As secondary rechargeable batteries that have been widely used, lithium ion batteries have penetrated into every aspect of our lives. For most users, the value of lithium-ion batteries is reflected in the discharge phase, and the charging phase is completely ineffective and forced to waste time. Because of this, manufacturers continue to improve the charging technology of lithium-ion batteries. SES Power remembers that it took at least 1-2 hours to fully charge a mobile phone ten years ago, and now a mobile phone only needs more than 20 minutes to be fully charged. It can be almost fully charged. The charging time of the first-generation Tesla electric vehicles often took more than an hour to get close to a full charge, but now it only takes about 40 minutes.

　　Of course, not all applications of lithium-ion batteries require fast charging. SES Power is good at lithium battery energy storage, UPS backup power, microgrid, home energy storage, etc. In fact, there is no urgent need for charging time. On the contrary, sometimes it is necessary to appropriately reduce the charging current to achieve the purpose of extending the charging time (because the cost of the charger is proportional to the output power). We use 12V, 24V, 36V, 48V series that can perfectly replace lead-acid batteries, 3Kw~20Kw (you can choose different capacity lithium batteries to match) home energy storage system, photovoltaic power generation energy storage system, large inverter (UPS) For supporting products, etc., it is necessary to consider the charging conditions in the design stage, and try to choose the most cost-effective charging solution.

　　Below we will look at the proposition of fast charging from a purely technical point of view for lithium-ion battery applications that require fast charging.

　　A: How fast is the charge to be called "quick charge"?

　　The basic demands of ordinary consumers for lithium-ion battery charging are:

　　1) Charge fast;

　　2) Do not affect the battery life;

　　3) To save money as much as possible, try to charge as much electricity as possible into my battery.

　　So fast can be called fast charge? There is no standard literature that gives specific values. Let’s refer to the standards of the electric vehicle industry for the time being. The entry-level fast charge is 3C, which theoretically takes 20 minutes to charge. Of course, this is unrealistic. We generally think that 80% full in 30 minutes can be recognized as fast charging. According to this idea, 80% of the lithium-ion battery is fully charged in 15 minutes, which is equivalent to working in a 3.2C state of charge.

　　B: What is the bottleneck of fast charging?

　　In the general proposition of fast charging, it is necessary to classify the surrounding areas, such as batteries, chargers, and power distribution facilities.

　　When we discuss the proposition of fast charging, the first thing that comes to our mind is whether there will be a problem with the battery. In fact, before there is a problem with the battery, the first problem is the charger and the power distribution line.

　　Tesla's charging pile, known as a super charging pile, has a power of 120kW. According to the parameters of Tesla Model S85D, 96s75p, 232.5Ah, and the highest 403V calculation, its 1.6C corresponds to the maximum demand power of 149.9kW. It can be seen from here that for long-range pure electric models, 1.6C or 80% full charge in 30 minutes has already constituted a test for the charging pile.

　　In the industry and national standards, it is not allowed to directly set up charging stations in the original residential electricity network, because the power consumption of one fast charging pile has already exceeded the electricity consumption of dozens of households. Therefore, charging stations all need to set up 10kV transformers separately, and not all distribution networks in a region have the margin to add more 10kV substations.

　　As for whether the battery can carry 1.6C or 3.2C charging requirements, it can be viewed from both macro and micro perspectives.

　　C: The macroscopic fast charging theory of lithium batteries

　　The reason why the title of this section is called "Macroscopic Fast Charging Theory" is that what directly determines the battery's fast charging capability is the properties of positive and negative materials, microstructure, electrolyte composition, additives, diaphragm properties, etc. inside the lithium battery. Content. Let's put aside for a moment what's almost entirely lithium-ion battery expertise, and then we'll stand entirely outside the battery and see how lithium-ion batteries can be fast-charged in third vision.

