Battery heating causes and techniques

What is battery thermal management

The optimal operating temperature of the battery is between 15-40°C. However, the operating temperature of the car is between -20°C and 55°C. However, the working temperature environment of the car is -20°C to 55°C, which requires thermal management of the battery, that is, heat dissipation for the battery in high temperature environment and battery heating in low temperature environment.

● Battery heat dissipation

When the temperature is too high, battery capacity will decay and the risk of thermal runaway increases. Therefore, when the temperature is too high, it needs to be dissipated.

● Battery heating

When the temperature is too low, the capacity and performance of the battery will be degraded, and if the battery is charged at this time, lithium precipitation will occur, which will lead to internal short circuit and thermal runaway. Therefore, when the temperature is too low, the battery heating heated.

● Temperature consistency

It is necessary to reduce the spatial variability of the temperature of the power cell as much as possible, that is, to ensure that the temperature difference between the cells is as small as possible.

The impact of low temperature environments on vehicles and batteries

The power battery is one of the most important parts of an electric vehicle and affects the performance of the vehicle in every way, including its range, maximum acceleration, lifespan and, of course, the most important – safety. Many factors affect the performance of the power cell, one of the biggest factors is the temperature. Many early electric vehicles in the winter range is only 70% of the original, and the driver does not turn on the air conditioning to warm up, because the fear of affecting the driving range.

In fact, low temperatures can also lead to a reduction in the discharge battery capacity. Lower battery temperatures completely inhibit the battery’s ability to discharge, affecting not only range, but even vehicle dynamics, energy recovery, etc.

Let’s take a common ternary 3.7 v lithium ion battery as an example:

The working principle of lithium-ion battery is essentially a redox reaction between the internal cathode, anode and electrolyte. The reaction rate of lithium embedded in the active material on the surface of the electrode slows down at low temperature, and the concentration of lithium ions inside the active material decreases, which will cause a decrease in the equilibrium potential of batteries, an increase in internal resistance, a decrease in discharge capacity, and even the freezing of the electrolyte and the inability of battery to discharge at extreme low temperatures, which greatly affects the low-temperature performance of the battery system and causes the decay of the power output performance and the reduction of the driving range of electric vehicles.

In addition, charging the batteries at low temperature is likely to form lithium deposits on the surface of the anode, which will pierce  battery separator and cause a short circuit to the cathode and battery anode, threatening the safety of use, and the safety of the electric vehicle battery system when charging at low temperature greatly restricts the promotion of electric vehicles in cold regions.

Solution

The performance of new energy vehicles without battery thermal management or poorly done thermal management will be greatly affected. Of course, with the development of technology, the electric vehicles nowadays, basically have battery thermal management system. And the ultimate purpose of the battery thermal management system is to keep the battery’s as much as possible at the most appropriate operating temperature.

The need for battery thermal management depends on the different battery types chosen for the vehicle and the temperature sensitivity of different batteries in terms of heat generation rate, energy efficiency and performance. Thermal management includes both warming and cooling, both of which are equally important. Battery heating technology is an important part of battery thermal management and is designed to bring the battery temperature up to optimal operating temperature quickly when the battery is cooler.

Blast battery heating

In fact, battery packs with blast battery heating are rare. BYD is said to have developed battery packs with this battery heating technology, using hot or cold air from external air conditioners to control the temperature inside the pack. However, this technology requires strict design of the air duct inside the battery pack, the battery heating speed is slow, and if the design is not perfect, it is easy to lead to high local temperature of  battery after battery heating.

Battery heating by heating equipment

The battery heating system consists of a heating element and an electrical circuit, of which the heating element is the most important part. Common heating elements are variable resistance heating elements and constant resistance heating elements, the former is usually called PTC (positive temperature coefficient), the latter is usually composed of metal heating wire heating film, such as silicone heating film, flexible electric heating film, etc.

This battery heating method usually heats up well and fast, but sometimes the battery heats up unevenly, and the temperature of the cells close to the heating source will be significantly higher than that of the cells far away from the heating source. Especially for the heating film, it is heated close to the surface of the battery module, so this battery heating method has certain requirements for the heat dissipation structure inside the battery pack.

PTC is widely used due to its safety in use, high thermal conversion efficiency, rapid temperature rise, no open flame, and automatic constant temperature. Its low cost is a favorable factor for the current high price of power batteries. However, the heating parts of PTC are large in size and will occupy a larger space inside the battery system.

Insulated flexible electric heating film is another type of heater that can be bent to any shape of the workpiece to ensure close contact with the workpiece and to ensure maximum heat transfer. Silicone heating film is a thin surface heater with flexibility, but it needs to be in close contact with the object to be heated, and this battery heating method is not as safe as PTC.

Battery heating by liquid circulation

Liquid-cooled battery packs occupy a mainstream position in current designs because of their good heating effect, uniform heat dissipation distribution, safety and reliability. The internal structure of the battery pack is usually designed as a waterway for heat dissipation, which distributes heat evenly to the interior of the pack to achieve uniform battery heating effect.

In terms of control principle, the already listed Xiaopeng G3 uses a more advanced liquid cooling control method in thermal management, and the G3 uses a more integrated HVAC controller for more precise temperature control of the batteries.

When the user starts charging the vehicle (whether fast or slow), the vehicle controller collects the battery temperature signal, and when the battery temperature is low and battery heating is required, the vehicle controller controls the coolant to enter the heating cycle. At this point there is usually a heat source to heat the circulating liquid, and then the liquid flows through the power battery interior to achieve battery heating. This is the principle of battery heating.

Battery heating scenarios and features

The main scenario of battery heating is the winter in northern cities of China. The battery temperature is low when the vehicle is started after sitting in the low temperature environment for a period of time, which seriously affects the driving experience of the vehicle, and if it goes to the charging pile for charging at this time, it will seriously affect the charging efficiency. Therefore, in the battery heating start and shutdown strategy, detailed temperature calibration is needed to achieve better results, not to waste resources, but also to meet the needs of the driver.

For power batteries, the performance and reliability of the battery system depends on the weakest cell, and the safety of the system depends on the most unstable cell, which determines that only the better the battery temperature consistency, the better the performance of the batteries. This is also the reason why most of the current battery pack designs are using liquid-cooled battery heating method.