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Knowledge of photovoltaic off-grid system batteries

9-24, 2021 / By MUST Power

The photovoltaic off-grid power generation system is composed of photovoltaic arrays, solar controllers, inverters, battery packs, loads, etc. The photovoltaic array converts solar energy into electric energy, charges the battery pack through the controller, and then supplies power to the load through the inverter. Because there is an extra battery between the photovoltaic and the inverter, there will be many changes in the current trend and equipment selection.


Diagram of off-grid power generation system

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

The current enters the battery and is discharged again, which will consume a certain amount and reduce the life of the battery. Does the user allow the current to pass through the battery and directly load it? In fact, this process is achievable, but it is not achieved by artifacts, but by light and automatic.

From the circuit principle, the same time, one charging time. That is, at the same time, the battery cannot be charged, charged, and charged at the same time. , When the solar power generation is insufficient, a discharge state will appear, and all photovoltaic power generation cannot be directly provided to the load 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 photovoltaic module is too small, and the third is the maximum allowable charging current of the battery. Under normal circumstances, the charging current of the battery = photovoltaic module power * MPPT efficiency / battery voltage, such as 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, the 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 an acid battery is generally 0.2C, that is to say, a 12V200AH battery, the maximum charging current is 200*0.2=40A, so it needs to be connected in parallel to meet the 100A current. Now the lithium battery has 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 the load is too small, and the third is the maximum allowable discharge current of 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 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, and the discharge current of some lead-carbon batteries can reach 1C. In the normal operation of the optical 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 the photovoltaic and the battery can supply power to the load at the same time.

How to design the battery cable

Off-grid inverters have overload capacity. For example, a 3kW off-grid inverter can support a 1kW motor to start, and the maximum instantaneous starting power can reach 6kW. Some people think that the energy of this instantaneous power must be external to the inverter. Provide, in fact, millisecond-level energy can not be provided by photovoltaic or battery, but the inverter can provide it. There are energy storage elements inside the inverter—capacitors and inductors, which can provide instantaneous power. The charging and discharging of the battery all use the same cable, so when designing, calculate the actual charging and discharging current, whichever is the largest, choose which one, such as a 5kW inverter, equipped with 4kW components, with a 3kW load, and battery 48V600AH, the maximum charging current of the inverter itself is 120A, and the maximum charging current of photovoltaic is 80A. When the load is maximum, the maximum discharge current of the battery is 65A. If the inverter does not support simultaneous charging of photovoltaic and mains, the cable is selected as 80A. 16 square meters, if photovoltaic and city electricity can be charged at the same time, the current can reach 120A, at this time the cable needs 25 square meters.

Summarize

When the photovoltaic output and the load power are similar or slightly larger, 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 photovoltaic power generation and load use are not in the same time period, such as photovoltaic power generation during the day, and the load uses electricity 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 charge and discharge current of the battery. The same inverter has different currents in different applications, so different calculations are required.