Why do car battery die in winter and how to keep it warm
Before the Spring Festival, colleagues from PCauto and GeekNEV drove 6 new energy vehicles for a “smart drive for thousands of miles” Spring Festival long-distance trip between Guangzhou and Wuhan.
Among them, the real battery life of pure electric models is about 50-60% of the NEDC nominal battery life under the smooth high-speed cruising condition in southern winter.
If the northern media runs the pure electric endurance test, this ratio will be lower. Therefore, we can find that the actual battery life of new energy vehicles has a very strong correlation with the outside temperature. When the outside temperature is overheated in summer, the power battery needs to be cooled down, and the air conditioner needs to be turned on in the car.
When the outside temperature is too low in winter, the power battery needs to be heated, and the heater needs to be turned on in the car. Today, let’s take a look at some things about battery heating:
Why you need to keep your battery warm
At present, all new energy vehicles are equipped with lithium-ion battery packs, including BEV pure electric, PEHV plug-in hybrid, REEV extended range, HEV non-plug-in hybrid, FCV hydrogen fuel cell, and lithium battery is very sensitive to temperature, the temperature is too hot and too cold, It will charge/discharge well in no time.
Generally speaking, the best working temperature of lithium-ion battery is around 20℃. For example, the constant temperature of power battery of MEB platform is set at 23℃. If the cell temperature drops too much due to the cooling of the outside world, the activity of the positive electrode material of the cell decreases.
The number of lithium ions moving inside the cell decreases, the ability of the charged ions to diffuse and move in the positive and negative materials becomes poor, the electric energy transfer speed decreases, the charged ions do not move smoothly, and the battery charge and discharge performance decreases.
Low temperature not only affects the efficiency of charge and discharge, but also generates lithium dendrites due to low temperature lithium precipitation, which affects the cycle life of the battery and even makes the battery scrapped in advance. As can be seen from the “SoC-Voltage” coordinate diagram below,
The discharge curves at extreme low temperature -30°C and ultra-low temperature -20°C are very steep. Only 20-60% of the SoC is available at -30°C, and only 15-80% of the SoC is available at -20°C. The voltage changes are very large.
Because the chemical reaction of battery charging and discharging will generate heat, in a non-extreme low temperature environment, the battery after working for dozens of minutes can rely on its own heat to maintain a mild and comfortable “body temperature”.
However, the power battery in the cold start state cannot rely on its own heat to resist the cold winter outside. At this time, we need to design insulation measures for the battery.
Here, take China as an example. Since China’s land area is so large and the north-south span is long, this gives different requirements for the power battery insulation strategy, and also allows them to form a “Southern Iron Lithium/Northern Ternary” battery technology route pattern.
The picture above is the voltage curve of lithium iron phosphate. The nominal voltage is only 3.2V (other power lithium batteries are 3.7V), and iron lithium is very cold.
In northern China, the average temperature in January is below 0°C, so the effect of using an electric vehicle equipped with lithium iron phosphate in winter is not satisfactory. The owners of the Model 3 lithium iron phosphate version should feel the deepest, and the gap between their electric cars and the ternary lithium version in winter is a bit big.
Mainstream battery insulation strategy
In areas with cold winters, the self-generated heat of the battery pack is not enough to maintain the normal operating temperature. Car companies basically use the following six solutions to solve the low temperature problem:
Passive insulation
Simply put, it is the shell itself at the three levels of the cell, the module, and the battery pack, as well as the supporting thermal insulation device.
Insulation materials must consider many factors, such as it must be a bad conductor of heat, must be flame retardant, must be insulated, preferably waterproof and dustproof, and can withstand high temperature and low temperature, the material cost should not be too high, and the weight should be lighter…
Generally, aerogel is released inside the battery pack for heat insulation, but aerogel is more interesting. It is the solid with the lowest density in the world (as low as 0.003g/cm2), and most of it is gas. Insulating heat conduction, NASA fell in love with this new material in the 1990s.
Lyriq’s Ultra Energy platform also advertised that its own aerogel insulation material can block the impact of 600 ℃ heat waves.
Mica is also a good thermal insulation material. The “mica battery” previously released by Lantu uses the “mica + aerogel” solution for thermal insulation. The other material is called “Telite”, which is a modified foam material, which is also used for heat insulation and is a relatively new type of material.
Under the wrapping of these materials, the self-generated heat of the battery cell can keep the battery in a more comfortable state for charging and discharging. Although passive thermal insulation material is a relatively “old” method, it is a must-have configuration for power batteries, and even simple new energy sources will be wrapped in a certain thermal insulation material.
