Some cool facts about batteries for off-grid photovoltaic systems

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

Schematic diagram of off-grid power generation system

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

When the current enters the battery and then is released, there will be a certain loss, which will reduce the life of the battery. So does the inverter have a function to allow the current to be used directly by the load without charging or discharging the battery? In fact, this process can be achieved, but it is not achieved by the inverter, but by the circuit supply automatically.

From the circuit principle, at the same time, the current can only flow in one direction. That is, at the same time, the battery is either charged or discharged, 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 a charging state, and all the load power is provided by the photovoltaic. When the solar power is less than the load power, the battery is in a discharging 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, and third, the maximum charging current allowed by the battery. Under normal circumstances, the charging current of the 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 = 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. Then it depends on the maximum charging current of the battery. Now the charging current of ordinary lead-acid batteries is generally 0.2C, that is, a 12V200AH battery, the maximum charging current is 200 * 0.2 = 40A, so 3 batteries must be connected in parallel to meet the current of 100A. Now there is a 48V100A version of lithium batteries, which can also be selected.

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 that the load is too small, and the third is the maximum discharge current allowed by 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, 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 = 3000/(48*0.96)=60A. It should be noted that the charge and discharge capacity of the battery may be different. For some lead-carbon batteries, the discharge current can reach 1C. When the light storage system is operating normally, if there is light, the battery current may not be calculated according to the above formula. The battery current should be less because it is possible that photovoltaic and battery power 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 the start-up of a 1kW motor, and the maximum instantaneous starting power can reach 6kW. Some people think that the energy of this instantaneous power must be provided by the outside of the inverter. In fact, millisecond-level energy cannot be provided by photovoltaics or batteries, but inverters can provide it. There are energy storage elements inside the inverter - capacitors and inductors, which can provide instantaneous power. The same cable is used for both charging and discharging the battery, so when designing, you need to calculate the actual charging and discharging current, and choose the one with the largest current. For example, a 5kW inverter is equipped with a 4kW component and a 3kW load. The battery is 48V600AH. The maximum charging current of the inverter itself is 120A, and the maximum photovoltaic charging current 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 AC power, select the cable according to 80A and use 16 square. If photovoltaic and AC power can be charged at the same time, the current can reach 120A. At this time, the cable should be 25 square.

Summarize

When the photovoltaic output and 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 efficiency is the highest; when the photovoltaic power generation and load use are not in the same time period, for example, the photovoltaic power generation during the day and the load use at night, the photovoltaic power generation must first enter the battery and then enter the load, and the off-grid system efficiency is low. The battery cable should be designed according to the maximum current of the battery charge and discharge. The same inverter has different currents in different application scenarios and needs to be calculated differently.