With the increasingly stringent global carbon emission requirements and the energy shortage in European countries caused by the Russian-Ukrainian war, the application of new energy, mainly represented by solar energy, has reached a top requirement . Solar power generation has serious instability, and the power generation changes significantly with the daytime, cloudy and sunny days, so the demand for energy storage is getting more and more important . As a basic small unit of life,  home energy storage battery has also become people's favorite. However, as a new product, this article will introduce the core components of home energy storage battery, and the current market status of these key components .

Although it is a home energy storage battery, it is basically the same as the battery with other application which are all composed of cells (battery cells), battery management system BMS, shell chassis, input and output terminals, structural parts, etc.. The voltage and capacity required by the home energy storage battery are solved by the series-parallel design of the battery cells. The battery cells are fixed inside the case with structural parts, and then connected to the input and output terminals after connecting to the BMS and finally encapsulating the case to form the required energy storage battery. Among these components, the real core is the battery cell and battery management system (BMS). The following are the details of these two components .

1. Lithium-ion battery cells

(1). According to the chemical material system, lithium-ion batteries are divided into ternary lithium iron batteries and lithium iron phosphate batteries. Due to high chemical activity and poor safety performance, ternary lithium batteries are gradually replaced by lithium iron phosphate batteries with higher safety performance and longer cycle life in the fields of electric vehicles and home energy storage batteries. Globally , except TESLA and LG Chem,which use ternary lithium batteries for home energy storage battery products, other manufacturers have basically chosen the lithium iron phosphate battery. Let the market lose patience and confidence in the ternary lithium battery.

(2). Lithium iron phosphate battery includes three structural forms, namely: cylindrical cell, prismatic cell and pouch cell. As a finished product of home energy storage battery, it does not care too much about the structure of its cell, but many buyers will prefer a certain form of product due to misleading information, and even think that lithium iron phosphate battery is prismatic cells to that of a few people. In fact, these three forms of cells can theoretically be used in home energy storage batteries, and the key lies in the design of the structure, the optimization of heat dissipation, and the control of costs.

a,  The advantages of cylindrical cells lie in the controllable cost, high degree of automation, high consistency, uniform model and flexible combination. For details, please refer to the analysis of the advantages of cylindrical cells , which will not be repeated here!

b,  Prismatic cells are highly customized products belonging to the EV market, and their parameters such as size, structure, and capacity are different. Cells below 200Ah are basically non-standard products, and the market versatility is very poor. Therefore, the resource flexibility of using square aluminum shells in the field of household energy storage batteries is relatively poor. At the same time, the cost per watt-hour of prismatic cells below 100 Ah is significantly higher than that of cylindrical cells (please note: the comparison here is at the same quality level).

c,  The theoretical process cost of pouch cells is higher, and the PACK process is more difficult. If the heat dissipation design is unreasonable, it will lead to problems such as flatulence. At present, there are not many manufacturers that apply to energy storage for home, especially battery pack factories basically do not consider using pouch cells to design home energy storage battery.

(3). The quality level of the cell

Regardless of the form of batteries, their quality levels are uneven. Roughly speaking, the same model of the same manufacturer will basically be divided into three categories: qualified products (A Grade), degraded products (B Grade) and recycled products , and what kind of quality level of cells is used by household energy storage battery manufacturers for sellers. It’s a "secret" from manufacture, but the product application experience will be very different.

a,  Qualified cells (A Grade): products that meet certain quality standards, but the so-called "standards" will also be different . For example, the standards of the automobile level will be much stricter than the general standards, but as a small home energy storage battery (Generally between 5--20KWH) It is not necessary to adopt automotive-grade standards, because this will significantly increase the cost. Generally, cells that meet the design capacity, voltage, self-discharge and more than 2000 cycles at room temperature of 100% DOD can be used in household energy storage batteries, but the cells need to be paired effectively. For the cycle of the battery, please refer to: Lithium-ion battery cycle life: Lithium-ion battery cycle life: how to estimate the service life of Powerwall LiFePO4 Battery? .

