Photovoltaic off-grid system battery several cold knowledge
Time:2024-03-28
Views:140
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.
Does photovoltaic power generation need to first enter the battery before entering the load
The current entering the battery and then discharging it will cause certain losses and reduce the lifespan of the battery. Does the inverter have a function that allows 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 by the automatic supply of the circuit.
From a circuit principle, at the same time, the current can only be in one direction. 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 charging 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 a battery=photovoltaic module power * MPPT efficiency/battery voltage. For example, 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 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 a regular lead-acid battery is generally 0.2C, which means that for a 12V200AH battery, the maximum charging current is 200 * 0.2=40A. Therefore, three batteries need to be connected in parallel to meet the current of 100A. Now, lithium batteries The battery is available in 48V100A versions and can also be selected.
Calculation of discharge current
The maximum discharge current of a battery is also determined by three factors: first, the maximum discharge current of the inverter itself; second, the load is too small; and third, 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. For example, if the load power is 3kW, the battery voltage is 48V, and the inverter efficiency is 0.96, 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 vary. Some lead carbon batteries can have a discharge current of up to 1C. When the photovoltaic energy storage system is operating normally, 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 lower because it is possible that both the photovoltaic and the battery supply power to the load simultaneously.
How to design the cables for batteries
Off grid inverters all have overload capacity. For example, a 3kW off grid inverter can support the starting 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. 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, which 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, and choose the one with the maximum. For example, a 5kW inverter with a 4kW component and a 3kW load, with a 48V600AH battery, the maximum charging current of the inverter itself is 120A, and 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 mains power, the cable is selected as 80A, using 16 square meters. If photovoltaic and mains power can be charged simultaneously, the current can reach 120A. At this time, the cable needs to use 25 square meters.
summary
When the photovoltaic output is similar to or slightly higher than the load power, 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 usage are not in the same time period, for example, photovoltaic power generation during the day and load usage 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 charging and discharging current of the battery. The same inverter may have different currents in different application scenarios and needs to be calculated differently.
Patrick Qi
Patrick Qi
Gitenno