In addition, the engineers have also developed a resistance/liquid/external heat source system that I will mention later when it does not work, but these active thermal insulation systems do not work for a long time. When the temperature is sufficient, the power supply will naturally be disconnected. At this time, the interior of the battery pack still needs to rely on passive thermal insulation materials to control the temperature.
Resistance heating system
Have you ever used “heat fast”? That’s it.
However, the traditional “heat fast” is very low-cost and does not have any protective equipment, which is equivalent to an RPG rocket launcher held in the hands of elementary school students.
There are generally two types of resistance heating systems for power batteries. The commonly used one is called PTC thermistor (Positive Temperature Coefficient), which is a semiconductor resistor whose resistance increases with temperature rise (below).
The other is a heating film, which is metal or silicon-based, and now there are conductive particles added to the polymer organic material to make a heating film. These films will be attached near the cell, and the heating effect is more uniform than PTC, and the volume is smaller (only as thin as 2mm).
There is also a heating device that uses the “Peltier effect”, which is more thermally efficient, but has not yet been mass-produced. A reader has asked before, if power consumption is used to heat the battery, wouldn’t the battery life be lower?
Good question. We said earlier that the charge and discharge performance of the battery at ultra-low temperature will be reduced by 50% or more. If I only use 5-10% of the power to heat the battery and let it recover 30% of the power, is it a profit? It is such a simple truth, and the heating system does not need to work all the time. When the temperature is almost the same, the self-generated heat left for the battery to work in the second half is to maintain the body temperature.
The temperature control device of the previous MEB platform, the heating part in the figure below uses a PTC thermistor (red box, one large and one small) and the optional heat pump (blue frame), the cooling part uses the R744 air conditioner compressor unit (blue frame, the same one).
Why are there two PTC heating devices? The big one is in the cabin, heating the car; the small one is in the front cabin, heating the battery. After the two devices are independent, they can be heated on demand, and the energy consumption will be lower.
Because of the presence of PTC, even in a low temperature environment of -10°C, the ID. product can be charged to nearly 80% in 1 hour.
Liquid heating system
In the previous article, we talked about liquid cooling solutions. Liquids can take away heat (liquid cooling) or bring heat (liquid heating), therefore, it can be changed to two temperature control systems, cold and hot, on the basis of the liquid-cooled power battery.
The liquid heating solution is actually a combination of “PTC + liquid circulation system” (equivalent to an electric water heater), which heats the liquid in the pack through PTC, and then transfers heat to each cell through the liquid circulation.
Compared with the pure PTC heating scheme, the “PTC + liquid circulation system” heats more uniformly, which can better ensure the consistency of the cells, delay the rate of capacity reduction, reduce the probability of thermal runaway, and improve the cycle life.
In the future, there may be mass production of immersion liquid heating solutions, and the heat transfer efficiency will be higher.
External heat source heating
In fact, it uses an internal combustion engine to generate heat by burning fossil fuels to heat the battery pack. A typical example is the “polar electric heating system” that can be installed on WM Motor EX5
In fact, it is a small single-cylinder piston internal combustion engine. It uses diesel fuel with higher fuel efficiency and is very useful in extremely low temperature environments below -30 °C.
The working process of this system is also quite simple. The small fuel tank with a capacity of 6L is filled with diesel oil, and the heat after the combustion of the diesel oil is used to heat the liquid medium in the liquid circuit of the battery, the liquid medium heats the battery. If it needs to be heated for about 1 hour after starting every day, 6L diesel is enough for more than 1 month.
The advantage of the external heat source heating system is that it does not need to consume the power of the battery itself to heat the battery.
Pulse self-heating
The pulse self-heating system has only recently been mass-produced. Both BYD and Changan are developing this technology. Among them, BYD arranges battery modules one-to-one，use battery pack 1 to charge battery pack 2, then quickly charge battery pack 1 with battery pack 2, and quickly jump left and right.
Because the internal resistance of the cell at low temperature is very large, the pulse charging will generate heat, and the heat is inside the cell, and there will be no transfer loss.
There is no information on how Changan’s pulse self-heating system is.
Heat pump
Heat pumps have become popular in the field of new energy vehicles in recent years. They use liquid-gas phase transition to exchange heat.
Outside and inside is an evaporator, which liquefies the gas and releases heat; outside the car is a condenser, which vaporizes the liquid for heat absorption; the pump is used to compress the gas.
Based on global environmental protection needs, countries are gradually banning R134a as a refrigerant for automotive air conditioners.
The main alternative refrigerant is R1234yf or CO2 (R744). The boiling point of CO2 is lower than that of R134a and R1234yf under standard atmospheric pressure, so the heating efficiency is higher in cold weather. Moreover, CO2 refrigerant is the most environmentally friendly, completely non-toxic, and does not need to be recycled for subsequent use.
