Detailed explanation of photovoltaic energy storage battery pack

In the photovoltaic energy storage system, the role of the battery is to store electrical energy. Due to the limited capacity of a single battery, the system usually combines multiple batteries in series and parallel to meet the designed voltage level and capacity requirements, so it is also called battery. Group. In a photovoltaic energy storage system, the initial cost of the battery pack and the photovoltaic module is the same, but the life of the battery pack is shorter. The technical parameters of the battery are very important to the system design. When selecting and designing, pay attention to the key parameters of the battery, such as battery capacity. , rated voltage, charge and discharge current, depth of discharge, number of cycles, etc.

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01 Battery capacity

The capacity of the battery is determined by the amount of active material in the battery, which is usually expressed in ampere-hour Ah or milliamp-hour mAh. For example, the nominal capacity of 250Ah (10hr, 1.80V/cell, 25°C) refers to the capacity released by discharging with a current of 25A for 10 hours at 25°C, reducing the voltage of a single battery to 1.80V.

The energy of the battery refers to the electrical energy that the battery can give under a certain discharge system, usually expressed in watt-hour (Wh). The energy of the battery is divided into theoretical energy and actual energy: for example, a 48V200Ah battery, the theoretical energy is 48*200=9600Wh, which is 9.6 kWh, which means the battery can store the amount of electricity. If the depth of discharge is 70%, the actual energy is 9600 *70%=6720 Wh, which is 6.72 kWh, which is the available power.

02 Rated voltage

The potential difference between the positive and negative electrodes of the battery is called the rated voltage of the battery. A common lithium iron phosphate battery has a rated voltage of 3.2V, and a 48V battery is made up of 15 cells connected in series.

The actual voltage of the battery is not a constant value. The voltage is high when there is no load, and the voltage will decrease when there is a load. When there is a sudden large current discharge, the voltage will also suddenly drop. There is an approximate linear relationship between the battery voltage and the remaining power. This simple association exists only in the case of no load. When a load is applied, the battery voltage is distorted by the voltage drop caused by the internal impedance of the battery.

03 Maximum charge and discharge current

The battery is bidirectional and has two states, charging and discharging. This current is limited. Different batteries have different maximum charge and discharge currents. The battery charging current is generally expressed as a multiple of the battery capacity C. For example, If the battery capacity is C=100Ah and the charging current is 0.15C, it is 0.15×100=15A.

04 Depth of discharge and cycle life

During the use of the battery, the percentage of the capacity released by the battery to its rated capacity is called the depth of discharge. The depth of discharge is closely related to battery life. The deeper the depth of discharge, the shorter the charging life.

A battery undergoes one charge and discharge, called a cycle (one cycle). Under certain discharge conditions, the number of cycles that the battery can withstand before the battery works to a specified capacity value is called cycle life.

The battery discharge depth is about 10% to 30% for shallow cycle discharge; the discharge depth is about 40% to 70% for medium cycle discharge; the discharge depth is about 80% to 90% for deep cycle discharge. The deeper the daily discharge depth of the long-term operation of the battery, the shorter the battery life, and the shallower the discharge depth, the longer the battery life.

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At present, the usual batteries in photovoltaic energy storage systems are electrochemical energy storage, which uses chemical elements as energy storage media, and the charging and discharging process is accompanied by chemical reactions or changes in the energy storage media. Mainly include lead-acid batteries, flow batteries, sodium-sulfur batteries, lithium-ion batteries, etc. The current applications are mainly lithium batteries and lead batteries.

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