1 low temperature battery performance degradation
1) the impact of low temperature on the battery discharge capacity
Capacity is one of the most important parameters of lithium batteries, the size of the curve with the temperature changes as shown below. Obviously, the capacity decreases as the temperature decreases. In addition to capacity, as the temperature decreases there are batteries Open circuit voltage. We all know that the battery contains energy is the product of the capacity and the terminal voltage, when both multipliers are down, the energy inside the battery must be a combination of both the descending effect.
The reduction of the cathode material's activity at low temperature makes it possible to reduce the amount of lithium ions that carry the discharge current due to the shift, which is the fundamental reason for the decrease in capacity.
2) the impact of low temperature on the battery resistance
Any charge, the battery resistance decreases with the temperature was significantly increased, the lower the charge of batteries, the greater the internal resistance, and this trend also changes with the temperature remains unchanged.
In low temperature, the positive and negative materials, the diffusion of charged ion mobility worse, pass through the electrode and the electrolyte passivation film becomes difficult, the speed of transmission in the electrolyte is also reduced, and in the transfer process will be additionally generated A lot of heat. Lithium ion reaches the negative, the negative diffusion within the negative material also becomes not smooth. The whole process, the movement of charged ions have become more difficult, in the external view, that is, the cell's internal resistance increased.
3) Low temperature battery charging and discharging efficiency
The charging efficiency at -20 ° C is only 65% at 15 ° C. The low temperature brings about the change in the electrochemical surface properties described earlier, with a significant increase in internal resistance, and a significant amount of electrical energy is dissipated over the internal resistance heating during discharge. We observed that the efficiency of Coulomb decreased.Electric car driving process, you will feel that looks almost the same amount of power, shortening the battery life at low temperatures.
4) lithium-ion battery internal side effects
Low temperature lithium battery performance degradation seriously, at the same time in the lithium-ion battery charging and discharging process there will be some side reactions in these side effects mainly lithium ion and electrolyte irreversible reaction will cause the capacity of lithium batteries decline, the battery performance further deterioration.
Due to the depletion of the conductive active material, the capacitance is decayed.According to the potential of positive and negative electrodes in the battery, these side reactions are more likely to occur on the negative side than the positive side.As the negative electrode potential is much lower than the positive electrode potential, The deposition of ions that react with the electrolyte solvent is deposited on the surface of the electrode to form an SEI film. The impedance of the SEI film is one of the factors that cause the overpotential of the negative electrode reaction. The deposits formed on the surface of the active material during charging increase the resistance . Reduce the effective surface area of active particles, increase the ionic resistance. Lithium battery available capacity and energy decline at the same time. Lithium battery charging process more prone to side reactions. Lithium battery charge begins, the lithium ions through the electrolyte to the negative Movement, so the potential difference between the electrode and the electrolyte is reduced, making lithium ion and the electrolyte more prone to irreversible side reactions.
2 preheating to form a basic consensus
In low temperature environment, the power battery to discharge itself to heat, even with a complete thermal management system, but the initial low-temperature phase of the discharge process will still cause harm to battery life, and even bring security risks.If only consider electric vehicles Owing to its own resources, without considering external sources, a chicken-and-egg problem arises, and the paradox can only be broken by jumping out of the peg of the vehicle itself, and as a result, it has long been possible to preheat it Sexual and feasible method.
The preheating methods that have been discussed include: PTC preheating, electro-thermal film preheating, liquid-liquid preheating, phase change material preheating, heat pipe preheating, and AC preheating. The so-called different preheating methods, Is the actual heating medium, the same is the external power supply to the basic idea of heating the battery.
3 Tesla how to do preheat?
Tesla pulled out with patents related to preheating, to see its basic idea of preheating and implementation of the following is a patent block diagram of the preheating system.
The following is the layout of the heating system in the vehicle schematic diagram, the red box at the bottom right, indicate the use of external power supply, and now the mainstream of the preheat the same way, or that the current popularity of these types of preheating program, Reference Tesla's idea Here is the details of several preheating systems described in the patent.
