Photovoltaic energy storage system batteries types and selection

It is foreseeable that the energy storage market is booming. As the name implies, energy storage is to store electrical energy. To store electrical energy, a medium or container is needed, and batteries are exactly the containers that store electrical energy. Generally, two types of batteries are mainly used in photovoltaic energy storage systems, namely lead-acid batteries and lithium batteries.

Lead acid battery

A lead-acid battery is a chemical energy storage device that uses lead and lead dioxide (PbO₂) as the active material of the cathode and anode of the battery, and uses dilute sulfuric acid as the electrolyte. In fact, it realizes the mutual conversion between electrical energy and chemical energy through electrochemical reactions. It is the preferred chemical power source for various energy storage systems, emergency power supplies, and soft/black start devices.

A single lead-acid battery cell has a nominal voltage of 2.0V, can be discharged to 1.5V, and can be charged to 2.4V. In practical applications, 6 cells are often connected in series to form a lead-acid battery module with a nominal voltage of 12V. On the basis of 12V, through proper series and parallel, the voltage value (such as 48V, 96V) accepted by the system can be obtained to realize normal charging and discharging work.

The main components of lead-acid batteries include: cathode and anode 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 batteries mainly include general flooded lead-acid batteries, colloid maintenance-free (for solar energy) lead-acid batteries, and lead-carbon batteries.

In actual use, the proportion of gel batteries and lead-carbon batteries is increasing. The over-discharge, self-recovery ability and low-temperature charge-discharge performance of gel batteries are better. Due to the addition of carbon (graphene) in the electrolyte, the lead-carbon battery prevents the sulfation of the anode and improves the problem of easy failure of the battery, thereby greatly improving the service life of the battery.

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 represented by C. The maximum charging current of a battery will be clearly marked in the battery specification parameters, usually 0.1C, 0.2C or 0.3C. The best charging current for lead-acid maintenance-free batteries should be about 0.1C, too large or too small charging current will affect the service life of the battery.

Lithium-battery

Lithium batteries are a type of batteries that use lithium metal or lithium alloys as cathode or anode materials and use non-aqueous electrolyte solutions. Lithium batteries are mainly divided into lithium metal batteries and lithium ion batteries. Lithium batteries generally refer to lithium-ion batteries, which are the best rechargeable batteries that can support charging and discharging.

Lithium-ion batteries use lithium alloy metal oxide as the cathode material and graphite as the anode material. The main body of lithium-ion batteries to store lithium is the anode material, which plays a key role in the performance of the battery’s charge and discharge efficiency and cycle times.

Lithium-ion batteries are mainly divided into lithium cobalt oxide batteries, lithium manganese oxide batteries, lithium nickel oxide batteries, lifepo4 batteries, and ternary lithium batteries according to different cathode materials.

Comprehensively weighing price, cost, performance, safety and other factors, in the actual application of energy storage systems, 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 cathode material, carbon as the anode 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 can be achieved through series and parallel.

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.

Selection of photovoltaic energy storage batteries

Many customers are still confused about how to choose a suitable photovoltaic energy storage battery in the actual system design, and they can choose from the following three aspects.

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, and quality assurance requirements.

The energy density of lithium batteries is higher, about 6 to 7 times that of lead-acid batteries, smaller in size, lighter in weight, and has a long cycle life, which is 1.5 to 5 times that of lead-acid batteries. Therefore, the batteries that are generally guaranteed by energy storage machine manufacturers are lithium batteries. However, the performance of lithium batteries is relatively poor at low temperatures, and the price is relatively high.

Capacity selection

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.

For example, a user needs to store 30kWh/day of electricity. He installed a set of 3KW off-grid energy storage system, and there is no utility power complementation, so it will take at least 2 days for his battery to be fully charged. It may not be full for a long time in the season of continuous rainy season.

In this case, it is necessary to consider the constraints of various factors in the system design stage, increase the installation capacity of photovoltaic modules, and increase the output capacity of energy storage control equipment.

As for lithium batteries, you can generally choose directly according to the capacity of a single PACK, such as a battery pack with a single PACK of 2.56KWh, 2 packs are 5.12KWh, and 10 packs are 25.6KWh.

High voltage battery or low voltage battery

There are high-voltage (HV) series and low-voltage (LV) series of energy storage inverters. Correspondingly, batteries are divided into high voltage battery and low voltage battery. Generally, an energy storage machine with single-phase output is paired with a low-voltage battery, and the terminal voltage is about 48V~52V.

At this time, if a lead-acid battery is used, four 12V can be connected in series to reach 48V. If the lithium battery can directly choose a battery pack with a voltage level of 48V. The three-phase grid-connected and off-grid integrated (Hybrid) energy storage machine can be used with a high-voltage battery, and the voltage is about 100V~550V. A more powerful all-in-one has a higher battery voltage.

Conclusion

From the above content, we can see that lead-acid batteries and lithium batteries have their own advantages and disadvantages. Generally speaking, lithium batteries are relatively high-end, but their prices are also expensive. Currently, they are more widely used in European, American and Australian markets.

New energy is the trend, and the huge market demand will become an inexhaustible driving force for the rapid development of technology. It is believed that the market will be lithium battery replacement lead acid and the price of lithium batteries will reach the range accepted by the public in the near future, and many fields including electric vehicles, photovoltaic energy storage systems, and portable power supplies will benefit from it.