the safety of the system depends on the most unstable individual cell) determines that only battery packs with better temperature consistency can deliver optimal performance.
This is why most battery pack designs currently employ liquid cooling.
Temperature of a Certain Cell – Maximum Discharge Current Curve
As shown in the above diagram, let’s assume that most of the cell temperatures are at 20 degrees, while cell B, due to slower heating, is only at 10 degrees. In this case, the entire battery pack has to accommodate cell B, and the discharge current is forced to drop from 140A to 100A, resulting in a one-third decrease in performance.
In fact, there are many more details and points to explore regarding battery pre-heating technology. For example, the heat dissipation of square cells, the optimal operating temperature range for different cell materials, the integration of cabin and battery thermal management, optimization of battery pack structure, and so on.
Each of these topics is worth in-depth research.