Preheating the system to achieve the form
The patent enumerates several forms of preheating the battery: a preheating system that preheats one or more power battery systems at the same time; preheating with a heater; reversing the use of a cooling system for preheating; Battery internal heat heating, such as the use of self-heating process of charging; Can absorb heat from the environment; You can use the motor heat to heat the battery; You can work in the battery during its heating; Can charge the battery during the heating process; During the charging process, the cooling system is used to reverse the operation of the battery to heat the battery. The preheating system can be an independent system or can work with one or some of the systems in the above block diagram.
Preheat system temperature acquisition and comparison
The preheat control system may include a comparator to compare the battery monitoring temperature collected from the original battery management system with the set temperature. If the battery temperature is lower than the ideal value, a feedback signal is provided. Alternatively, a temperature may be set separately Detected sensor system to collect the actual battery temperature; thermocouples and other known temperature sensors can be used here.
The temperature comparator can feed back the information that the temperature is below the ideal value in a number of ways: The temperature comparator can continuously send signals whose temperature is below the ideal and can also send intermittently.
Preheat the control loop
The warm-up control loop needs to provide the controller with a warm-up time before the vehicle starts, during which time the preheat enable signal is continuously sent.
The preheating control loop needs to give the starting time of the vehicle. The starting time can be given directly from the preset schedule in the program. According to the historical accumulation of starting time, the system estimates a time by itself and can calculate it according to the historical data according to the statistical rule One time, for example, if the system observes that the car's owner has started the car at 8:00 am in the past few weeks, then it is also predictable for the upcoming morning, with the car starting at 8:00 .
Heating time
When to start the preheating, in addition to need to consider the car's start-up time, but also need to determine the length of preheating, that is, from the starting temperature for how long to reach the ideal temperature can be heated in strict accordance with the car start time minus the time required for preheating , Turn on the preheating system, this strategy does not consider the battery needs to maintain a certain temperature at an ideal temperature in order to achieve the issue of thermal balance If you want to achieve better preheating effect, you need to set aside the holding time.
Continuous control loop
The patent called "further control system" corresponds to the previous system block diagram, and we temporarily translate it into a "continuous control loop." The continuous control loop needs to be coupled with multiple systems in the system block diagram, at least with the preheat control loop and Heating system with the heating system includes heating devices, which can be resistance heaters or other types of heaters. Continuous control loop through the temperature signal is lower than the ideal and the heating system is linked to the heating system requires continuous heating; continuous control loop can also be passed The preheat system enable signal acts on the heating system to indicate that it continues to operate; the heating system can also be an integral part of the overall air conditioning system, relying on the coolant to heat the battery.
Cost evaluation loop
How much power it takes to heat the battery, compared to the savings in battery power and other overall effects, is it cost-effective, and a system that makes estimates in this area is named 'cost estimating circuit', for the moment as a cost-assessment loop, The previous literature is rare, is a consideration of the economic benefits of a design .Cost assessment results can be used to preheat the control system, to determine whether to preheat.Cost assessment can be based on previous operating experience Data, municipal electricity price fluctuations, etc., the current price of low battery temperature on battery discharge capacity
Capacity is one of the most important parameters of lithium batteries, the size of the curve with the temperature changes as shown below. Obviously, the capacity decreases as the temperature decreases. In addition to capacity, as the temperature decreases there are batteries Open circuit voltage. We all know that the battery contains energy is the product of the capacity and the terminal voltage, when both multipliers are down, the energy inside the battery must be a combination of both the descending effect.
The reduction of the cathode material's activity at low temperature makes it possible to reduce the amount of lithium ions that carry the discharge current due to the shift, which is the fundamental reason for the decrease in capacity.
2) the impact of low temperature on the battery resistance
Any charge, the battery resistance decreases with the temperature was significantly increased, the lower the charge of batteries, the greater the internal resistance, and this trend also changes with the temperature remains unchanged.
