About photovoltaic off-grid system battery need to know some little knowledge

Today Guangxi photovoltaic power generation small series to share the common pv off-grid system battery some cold knowledge, I hope to help you.

Whether the photovoltaic power generation must first into the battery and then into the load

Current into the battery, and then out, there is a certain loss, and will reduce the impact of battery life. So does the inverter have a function that allows the current to be used directly to the load without passing through the battery charge and discharge? In fact, this process can be achieved, but not by the inverter to achieve, but by the automatic circuit supply to achieve.

From the circuit principle, the current can only be in one direction at a time. At the same time, the battery is either charged or discharged, the battery can not charge and discharge at the same time. Therefore, when the solar power is greater than the load power, the battery is in the charging state, and all the electrical energy of the load is provided by photovoltaic. When the solar power is less than the load power, the battery is in the 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 the battery is determined by three aspects: first, the maximum charging current of the inverter itself; second, the photovoltaic module is too small; third, the maximum charging current allowed by the battery. Under normal circumstances, the battery charging current = * MPPT efficiency/battery voltage, photovoltaic modules power components such as power is 5.4 kW, the efficiency of the controller is 0.96, is 48 v battery voltage, then the maximum charging current = 0.96/48 = 5400 * 108 a, mains charging basically is calculated based on maximum charging current inverter, if the inverter is the maximum charging current 100 a, then the current limited to 100 a, then saw the biggest battery charging current, now ordinary lead-acid battery charging current is generally 0.2 C, That is to say, for a 12V200AH battery, the maximum charging current is 200*0.2=40A, so three parallel batteries are needed to meet the current of 100A. Now, lithium batteries have 48V100A version, which can also be selected.

Calculation of discharge current

The maximum discharge current of the battery is also determined by three aspects: first, the maximum discharge current of the inverter itself; second, the load is too small; third, the maximum discharge current allowed by the battery. Under normal circumstances, the battery discharge current determined by load, battery discharge current = * load power/battery voltage inverter efficiency, such as load power is 3 kw, 48 v battery voltage is, the inverter efficiency is 0.96, is the world's largest charging current = 3000 / (48 * 0.96) = 60 a, pay attention to the battery charge and discharge capacity may be different, some lead carbon batteries, discharge current can reach 1 c. 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, the current of the battery will be less, because it is possible for the photovoltaic and battery to supply the load at the same time.

How to design the cable of accumulator

Off-grid inverter has overload capacity, such as a 3 kw off-grid inverter, can support a 1 kw motor start, start the instantaneous power can reach 6 kw, some people think that the instantaneous power of the energy provided by inverter external, actually the energy of the millisecond, regardless of the photovoltaic (pv) when the battery can also provide, but can provide inverter, inverter internal - capacitance and inductance energy storage component, can provide instantaneous power. Storage battery charging and discharging are all use the same cable, so, when the design to calculate the charge and discharge current, which is the largest, is to choose which, such as a 5 kw inverter, match the component 4 kw, 3 kw load, 48 v600ah storage battery, inverter own maximum charging current is 120 a, photovoltaic maximum charging current is 80 a, maximum load, maximum discharge current is 65 a battery, if does not support the photovoltaic inverter and mains charging at the same time, the cable according to 80 a.