A Few Cold Facts About Off-Grid Storage Batteries

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Photovoltaic off-grid power generation system, consisting of photovoltaic array, solar controller, inverter, off grid storage batteries, load, etc. The PV array converts solar energy into electrical energy, charges the battery bank through the controller, and then supplies power to the load through the inverter. Since there is an extra battery between the PV and inverter, there will be many changes in the current direction and equipment selection.

 What is off grid energy storage?

Off-grid PV energy storage system consists of PV power generation, energy storage system and inverter, etc. organically combined together to form a single controllable unit, which can directly use PV modules to charge the off grid storage batteries to meet the demand of the electricity-using load.

A common off-grid energy storage system is the standby power system (UPS), which is widely used in areas with frequent power outages, unstable power grids and no power grids, or systems with high requirements for grid power supply assurance.

 Off-grid photovoltaic power generation system is mainly suitable for some special and away from the grid some electricity load, in some communication, border guard post, islands or field work occasions and people still have a lot of market demand space.

Does photovoltaic power have to go to the off grid storage batteries before it goes to the load?

The current enters the battery and then discharges it, there are certain losses and it will reduce the life of the battery. So does the inverter have a function that allows the current to be used directly to the load without going through the battery charge and discharge? In fact, this process can be achieved, just not by the inverter to achieve, but by the circuit supply automatically to achieve.

 From the circuit principle, the current can only go in one direction at the same moment. That is, at the same moment, the battery is either charging or discharging, the battery can not be charged and discharged at the same time. Therefore, when the solar power is greater than the load power, the battery is in the charging state, all the load power is provided by from the photovoltaic. When the solar power is less than the load power, the battery is in the discharge state, all the PV power is not provided directly to the load through the off grid storage batteries.

 How to calculate the off grid energy storage batteries 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 PV module is too small, and third, the maximum charging current allowed by the battery. Under normal circumstances, the battery charging current = PV module power * MPPT efficiency / battery voltage, such as module power is 5.4kW, the efficiency of the controller is 0.96, battery voltage is 48V, then the maximum charging current = 5400 * 0.96 / 48 = 108A, utility charging is basically calculated according to the maximum charging current of the inverter, if the maximum inverter charging current is 100A, it will limit this current to 100A, then it depends on the maximum charging current of the battery, now the ordinary lead-acid battery charging current is generally 0.2C, that is to say, a 12V200AH battery, the maximum charging current is 200 * 0.2 = 40A, so it is necessary to connect three in parallel to meet the 100A current, now the lithium battery, there are 48V100A version, you can also choose.

 How to calculate the 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 the load is too small, and the third is the maximum discharge current allowed by the battery. Under normal circumstances, the battery discharge current is determined by the load, the battery discharge current = load power / battery voltage * inverter efficiency, such as load power is 3kW, battery voltage is 48V, inverter efficiency is 0.96, then the maximum charge current = 3000 / (48 * 0.96) = 60A, pay attention to the battery charging and discharging capacity may be different, some lead carbon batteries, the discharge current can reach 1C. In the normal operation of the light storage system, if there is light, the off grid storage batteries’ current may not be calculated according to the above formula, the battery current to be less, because there may be photovoltaic and battery power supply to the load at the same time.

 How to design the cable of the off grid energy storage batteries？

Off-grid inverters have overload capability. For example, a 3kW off-grid inverter can support a 1kW motor to start, and the maximum start instantaneous power can reach 6kW. Some people think that the energy of this instantaneous power has to be provided by the outside of the inverter, but in fact, the millisecond energy cannot be provided by the battery no matter the PV or the time, but the inverter can provide it. Capacitor and inductor, can provide instantaneous power. Battery charging and discharging are used in the same cable, so in the design, to calculate the actual charging and discharging current, which is the largest, choose which, such as a 5kW inverter, with 4kW components, with a 3kW load, battery 48V600AH, the inverter’s own maximum charging current is 120A, the maximum PV charging current is 80A, the maximum load, the maximum battery discharge current is 65A, if the inverter does not support PV and utility charging at the same time, the cable is selected according to 80A, with 16 square, if PV and utility can be charged at the same time, the off grid storage batteries’ current can reach 120A, this time the cable should be 25 square.