[Pacific Auto Network] Since humans began to use electricity, anxiety about the use of electricity has always existed. Even the appearance of batteries for storing electric energy only slightly eased this anxiety. Just like the hot electric car in the world now, even if there is a fresh experience, it is still impossible to avoid the anxiety caused by battery life.
The direct impact of batteries on electric vehicles has also made major new energy sources and battery supplies rack their brains to promote ways to increase battery life. However, whether it is the physical battery stacking scheme, the method of changing the electrolyte composition or even the material form of the battery, the battery life can’t even show a normal state in the face of low temperature. How to keep the battery warm and warm at low temperature has become the key to fight against this "battery killer".
In the process of developing and using the battery, we already know that the battery has a normal working temperature range, and the actual use effect of the battery will be greatly reduced in a low temperature environment, so the battery needs a good thermal management to keep it warm in this environment.
On April 21st, Changan Deep Blue Brand, a subsidiary of Changan Automobile, held a deep blue technology sharing meeting, at which an item named "Micronucleus High Frequency Pulse Heating Technology" was announced.
Just listening to this name, we know that this technology is definitely aimed at the battery thermal management system, especially the word pulse heating exists. Looking back at the technology announced by Chang ‘an this time, in fact, its idea is not complicated. Under the condition of constant heating, let the battery pack heat up more quickly and evenly, so that the battery pack can reach a suitable working environment at low temperature as soon as possible.
Contemporary Amperex Technology Co., Limited patent CN 108711662 B pulse heating device
In principle, it is actually based on a patented technology published by Contemporary Amperex Technology Co., Limited. This patent takes advantage of the characteristic that the internal resistance increases due to low temperature. By installing devices that can generate oscillating current at both ends of the battery, the current passes through the battery cells with large internal resistance, so that a lot of heat is generated inside the battery, and finally the battery temperature rises rapidly.
Although this heating method can make Contemporary Amperex Technology Co., Limited’s battery pack reach the heating efficiency of 4℃/min, it is easy for the lithium ions in the lithium battery to form dendritic metallic lithium in the process of reduction on the negative electrode, that is, "lithium dendrites". The growth of "lithium dendrite" to a certain extent will affect it slightly, and it will cause internal short circuit of lithium battery, which will seriously threaten personal safety.
Therefore, in order to avoid lithium dendrites on the negative electrode of the battery due to frequent over-charging, Changan slightly improved this technology based on Contemporary Amperex Technology Co., Limited’s technology, and chose to use alternating current to generate current for heating the battery pack.
Why do you have to mention alternating current? In the previous battery self-heating technology, the current generated was DC. According to the physical definition, the current whose magnitude and direction do not change in unit time is called direct current. Looking back at the conditions for the generation of "lithium dendrites", it is too late for the negative electrode to treat lithium ions during the discharge process, resulting in the appearance of metallic lithium.
The negative electrode of the battery that needs breathing time is easy to reach the limit in the face of constant output of direct current, and then it is easy to appear "lithium dendrite". Therefore, in order to weaken this "invariability", it is necessary to give the negative electrode some breathing space, and alternating current with periodic changes in the magnitude and direction of current per unit time is more suitable for this work.
Unlike direct current, alternating current does not always keep a constant value, but it always keeps a periodic change of positive value -0- negative value -0- positive value. It is precisely because of this unsteady characteristic of alternating current that the burden on the negative electrode of the battery can be reduced, thus reducing the probability of lithium dendrites.
At the same time, Chang ‘an also mentioned the semiconductor IGBT (insulated gate bipolar transistor) at the meeting. IGBT is a switch that is either on or off. It has no function of amplifying voltage. When it is on, it can be regarded as a conductor, and when it is off, it can be regarded as an open circuit. Coupled with the cooperation of the motor and BMS system, random high-frequency current charging and discharging switching can be realized, which further reduces the occurrence of lithium dendrite.