　　C1: There is an optimal charging current for the battery

　　In 1972, American scientist J.A. Mas proposed that the battery has an optimal charging curve and Mas's third law during the charging process. It should be noted that this theory is proposed for lead-acid batteries, and the boundary condition that defines the maximum acceptable charging current is a small amount. The production of side reaction gases is obviously related to the specific reaction type.

　　But the idea that there is an optimal solution in the system is universal. Specific to lithium batteries, the boundary conditions that define their maximum acceptable current can be redefined. Based on the conclusions of some research literature, its optimal value is still a curve trend similar to Maas' law.

　　It is worth noting that the boundary conditions of the maximum acceptable charging current of lithium batteries need to consider not only the factors of lithium battery cells, but also system-level factors, such as different heat dissipation capabilities, the maximum acceptable charging current of the system is different. .

　　Then we will continue the discussion on this basis for the time being.

　　The formula description of Maas' theorem:

　　I =I0*e^αt

　　In the formula; I0 is the initial charging current of the battery; α is the charging acceptance rate; t is the charging time. The values of I0 and α are related to battery type, structure and condition.

　　The current research on battery charging methods is mainly based on the optimal charging curve. As shown in the figure below, if the charging current exceeds this optimal charging curve, it will not only not improve the charging rate, but also increase the gas evolution of the battery; if it is less than this optimal charging curve, although it will not cause damage to the battery, it will prolong the charging. time, reducing the charging efficiency.

　　The elaboration of this theory consists of three levels (Maas's third law):

　　① For any given discharge current, the current acceptance ratio α when the battery is charged is inversely proportional to the square root of the capacity released by the battery;

　　② For any given discharge amount, α is proportional to the logarithm of the discharge current Id;

　　③ After the battery is discharged at different discharge rates, its final allowable charging current It (acceptance capacity) is the sum of the allowable charging currents at each discharge rate.

　　The above theorem is the source of the concept of charge acceptance. The charging acceptance capacity is the maximum capacity that can be used for a rechargeable battery with a certain amount of charge under a specific environment and condition without causing undue side reactions and adverse effects on the life and performance of the battery cell. recharging current.

　　SES Power will further understand the three laws of mas for you:

　　The first law is that after the battery discharges a certain amount of power, its charge acceptance capacity is related to the current charge capacity. The lower the charge capacity, the higher the charge acceptance capacity.

　　The second law, during the charging process, pulse discharge occurs, which helps to improve the real-time acceptable current value of the battery;

　　The third law, the charge acceptance capacity will be affected by the superposition of the charge and discharge conditions before the charging time.

　　If the Maas theory is also applicable to lithium batteries, the reverse pulse charging (hereinafter referred to as the Reflex fast charging method) can be explained by depolarization. Support for the pulse method.

　　Of course, the intelligent charging method that can really apply the Maas theory is to track the battery parameters, so that the charging current value always follows the Maas curve of the lithium battery, so that the charging efficiency can be maximized within the safety boundary.

　　C2: Common fast charging methods

　　There are many charging methods for lithium batteries, and for the requirements of fast charging, the main methods include pulse charging, Reflex charging, and intelligent charging. Different battery types have different charging methods.

　　C2.1 Pulse charging

　　The standard pulse charging method in the literature is that the pulse phase is set after the charging reaches the upper limit voltage of 4.2V, and continues above 4.2V (taking the ternary lithium battery as an example). Not to mention the rationality of its specific parameter settings for the time being, because there will be differences between different types of batteries.

　　Let's focus on the pulse implementation process. The following is the pulse charging curve, which mainly includes three stages: pre-charge, constant current charging and pulse charging.

　　During constant current charging, the battery is charged with a constant current, and part of the energy is transferred to the inside of the battery.

　　When the battery voltage rises to the upper limit voltage (4.2 V), the pulse charging mode is entered: the battery is charged intermittently with a pulse current of 1 C.

