Today, let’s talk about the most important part of the energy storage system – the battery cell. The most fundamental unit responsible for energy storage is the battery cell, similar to how the most important part of a school is its students.
To manage a school well, you need to know how to manage students. To manage students, you need to know about their temperaments, characteristics, and so on. What are the temperaments and characteristics of battery cells? What are the basic concepts about battery cells?
Cell material
The most popular and economical energy storage chemical battery at present is lithium iron phosphate battery, which includes the basic structure of positive electrode material, negative electrode material, electrolyte, separator, etc.
The positive electrode material is composed of lithium iron oxide, which has good thermal and chemical stability. Among the positive electrode materials, lithium iron phosphate is considered a good student, with stable personality, low raw material price, medium to high energy density, relatively environmentally friendly, long service life, and excellent performance compared to students who are specialized in ternary lithium. That’s why lithium iron phosphate can become the backbone of the energy storage industry.
The negative electrode material is generally graphite, or some hard carbon and soft carbon. Graphite has low cost, abundant resources, mature technology, and is relatively stable with long cycle life, etc.
The electrolyte is generally composed of lithium hexafluorophosphate (LiPF ₆) and carbonate solvents (EC/DMC/EMC, etc.). Its function is to conduct lithium ions between the positive and negative electrodes. The essence of lithium battery charging and discharging is that lithium ions move back and forth to both sides of the positive and negative electrodes during charging and discharging. Diaphragm is usually a polyolefin microporous membrane, which prevents short circuiting between positive and negative electrodes and allows lithium ions to pass through. One end can discharge positive and the other end can discharge negative due to the good isolation type of the diaphragm. If it is not good, self discharge will occur, and one typical situation is that the battery will lose power even when not in use.
4H,2H,1H
Do you often see the words 0.5C, 1C, 2C, 4H, 2H, 1H? What do they mean? The H in 1H means’ hour ‘in English, which means one hour. If 1H is marked, it indicates that the energy storage system is designed to discharge continuously within one hour and complete the discharge. 2H is equivalent to discharging in 2 hours. The most common system in China currently is the 2H system, which means a continuous discharge for 2 hours. With the development of energy storage demand for wind and solar power generation, energy storage systems are gradually moving towards 4H and 8H systems, which are long-term energy storage systems.
0.5C,1C,2C
The charging speed of mobile phones is getting faster and faster. Previously, it took 2 hours to charge a phone, but now it can be fully charged in half an hour. Many electric vehicles have also started to have overcharging and flash charging power. Lei Jun said during the release of Yu7 that the highest power can reach 5.2C.
The meaning of this C in English is capacity. The complete meaning is the battery charging and discharging capacity ratio. 1C means to fully charge or discharge electricity in one hour. 2C is twice the battery capacity rate, which means that charging or discharging can be completed in half an hour. The faster the charging and discharging speed, the easier it is for the battery to generate heat, and the shorter its lifespan.
Cycle life
In the promotional materials of energy storage system manufacturers, you can often see that there are 4000 cycles, 6000 cycles, 8000 cycles, and even 12000 cycles of service life. What does this mean?
The so-called cycle is the action of completing charging and discharging. The battery life of a mobile phone is usually 1000 cycles, which means that after being charged 1000 times, the battery life of the phone is almost at its end. At first, it can last for 6 hours, but after 1000 cycles, it may only last for 2 hours. At this time, you should replace the battery.
The manufacturer says that the energy storage system that can achieve a cycle life of 8000 times must be a long-term energy storage system, which is a 4H, 8H or 0.25C, 0.125C system design.
DOD
The word DOD is also a high-frequency term in energy storage, which means Depth of Discharge. How do you understand this? For example, when your phone shows that the remaining battery level is 20%, you become anxious about using electricity and then go to charge it. At this time, the discharge depth of the phone is 80%. Personally, I suggest that the phone should be charged when it shows 20%, which is beneficial for the battery life of the phone.
SOC
The remaining battery level of the phone mentioned earlier is 20%, so the SOC of this phone is 20%, which means the remaining battery level.
Ah
In the field of batteries, when expressing battery capacity, most people use Ah. Mobile phone capacity likes to use mAh, and power banks also like to use mAh. What is the specification of your power bank? 10000mAh？ Or 20000mAh? How much is it still? Please let me know in the comment section.
Friends who often travel by plane know that airport security personnel often check the capacity of power banks. The maximum capacity of power banks that can be carried on a plane is 100Wh. Why not use how many mAh as the judgment standard? MAh is composed of mA * h, which is the product of current and time. This cannot represent energy. What can represent energy is watt hours or wh (w * h), where w is the product of current and voltage. That is to say, wh can also be written as V * A * h, and friends who understand physics can see that Ah lacks voltage here.
The voltage of different batteries varies, and the voltage of battery SOC varies from 100% (4.2V) to 0% (3V). The standard voltage of a typical power bank is 3.7V, so a 10000mAh power bank is generally 37Wh. The maximum power bank that can be purchased on the market and carried on airplanes is 20000mAh, which is 74Wh.
Wh
In order to better measure the amount of energy charged and discharged by an energy storage system, Wh is generally used instead of Ah as the nominal energy capacity. An integrated container energy storage system usually uses MWh as the unit of energy storage capacity, such as 5MWh (5000 kWh), 6.3MWh (6300 kWh), and so on.