b,  Degraded cells (B Grade), also known as A Grade products: usually the channels are factories and traders, and the products of the factories will be relatively reliable to know the general reasons for their downgrades, but the sources of goods for traders are difficult to trace, and Can't really know the reason for "why it became a B Grade". At the same time, there are many types of degraded products, which may be caused by insufficient capacity, excessive internal resistance, excessive self-discharge, and poor internal moisture control. Energy storage batteries have high requirements on the identification ability of manufacturing factories.

c,  Recycled cells: There are uncirculated and circulating recycled batteries. As for the uncirculated disassembled cores, please refer to the above degraded products, but the difference is that it is difficult to find out the reason for the uncirculated battery. As for the circulating dismantling products, there is a lot of controversy. On the one hand, the state encourages cascade utilization. On the other hand, after a certain percentage of the initial capacity (usually 80% of the initial capacity) is recycled, the battery life will be terminated, and many people in the market believe that There is still 80% of the capacity available (refer to: Lithium-ion battery cycle life: Lithium-ion battery cycle life: how to estimate the service life of Powerwall LiFePO4 Battery? ), in addition to the uncertainty of the remaining capacity, at the same time the cycle of dismantling There is no way to judge the consistency of the machine, resulting in poor recognition of packaging and no guarantee of quality. It is not recommended to use some recycled cells with serious capacity degradation on household energy storage batteries with high safety performance requirements.

2. Battery Management System (BMS)

In the energy storage battery, the battery management system BMS is one of the most important components, which determines the safety, life, and matching functions and performance of the battery. It can even be said that the BMS is the "brain" of the energy storage battery. quite crucial role. As one of the core components of household energy storage batteries, how to identify or identify the quality of BMS has become a difficult problem for many businesses. In the author's opinion, it is nothing more than the following points:

(1). All lithium-ion batteries need to increase external circuit protection, which is different from lead-acid batteries and nickel-metal hydride, nickel-chromium batteries. But the protection circuit is also different, divided into hardware solution protection circuit (commonly known as: protection board) and software intelligent control circuit (that is, what we call BMS). The function of the protection board is relatively simple, mainly to protect the basic functions of lithium batteries such as overcharge and over discharge, over current, over temperature and short circuit. The degree of intelligence is not high, and it is basically a passive protection form. The BMS is relatively intelligent, and can calculate the battery's state of charge, SOC, and state of health, SOH, etc., and can even actively intervene after learning, and conduct effective management through monitoring data in turn. At present, in order to pursue cost, many manufacturers in the world use protective plates to passively protect energy storage batteries, which is a dangerous practice. Customers need to evaluate and use them carefully, especially for some large energy (such as more than 5KWH).

(2). The core technology of BMS---SOX algorithm. For the battery's state of charge SOC and state of health SOH, only accurate calculation can be effectively managed. Therefore, the SOX algorithm has become one of the core technologies of BMS, especially for the lithium iron phosphate battery system, which is more common for household energy storage batteries. more difficult. The traditional algorithms are based on the "Ampere-hour integration plus voltage calibration" method, which is also widely used in the world. However, for lithium iron phosphate, the accuracy of this algorithm is poor, and the calibration voltage changes greatly, which leads to a lot of problems in actual use. Large errors, especially after cycling for a period of time, will become more and more obvious. At present, there are many algorithm technologies on the market, but the actual application is relatively small, which is also one of the technical bottlenecks in the development of new fields such as new energy vehicles and energy storage.

(3). BMS communication: In the actual use of the home energy storage battery, the energy storage battery is connected to the inverter (or the charging and inverter integrated machine). In the intelligent control and management system, how to let the inverter know the energy storage real-time status of batteries for better use of energy storage batteries? This requires a communication protocol to solve. The communication protocol is simply understood as a pre-agreed dialogue language, and there is no novel technology. However, as a BMS of an energy storage battery, it is very critical to meet the communication of the inverters. Because the inverters exist in the world for a longer time than the energy storage batteries, the energy storage batteries are required to be compatible with different versions of the inverter. There is no technical threshold but requires a lot of accumulation.

To sum up, as the two core components of home energy storage battery, cells and BMS are very important, especially in the current world, which brings many uncertain choices to the market, and even vicious competition. It will be a fatal blow to the entire industry, so it is necessary for customers, manufacturers, etc. to standardize products so that the market can enter a healthy and orderly development path.