　　During the constant charging time Tc, the battery voltage will continue to rise, and the voltage will slowly drop when charging stops.

　　When the battery voltage drops to the upper limit voltage (4.2 V), the battery is charged with the same current value to start the next charging cycle, and so on until the battery is fully charged.

　　During the pulse charging process, the battery voltage drop speed will gradually slow down, and the charging stop time T0 will become longer. When the constant current charging duty cycle is as low as 5% to 10%, the battery is considered to be fully charged and the charging is terminated.

　　Compared with the conventional charging method, the pulse charging can be charged with a larger current, and the concentration polarization and ohmic polarization of the battery will be eliminated during the charging stop period, so that the next round of charging can be carried out more smoothly, and the charging speed is fast. The temperature change is small and the impact on battery life is small, so it is widely used at present.

　　But the disadvantage is obvious: a power supply with limited current capability is required, which increases the cost of the pulse charging method.

　　C2.2 Intermittent charging method

　　The lithium battery intermittent charging method includes variable current intermittent charging method and variable voltage intermittent charging method.

　　C2.2.1 Variable current intermittent charging method

　　The characteristic of the variable current intermittent charging method is to change the constant current charging to the voltage limiting variable current intermittent charging.

　　In the first stage, the battery is charged with a larger current value, and the charging is stopped when the battery voltage reaches the cut-off voltage V0, and the battery voltage drops sharply at this time.

　　In the second stage, after a period of off-charging time, the charging is continued with a reduced charging current.

　　In the third stage, when the battery voltage rises to the cut-off voltage V0 again, stop charging, so the charging current for several times (generally about 3 to 4 times) will reduce the set cut-off current value.

　　Finally, it enters the constant voltage charging stage, and charges the battery with a constant voltage until the charging current decreases to the lower limit value, and the charging ends.

　　In the main charging stage of the variable current intermittent charging method, under the condition of limited charging voltage, the intermittent mode of gradually decreasing current is adopted to increase the charging current, that is, the charging process is accelerated and the charging time is shortened. However, this charging mode circuit is relatively complex and expensive, and is generally only considered for high-power fast charging.

　　C2.2.2 Variable voltage intermittent charging

　　On the basis of the variable current intermittent charging method, some people have studied the variable voltage intermittent charging method. The difference between the two lies in the charging process in the first stage, which replaces the intermittent constant current with the intermittent constant voltage.

　　In each constant voltage charging stage, due to the constant voltage, the charging current naturally decreases according to an exponential law, which is in line with the characteristic that the acceptable rate of battery current gradually decreases with the progress of charging.

　　C2.2.3 Reflex fast charging method

　　The Reflex fast charging method, also known as the reflex charging method or the "hiccup" charging method. Each working cycle of this method includes three stages: forward charge, reverse instantaneous discharge and stop charge.

　　It solves the phenomenon of battery polarization to a large extent and speeds up the charging speed. But reverse discharge will shorten lithium battery life. SES Power is here to remind you that lead-acid batteries are the biggest beneficiaries of this approach.

　　C2.2.4 Intelligent charging method

　　Intelligent charging is a more advanced charging method at present. The main principle is to apply du/dt and di/dt control technology: by checking the increment of battery voltage and current to judge the battery charging state, dynamically track the acceptable charging current of the battery, and make charging the current is always around the maximum charging curve that the battery can accept.

　　This kind of intelligent method generally combines advanced algorithm technology such as neural network and fuzzy control to realize the automatic optimization of the system.

　　At the end of the article, SES Power needs to remind you that many customers often ask whether they can use the original lead-acid charger for lithium-ion batteries after purchasing the 12V100Ah and 24V100Ah (lead-acid replacement products) of our lithium iron phosphate batteries. The answer to the question of charging is: the characteristics of lithium-ion batteries are different from those of lead-acid batteries. Do not use lead-acid battery chargers to charge lithium-ion batteries, even if the charging voltage parameters are the same!