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Photovoltaic off-grid power generation system consists of 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 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 from the photovoltaic. When the solar power is less than the load power, the battery is in a discharge 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 that the photovoltaic module is too small, 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.
How to design the battery cable
Off-grid inverters have overload capability. For example, a 3kW off-grid inverter can support a 1kW motor to start, and the maximum instantaneous power can reach 6kW. Some people think that the energy of this instantaneous power should be provided by the inverter. Provide, in fact, millisecond-level energy, regardless of whether the photovoltaic power is restored or not, the battery cannot provide it, but the inverter can provide it. There are energy storage elements inside the inverter-capacitors and inductors, which can provide instantaneous power. The same cable is used for battery charging and discharging, so when designing, it is necessary to calculate the actual charging and discharging current, whichever is the largest, choose whichever one, such as a 5kW inverter, equipped with 4kW components, with a 3kW load, and the battery 48V600AH, the maximum charging current of the inverter itself is 120A, the maximum charging current of the photovoltaic is 80A, and the maximum discharge current of the battery is 65A when the load is maximum. 16 square meters, if the photovoltaic and mains can be charged at the same time, the current can reach 120A, and the cable will use 25 square meters at this time.
When the photovoltaic output is similar to or slightly larger than the load power, the photovoltaic current can be directly supplied to the load without passing through the battery, and the off-grid system has the highest efficiency; when the photovoltaic power generation and the load are not in the same time period, for example, photovoltaic power generation during the day and load power consumption at night , At this time, photovoltaic power generation must first enter the battery and then enter the load, and the efficiency of the off-grid system is low. The cable of the battery should be designed according to the maximum charging and discharging current of the battery. The current of the same inverter is different in different applications, so different calculations are required.