In low temperature, the positive and negative materials, the diffusion of charged ion mobility worse, pass through the electrode and the electrolyte passivation film becomes difficult, the speed of transmission in the electrolyte is also reduced, and in the transfer process will be additionally generated A lot of heat. Lithium ion reaches the negative, the negative diffusion within the material also becomes not smooth. The whole process, the movement of ions have become more difficult, in the external view, that is, the resistance of the cell increased.
3) Low temperature battery charging and discharging efficiency
The charging efficiency at -20 ° C is only 65% at 15 ° C. The low temperature brings about the change in the electrochemical surface properties described earlier, with a significant increase in internal resistance, and a significant amount of electrical energy is dissipated over the internal resistance heating during discharge. We observed that the efficiency of Coulomb decreased.Electric car driving process, you will feel that looks almost the same amount of power, shortening the battery life at low temperatures.
4) lithium-ion battery internal side effects
Lithium battery performance degradation at low temperature serious, at the same time in the lithium-ion battery charging and discharging process there will be some side reactions in these side effects mainly lithium ion and electrolyte irreversible reaction will cause lithium battery capacity decline, the battery performance further deterioration.
Due to the depletion of the conductive active material, the capacitance is decayed.According to the potential of positive and negative electrodes in the battery, these side reactions are more likely to occur on the negative side than the positive side.As the negative electrode potential is much lower than the positive electrode potential, The deposition of ions that react with the electrolyte solvent is deposited on the surface of the electrode to form an SEI film. The impedance of the SEI film is one of the factors that cause the overpotential of the negative electrode reaction. The deposits formed on the surface of the active material during charging increase the resistance . Reduce the effective surface area of active particles, increase the ionic resistance. Lithium battery available capacity and energy decline at the same time. Lithium battery charging process more prone to side reactions. Lithium battery charge begins, the lithium ions through the electrolyte to the negative Movement, so the potential difference between the electrode and the electrolyte is reduced, making lithium ion and the electrolyte more prone to irreversible side reactions.
2 preheating to form a basic consensus
In low temperature environment, the power battery to discharge itself to heat, even with a complete thermal management system, but the initial low-temperature phase of the discharge process will still cause harm to the battery life, and even bring security risks.If only consider electric vehicles Owing to its own resources, without considering external sources, a chicken-and-egg problem arises, and the paradox can only be broken by jumping out of the peg of the vehicle itself, and as a result, it has long been possible to preheat it Sexual and feasible method.
The preheating methods that have been discussed include: PTC preheating, electro-thermal film preheating, liquid-liquid preheating, phase change material preheating, heat pipe preheating, and AC preheating. The so-called different preheating methods, Is the actual heating medium, the same is the external power supply to the basic idea of heating the battery.
3 Tesla how to do preheat?
Tesla pulled out with patents related to preheating, to see its basic idea of preheating and implementation of the following is a patent block diagram of the preheating system.
The following is the layout of the heating system in the vehicle schematic diagram, the red box at the bottom right, indicate the use of external power supply, and now the mainstream of the preheat the same way, or that the current popularity of these types of preheating program, Reference Tesla's idea Here is the details of several preheating systems described in the patent.
Preheating the system to achieve the form
The patent enumerates several forms of preheating the battery: a preheating system that preheats one or more power battery systems at the same time; preheating with a heater; reversing the use of a cooling system for preheating; Battery internal heat heating, such as the use of self-heating process of charging; Can absorb heat from the environment; You can use the motor heat to heat the battery; You can work in the battery during its heating; Can charge the battery during the heating process; During the charging process, the cooling system is used to reverse the operation of the battery to heat the battery. The preheating system can be an independent system or can work with one or some of the systems in the above block diagram.
