Embrace Solar

jack@chinasolarmanufacturer.com

# Knowledges of PV Off-grid System Battery Photovoltaic off-grid power generation system consists of a photovoltaic array, solar controller, inverter, battery pack, load, and so on. The photovoltaic array converts solar energy into electrical energy, charges the battery pack through the controller, and then supplies power to the load through the inverter. Since there is an extra battery between the photovoltaic and the inverter, there will be many changes in the current direction and equipment selection.

Does photovoltaic power generation have to enter the battery first and then enter the load?

When the current enters the battery and is released again, there will be a certain loss, and it will reduce the life of the battery. So does the inverter have a function to allow the current to be used directly by the load without going through the battery charge and discharge? In fact, this process can be realized, but it is not realized by the inverter, but automatically realized by the circuit supply.

From the circuit principle, at the same time, the current can only go in one direction. That is, at the same moment, the battery is either charged or discharged, and the battery cannot be charged and discharged at the same time. Therefore, when the solar power is greater than the load power, the storage battery is in a charging state, and all the electrical energy of the load is provided by the photovoltaic. When the solar power is less than the load power, the battery is in a discharged state, and all photovoltaic power generation is directly provided to the load without going through the battery.

Calculation of battery charging current

The maximum charging current of the battery is determined by three aspects: one is the maximum charging current of the inverter itself, the other is the size of the photovoltaic module, and the third is the maximum charging current allowed by the battery. Under normal circumstances, the charging current of the battery = photovoltaic module power * MPPT efficiency/battery voltage, if the module power is 5.4kW, the efficiency of the controller is 0.96, and the battery voltage is 48V, then the maximum charging current = 5400*0.96/48= 108A, mains charging is basically calculated according to the maximum charging current of the inverter. If the maximum charging current of the inverter is 100A, this current will be limited to 100A, and then it depends on the maximum charging current of the battery. Now ordinary lead The charging current of the acid battery is generally 0.2C, that is to say, for a 12V200AH battery, the maximum charging current is 200*0.2=40A, so three batteries need to be connected in parallel to meet the current of 100A. Now lithium batteries have a 48V100A version, and you can also choose .

Calculation of discharge current

The maximum discharge current of the battery is also determined by three aspects: one is the maximum discharge current of the inverter itself, the other is that the load is too small, and the third is the maximum discharge current allowed by the battery. Under normal circumstances, the discharge current of the battery is determined by the load. The discharge current of the battery = load power/battery voltage * inverter efficiency. For example, the load power is 3kW, the battery voltage is 48V, and the inverter efficiency is 0.96. When charging, the maximum charging current = 3000/(48*0.96) = 60A. It should be noted that the charging and discharging capacity of the battery may be different. For some lead-carbon batteries, the discharging current can reach 1C. When the photovoltaic energy storage system is running normally if there is sunlight, the current of the battery may not be calculated according to the above formula, and the current of the battery should be less because it is possible that the photovoltaic and the battery supply power to the load at the same time.