"Energy storage", as the name suggests, is to store electrical energy. To store electrical energy, you need a medium or container, and a battery is the container that stores electrical energy. Generally, two types of batteries are mainly used in photovoltaic energy storage systems, namely lead-acid batteries and lithium batteries.
1. Lead-acid battery
Lead-acid batteries are chemical energy storage devices that use lead and lead dioxide (PbO₂) as the active materials of the negative and positive electrodes of the battery, and dilute sulfuric acid as the electrolyte. In fact, the mutual conversion between electrical energy and chemical energy is achieved through electrochemical reactions; It is the preferred chemical power source for various energy storage systems, emergency power supplies, and soft/black start devices.
The nominal voltage of a single-cell lead-acid battery is 2.0V. It can be discharged to 1.5V and charged to 2.4V. In practical applications, six single-cell lead-acid batteries are often connected in series to form a lead-acid battery with a nominal voltage of 12V. The battery module, on the basis of 12V, can obtain the voltage value (such as 48V, 96V) accepted by the system through proper series and parallel connection on the basis of 12V, so as to realize normal charging and discharging work.
The main components of lead-acid batteries include: positive and negative poles, plates, separators, electrolyte, containers, etc. Because of the large amount of chemical solution in the battery module, it is generally relatively heavy.
Lead-acid battery internal structure diagram
The charging modes of lead-acid batteries generally have three modes: constant current, constant voltage and floating charge, also known as three-stage charging. Charging current is a very important parameter, and the unit is generally expressed in C. The maximum charging current of a battery will be clearly marked in the battery specifications, usually 0.1C, 0.2C or 0.3C; for example, the capacity C of a certain battery is 200Ah, the charging current is 0.1C, then it is 0.1×200=20A, and 0.2C, then it is 0.2×200=40A; the optimal charging current for lead-acid maintenance-free batteries should be about 0.1C, and the charging current will be too large or too small. Affect battery life.
① Good safety performance. Lead-acid batteries are easy to maintain, and can continue to be used after damage, with a little repair.
② The price is cheap. The price of lead-acid batteries is about 1/3 of that of lithium batteries, which shows that they are cheap.
③ Recyclable and reverse repair. Lead-acid batteries are highly recyclable. After the lead-acid battery wears out, they can go to the electric vehicle shop to replace the old battery with a new one. After making up the price difference, they can replace a new set of batteries, which can minimize the loss, which is not a small benefit for users. Lead in batteries can also be recycled to reduce environmental pollution.
Heavy weight, limited battery capacity, and short lifespan. The service life of lead-acid batteries is generally about two years after 400 deep charges and deep discharges.
2. lithium battery:
Lithium batteries are a type of battery that uses lithium metal or lithium alloy as positive/negative electrode materials and uses non-aqueous electrolyte solutions; it is mainly divided into lithium metal batteries and lithium-ion batteries. Lithium batteries generally refer to lithium-ion batteries, which are secondary batteries that can support charging and discharging. Lithium-ion batteries use lithium alloy metal oxide as the positive electrode material and graphite as the negative electrode material. The main body of lithium-ion batteries storing lithium is the negative electrode material, which plays a key role in the battery's charge and discharge efficiency, cycle times and other properties.
Schematic diagram of the internal structure of lithium battery
Lithium-ion batteries are mainly divided into lithium cobalt oxide batteries, lithium manganate batteries, lithium nickel oxide batteries, lithium iron phosphate batteries, ternary lithium batteries, etc. according to different cathode materials.Comprehensively weighing factors such as price, cost, performance, and safety, in actual energy storage system applications, more lithium iron phosphate batteries are used. Lithium iron phosphate batteries are generally considered to contain no heavy metals and rare metals, and are non-toxic, non-polluting, green and environmentally friendly batteries. It uses lithium iron phosphate (LiFePO4) as the positive electrode material and carbon as the negative electrode material. The rated voltage of the monomer is 3.2V, and the charging cut-off voltage is about 3.6V~3.65V; the required voltage and capacity are achieved through series and parallel.
① Light weight and small size. The volume of the lithium battery is 2/3 of the lead-acid battery, and the weight is about 1/3 of the lead-acid battery. Lithium batteries with the same volume have a higher capacity than lead-acid batteries. The weight reduction makes electric vehicles lighter, while increasing range by about 10%.
②Long life. The average life of lithium batteries is about 4 years, which is about 1.5 times that of lead-acid batteries. It can be charged and discharged more than 500 times. The battery has no memory and is highly shock resistant.
③ Good overcharge resistance and charge and discharge resistance. At room temperature, the lithium battery can be charged continuously for 48 hours without accidents such as battery expansion, leakage, and rupture, and the capacity remains above 95%. And under the special charger, it can charge and discharge quickly.
The production cost of lithium battery is high, and the production equipment is expensive. Labor costs account for about 40% of production costs, and the price is about three times that of lead-acid batteries. The price-performance ratio brought by three times the price is not high, making people feel flashy. In addition, lithium batteries cannot be recycled and can only be discarded after they are worn out.
Lithium battery pack:
The application of lithium batteries should also include BMS (Battery Management System), that is, the battery management system. A BMS system generally includes a central control unit, an acquisition unit, a display unit, a data recording unit and an external expansion unit.
The main functions of BMS are:
① Single battery parameter monitoring ② SOC (state of charge) calculation ③ Temperature monitoring and management ④ Battery energy balance control ⑤ Fault diagnosis alarm ⑥ Information display ⑦ Data recording ⑧ Communication with the host computer
3. Selection of energy storage batteries
Many customers are still confused about how to choose a suitable energy storage battery in the actual system design, and they can choose from the following three aspects.
Photovoltaic off-grid energy storage system (picture)
Lead acid battery or lithium battery
First of all, batteries account for a high proportion of energy storage systems. For energy storage inverters that support both lead-acid batteries and lithium batteries, which one to choose depends on investment willingness, project type, project location, warranty requirements, etc. The energy density of lithium batteries (ρ=E/V) is higher, about 6 to 7 times that of lead-acid batteries, smaller in size, lighter in weight, and has a long cycle life, 1.5 to 5 times that of lead-acid batteries. Therefore, the batteries that are generally guaranteed by energy storage machine manufacturers are all lithium batteries. However, the performance of lithium batteries is relatively poor at low temperatures, and the price is relatively high, usually 2 to 4 times that of lead-acid batteries.
Secondly, the capacity selection of the battery is mainly determined according to the user's power storage needs, and at the same time, the system capacity must be taken into consideration.
high voltage battery or low voltage battery
Energy storage inverters include high-voltage (HV) series and low-voltage (LV) series. Correspondingly, batteries are also divided into high-voltage and low-voltage. Generally, single-phase output energy storage machines are equipped with low-voltage batteries, and the terminal voltage is about 48V~52V. At this time, if you choose a lead-acid battery, you can connect four 12V batteries in series to reach 48V; if you use a lithium battery, you can directly choose a battery pack with a voltage level of 48V;
From the above content, we can see that lead-acid batteries and lithium batteries have their own pros and cons. Generally speaking, lithium batteries are relatively high-end, but they are also expensive. They are currently used more in overseas markets in Europe, America and Australia. New energy is the trend, and huge market demand will become an inexhaustible driving force for the rapid development of technology. It is believed that the price of lithium batteries will reach a range acceptable to the public in the near future.