Preheat system temperature acquisition and comparison
The preheat control system may include a comparator to compare the battery monitoring temperature collected from the original battery management system with the set temperature. If the battery temperature is lower than the ideal value, a feedback signal is provided. Alternatively, a temperature may be set separately Detected sensor system to collect the actual battery temperature; thermocouples and other known temperature sensors can be used here.
The temperature comparator can feed back the information that the temperature is below the ideal value in a number of ways: The temperature comparator can continuously send signals whose temperature is below the ideal and can also send intermittently.
Preheat the control loop
The warm-up control loop needs to provide the controller with a warm-up time before the vehicle starts, during which time the preheat enable signal is continuously sent.
The preheating control loop needs to give the starting time of the vehicle. The starting time can be given directly by the preset schedule in the program. According to the historical accumulation of starting time, the system estimates a time by itself and can calculate it according to the historical data according to the statistical rule One time, for example, if the system observes that the car's owner has started the car at 8:00 am in the past few weeks, then it is also predictable for the upcoming morning, with the car starting at 8:00 .
Heating time
When to start the preheating, in addition to need to consider the car's start-up time, but also need to determine the length of preheating, that is, from the starting temperature for how long to reach the ideal temperature can be heated in strict accordance with the car start time minus the time required for preheating , Turn on the preheating system, this strategy does not consider the battery needs to maintain a certain temperature at an ideal temperature in order to achieve the issue of thermal balance If you want to achieve better preheating effect, you need to set aside the holding time.
Continuous control loop
The patent called "further control system" corresponds to the previous system block diagram, and we temporarily translate it into a "continuous control loop." The continuous control loop needs to be coupled with multiple systems in the system block diagram, at least with the preheat control loop and Heating system with the heating system includes heating devices, which can be resistance heaters or other types of heaters. Continuous control loop through the temperature signal is lower than the ideal and the heating system is linked to the heating system requires continuous heating; continuous control loop can also be passed The preheat system enable signal acts on the heating system to indicate that it continues to operate; the heating system can also be an integral part of the overall air conditioning system, relying on the coolant to heat the battery.
Cost evaluation loop
How much power it takes to heat the battery, compared to the savings in battery power and other overall effects, is it cost-effective, and a system that makes estimates in this area is named 'cost estimating circuit', for the moment as a cost-assessment loop, The previous literature is rare, is a consideration of the economic benefits of a design .Cost assessment results can be used to preheat the control system, to determine whether to preheat.Cost assessment can be based on previous operating experience Data, electricity price fluctuation plan of municipal system, etc., current electricity price (area with floating electricity price), estimated electricity consumption, etc. The pre-heating cost is compared with the system default cost criterion, and below the preset value, the preheating System enable signal.
Warm-up enable signal
Preheat enable signal can be set in continuous control loop in the form of software; it can work in the form of firmware; can use hardware directly; can use mechanical switch;
Synthesis
According to the statement of Tesla in the patent literature, it should be understood as a whole that the control system should issue a heating system enable signal at an appropriate time according to the scheduled departure time, and start the preheating system to start heating. After that, Use a temperature comparison circuit to monitor the battery temperature in real time and compare the difference between the actual battery temperature and the set temperature as long as the actual feedback temperature is lower than the ideal temperature and the heating enable signal is continuously sent out. It has been determined that the actual temperature signal has reached or even exceeded the ideal temperature, then stop sending the heating enable signal.
At the end of 2017, Tesla was delayed in announcing the addition of a warm-up feature, so did not design the warm-up feature in the previous car? Remember the first day of the 2016 New Year, a Tesla Model S in Norway Charging the super charging station in the process of charging the entire car burned a burning electric vehicle body temperature dropped by 3 degrees Now think about it, to sell to Norway that latitude Tesla, was not preheated Function? However, look at the heating of the patent application time, in March 2008, the public time is June 2010, it has been nearly a decade. Is that preheating is not necessary, or for any other reason? It is unclear.A understanding of the children's shoes, you can talk about Kazakhstan.This Tesla's preheating feature will enable the patent, or another way, we will see.
