Photovoltaic off-grid power generation system, consisting of PV array, solar controller, inverter, battery bank, load, etc. The PV array converts solar energy into electrical energy, charges the battery bank through the controller, and then supplies power to the load through the inverter. Since there is an extra battery between the PV and inverter, there will be many changes in the current direction and equipment selection.
Schematic diagram of the off-grid power generation system
Does PV power generation have to go into the battery before going to the load?
If the current enters the battery and then discharges it, there is a certain loss and it will reduce the life of the battery. So does the inverter have a function that allows the current to go directly to the load without charging and discharging the battery? In fact, this process can be achieved, just not by the inverter, but by the circuit supply automatically to achieve.
From the circuit principle, the current can only go in one direction at the same moment. That is, at the same moment, the battery is either charging or discharging, and the battery cannot be charged and discharged at the same time. Therefore, when the solar power is greater than the load power, the battery is in the charging state, the load all the power provided by from the photovoltaic. When the solar power is less than the load power, the battery is in the discharging state, all the photovoltaic power is provided directly to the load without the battery.
Calculation of battery charging current
The maximum charging current of the battery is determined by three aspects: first, the maximum charging current of the inverter itself, second, the PV module is too small, and third, the maximum charging current allowed by the battery. Under normal circumstances, the battery charging current = PV module power * MPPT efficiency / battery voltage, such as module power is 5.4kW, the efficiency of the controller is 0.96, battery voltage is 48V, then the maximum charging current = 5400 * 0.96 / 48 = 108A, utility charging is basically calculated according to the maximum charging current of the inverter, if the maximum inverter charging current is 100A, it will limit this current to 100A, then it depends on the maximum charging current of the battery, now the ordinary lead-acid battery charging current is generally 0.2C, that is to say, a 12V200AH battery, the maximum charging current is 200 * 0.2 = 40A, so it is necessary to connect three in parallel to meet the 100A current, now the lithium battery, there are 48V100A version, 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 second is the load is too small, and the third is the maximum discharge current allowed by the battery. Under normal circumstances, the battery discharge current is determined by the load, the battery discharge current = load power / battery voltage * inverter efficiency, such as load power is 3kW, battery voltage is 48V, inverter efficiency is 0.96, then the maximum charge current = 3000 / (48 * 0.96) = 60A, pay attention to the battery charging and discharging capacity may not be the same, some In the normal operation of the light storage system, if there is light, the battery current may not be calculated according to the above formula, the battery current to be less, because there may be photovoltaic and battery power to the load at the same time.
How to design the battery cables
Off-grid inverters have overload capacity, such as a 3kW off-grid inverter, can support a 1kW motor start, the maximum start instantaneous power can reach 6kW, some people think that this instantaneous power energy to be provided by the inverter external, in fact, millisecond energy, whether photovoltaic or time battery can not provide, but the inverter can provide, the inverter internal energy storage components -Capacitor and inductor, can provide instantaneous power. Battery charging and discharging are used in the same cable, so in the design, to calculate the actual charging and discharging current, which is the largest, choose which, such as a 5kW inverter, with 4kW components, with a 3kW load, battery 48V600AH, the inverter's own maximum charging current is 120A, the maximum PV charging current is 80A, the maximum load, the maximum battery discharge current is 65A, if the inverter does not support PV and utility charging at the same time, the cable is selected according to 80A, with 16 square, if PV and utility can be charged at the same time, the current can reach 120A, this time the cable should be 25 square.
Summarize
When the PV output and load power is about the same or slightly larger, PV current can not go through the battery, directly supply the load, off-grid system efficiency is the highest; when the PV power generation and load time is not in the same time period, such as PV power generation during the day, load power at night, then the PV power must first enter the battery and then enter the load, off-grid system efficiency is lower. The battery cable should be designed according to the maximum battery charge and discharge current, the same inverter in different applications, the current is not the same, need to be calculated differently.