The efficiency of the heat pump is much higher than that of PTC heating, but the power of the heat pump is not high. I have seen the dismantling of the MEB platform before, in the case of extremely low temperature, the heat pump air conditioner and PTC will work together, and the PTC that consumes electricity will be stopped after the temperature rise is sufficient.
The last two sentences of this section are: thermal insulation and thermal insulation are two independent items with strong correlation. Thermal insulation is a must for safety. The stronger the thermal insulation performance, the better; but the stronger the thermal insulation, the better.
High-latitude users of course hope that the battery’s thermal insulation performance is full, but if the low-latitude user’s battery is too warm, the performance and cost of the cooling device will also increase, otherwise the heat will not be able to be discharged smoothly in summer.
Winter battery insulation and other things for car use
Intelligent preheating function
When charging at low temperature, the active material inside the battery is very lazy, generally speaking, it can only be charged by trickle. At this time, it is very dangerous to charge with high current. New energy vehicles with poor electronic control may be at risk when fast charging in northern winter.
However, the power of new energy vehicles is very precious. When the vehicle is stationary, the heating device will not be activated all the time to maintain the battery temperature. If you suddenly want to travel or suddenly connect to the power grid for charging,
Cold-started batteries will encounter ultra-low temperature failures, ranging from a large reduction in power, or failure to start or production of lithium dendrites to reduce cycle life.
At present, mid-to-high-end new energy vehicles will be equipped with a battery pre-heating function. A little before your APP makes an appointment to use the vehicle, turn on the battery heating device to warm it up, and even turn on the cabin heater in advance.
How to charge efficiently
There is no doubt that the temperature of the open-air parking lot is far lower than the temperature inside the basement. Therefore, if you are a pure electric vehicle owner, you can choose between the open air and the basement, and please choose the basement first (except when the parking fee in the basement is too high).
Because of the low temperature in winter, the charging speed is not only very slow, but also the battery is easily damaged when charging at ultra-low temperature. Under the basement, not only the temperature is higher, the charging speed can be improved, and the battery safety is also guaranteed.
In winter, the melting snow will soak the charging equipment. If you are operating on a public charging pile that you are not familiar with, you may wish to wear a pair of plastic insulating gloves (or extra-large insulating gloves that can be put over the cold gloves).
On the one hand, it will not stain your hands or cotton gloves, and on the other hand, it can prevent electric shock. The maintenance of public facilities in China is worrying, and the management of relevant departments or profit-making institutions is not effective. Therefore, the daily use of charging piles is still very serious, and the insulation is not in place.
In addition, charging is generally slow in winter, and many public charging piles use “one-pile multi-line” charging parking spaces. Since “one pile of multiple lines” will divide a part of the power, when charging, you might as well choose a pile position where both charging guns are idle to charge, so that the charging time can be shortened.
Temperature consistency
When the cells leave the factory, there will be differences in consistency, and they will be balanced by electronic control later.
The cooling system and the heating system will also bring consistency problems to the cells in the later stage.
Whether it is gas, liquid, or solid heat transfer, there must be the hottest point and the coldest point in a heat transfer channel. The cells above these two points will have completely different aging rates. Overheated/cooled cells will age very quickly, their capacity will drop faster, and there will be insufficient voltage in the later stage.
Different types of power systems
All new energy vehicles are equipped with power batteries. Among them, BEV pure electric vehicles have the largest power battery and no other power systems. Therefore, they are most afraid of low temperature in winter and need to consume their own power to keep warm.
PHEV plug-in hybrid vehicles (medium-sized batteries), REEV extended-range hybrid vehicles (small/medium-sized batteries), and HEV non-plug-in hybrid vehicles (small-sized batteries) are all equipped with internal combustion engines, so there is no need to think too much about the low temperature in winter.
FCV hydrogen fuel cell vehicles also need heat preservation, because the power system is installed in a small power battery as the energy center, so there is also the problem of heat preservation.
Never drive violently
Low temperature discharge hurts the battery like low temperature charging, and those pure electric vehicles equipped with electric heating systems or external heat sources are not immune.
Because when the car starts cold, the heating system has not yet entered the state, and the temperature inside the battery is still very low. At this time, do not drive violently. Even gasoline cars that do not have a smooth cold start are low-energy in winter.
In addition, because of driving in winter, heating is required inside and outside the car, and components such as exterior mirror heating, rear window heating wire, air conditioning heater, seat heating and other components will consume a lot of electricity. At this time, if you do not drive the car according to the standard operation mode, the power consumption will be far faster than you think.
Conclusion
Therefore, when purchasing batteries, pay attention to whether the temperature of the region in winter exceeds the operating temperature range of the battery. If you are in a place where the temperature is low in winter, you can buy the low-temperature lithium battery with the heat preservation device mentioned above.
