The photovoltaic off grid power generation system consists of a photovoltaic array, solar controller, inverter, battery pack, load, etc. The photovoltaic array converts solar energy into electrical energy, charges the battery pack through a controller, and then supplies power to the load through an inverter. Due to the addition of an additional battery between the photovoltaic and inverter, there will be many changes in current direction and equipment selection.

Schematic diagram of off grid power generation system

Do photovoltaic power generation must first enter the battery before entering the load

The current entering the battery and then releasing it will cause certain losses and reduce the lifespan of the battery. So does the inverter have a function of allowing current to be directly used by the load without passing through the battery for charging and discharging? In fact, this process can be achieved, but it is not achieved by the inverter, but automatically by the circuit supply.

In terms of circuit principle, the current can only flow in one direction at the same time. At the same time, the battery either charges or discharges, and the battery cannot charge and discharge simultaneously. Therefore, when the solar power is greater than the load power, the battery is in a charged state, and all the electrical energy of the load is provided by the photovoltaic system. 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 passing through the battery.

Calculation of battery charging current

The maximum charging current of a battery is determined by three factors: the maximum charging current of the inverter itself, the small size of the photovoltaic module, and the maximum allowable charging current of 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 controller efficiency is 0.96, and the battery voltage is 48V, then the maximum charging current is 5400 * 0.96/48=108A. Municipal charging is basically calculated based on the maximum charging current of the inverter. If the maximum charging current of the inverter is 100A, this current will be limited to 100A, Looking at the maximum charging current of the battery, the charging current of ordinary lead-acid batteries is generally 0.2C, which means that 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. Currently, there are 48V100A versions of lithium batteries that can also be selected.

Calculation of discharge current

The maximum discharge current of a battery is also determined by three factors: firstly, the maximum discharge current of the inverter itself, secondly, the load is too small, and thirdly, the maximum allowable discharge current of the battery. Under normal circumstances, the discharge current of a battery is determined by the load. The discharge current of the battery is equal to the load power/battery voltage * inverter efficiency. If the load power is 3kW, the battery voltage is 48V, and the inverter efficiency is 0.96, then the maximum charging current at this time is 3000/(48 * 0.96)=60A. It should be noted that the charging and discharging capacitance of the battery may be different. Some lead carbon batteries can have a discharge current of up to 1C. During the normal operation of the photovoltaic energy storage system, if there is light, 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 both the photovoltaic and battery supply power to the load.

How to design the cables for the battery

Off grid inverters all have overload capacity. For example, a 3kW off grid inverter can support the start of a 1kW motor, with a maximum instantaneous power of 6kW. Some people believe that the energy of this instantaneous power needs to be provided externally by the inverter, but in fact, millisecond level energy cannot be provided by the photovoltaic or battery, but the inverter can provide it. There are energy storage components inside the inverter - capacitors and inductors, Both can provide instantaneous power. The charging and discharging of batteries all use the same cable, so when designing, it is necessary to calculate the actual charging and discharging current. Choose which one is the largest, such as a 5kW inverter with a 4kW component and a 3kW load. The battery is 48V600AH, and the maximum charging current of the inverter itself is 120A. The maximum charging current of the photovoltaic system is 80A. When the load is maximum, the maximum discharging current of the battery is 65A. If the inverter does not support simultaneous charging of photovoltaic and municipal power, The cable is selected based on 80A, using 16 square meters. If the photovoltaic and mains can be charged simultaneously, the current can reach 120A, and at this time, the cable needs to use 25 square meters.

summary

When the photovoltaic output and load power are similar or slightly higher, the photovoltaic current can be directly supplied to the load without passing through the battery, resulting in the highest efficiency of the off grid system; When photovoltaic power generation and load consumption are not in the same time period, such as photovoltaic power generation during the day and load consumption at night, photovoltaic power generation must first enter the battery before entering the load, and the efficiency of the off grid system is relatively low. The cables of the battery should be designed according to the maximum current of battery charging and discharging. The same inverter may have different currents in different applications and needs to be differentiated.