The dark blue C385 power battery pack officially announced by Chang ‘an can keep the heating rate of 4℃/ at the ambient temperature of minus 30℃, and can improve the power performance by 50% and shorten the charging time by 15% at the ambient temperature of minus 30℃. From the data point of view, the improved "battery self-heating" technology is not only more efficient, but also has a longer battery life, which is quite good news for users in low temperature areas.
At present, the mainstream types of batteries on the market can be divided into two types according to the element types, namely ternary lithium batteries and lithium iron phosphate batteries. The biggest difference between them is the different cathode materials used.
Lithium iron phosphate battery uses Ferrous lithium phosphate (PO4) as cathode material. Its advantage is that it is very safe at high temperature or overcharge, but its disadvantage is that at low temperature (below -10℃), the lithium phosphate battery decays very quickly. After less than 100 charge and discharge cycles, the battery capacity will drop to 20% of the initial capacity, which is basically insulated from the use in cold areas.
Ternary lithium batteries are lithium batteries made of lithium nickel cobalt manganate (Li(NiCoMn)O2, NCM) or lithium nickel cobalt aluminate (NCA). Nickel salt, cobalt salt and manganese salt are adjusted in different proportions, so they are called "ternary". For example, NCM811 in Contemporary Amperex Technology Co., Limited means that the ratio of nickel, cobalt and manganese is 8:1:1.
The advantage of ternary lithium battery is its high energy density, both of which are made in Contemporary Amperex Technology Co., Limited. The energy density of its lithium iron phosphate battery is 178Wh/kg, while that of NCM523 is 200Wh/kg and that of NCM811 is 240Wh/kg. In terms of low temperature, ternary lithium battery can also maintain normal battery capacity at -30℃, which is more suitable for the use conditions in the northern low temperature area. The disadvantage is that the ternary material of ternary lithium battery will decompose at 200℃ at high temperature, and it is easy to burn or explode at high temperature.
Although the materials, advantages and disadvantages of the above two batteries are different, from a microscopic point of view, their working principles are also the process of lithium ion migration back and forth between the positive and negative electrodes.
In the low temperature environment, the activity of the anode and cathode materials of the battery decreases, and the conductivity of the electrolyte as a bridge also decreases. Therefore, when the battery is charged and discharged, there will be resistance inside, which is called internal resistance. When the internal resistance of the battery increases, during the normal use of the battery, a lot of Joule heat will be generated to cause the battery temperature to rise. Experiments show that when the environment is below 0℃, the internal resistance will increase by about 15% for every 10℃ drop in temperature.
Being hindered by internal resistance, trying to exert force can only lead to excessive discharge of the battery, and the electric energy is continuously converted into heat energy, which not only reduces the power, can’t output power normally, but also easily affects the safety of the battery. All these results are caused by the low temperature environment.
In order to solve this problem, in addition to the latest "pulse self-heating" technology mentioned above, in fact, suppliers and manufacturers have done a lot of "warm" measures.
PTC element
Heating film
At present, there are several common schemes. The first one is PTC and heating film selected by most pure electric vehicles. The idea of this scheme is to heat the battery through external electric heating elements to improve the battery temperature. There are two kinds of PTC: water heating and air heating. Water heating heats the coolant through PTC and then exchanges heat with radiator. Air heating means that after the warm air is turned on, the cold air directly exchanges heat with PTC and finally blows out warm air. The heating film is like covering the battery with a conductive heating quilt, but both of them have obvious shortcomings. PTC is easy to cause uneven heating and occupy the space of the battery compartment. Because of the safety, the overall cost of the heating film is not low, and the actual heating efficiency is not high.
Another scheme is the liquid cooling circulation system, which is like adding a set of heating to the battery pack, and heating the coolant to obtain a long-term heat source. Another scheme is heat pump air conditioning. The whole principle is like the forced extraction of atmospheric heat into the car, but when the ambient temperature is too low, the heat pump is easy to fail, so a "magic change" is also made, which not only extracts the heat from the outside air, but also collects the waste heat generated by the power battery system, the drive system and the PCS power electronics. The whole system relies on the eight-way reversing valve for complex heat extraction, so as to improve the overall efficiency of the heat pump air conditioning.
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