 August 23, 2023

# A Few Things to Know About Energy Storage Batteries for PV Off-Grid Systems

Photovoltaic off-grid power generation systems, consisting of solar modules, solar controllers, inverters, storage batteries, loads, etc. The solar module converts solar energy into electrical energy, charges the storage battery through the controller and then supplies the load through the inverter. As there is an additional storage battery between the PV and the inverter, many changes are created in terms of current direction and equipment shape. ## 01 Does photovoltaic power generation have to enter the energy storage battery first and then enter the load?

The current enters the energy storage battery, and then discharges it, there is a certain loss, and it will reduce the life of the energy storage battery. So does the inverter have a function that allows the current to be used directly by the load without being charged and discharged by the energy storage battery? In fact, this process can be realized, but it is not realized by the inverter, but automatically realized by the circuit supply.

From the circuit principle, the current can only go in one direction at the same time. That is, at the same time, the energy storage battery is either charged or discharged, and the energy storage battery cannot be charged and discharged at the same time. Therefore, when the solar power is greater than the load power, the energy storage battery is in a charged state, and all the electrical energy of the load is provided by photovoltaics. When the solar power is less than the load power, the energy storage battery is in a state of discharge, and all photovoltaic power generation is directly supplied to the load without passing through the energy storage battery.

## 02 Calculation of charging current of energy storage battery

The maximum charging current of the energy storage battery is determined by three aspects: one is the maximum charging current of the inverter itself, the second is the size of the photovoltaic module, and the third is the maximum charging current allowed by the energy storage battery. Under normal circumstances, the charging current of the energy storage battery = photovoltaic module power * MPPT efficiency / energy storage battery voltage, if the module power is 5.4KW, the efficiency of the controller is 0.96, and the energy storage 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, the current will be limited to 100A, and then look at the energy storage battery. The maximum charging current of the 48V100AH power wall battery produced by Improve, the standard charging current is 0.2C, that is 100*0.2=20A, and the fast charging current is 0.5C ( 50A ), which is no problem.

## 03 Calculation of discharge current

The maximum discharge current of the energy storage battery is also determined by three aspects: one is the maximum discharge current of the inverter itself, the second is the size of the load, and the third is the maximum discharge current allowed by the energy storage battery. Under normal circumstances, the discharge current of the energy storage battery is determined by the load, the discharge current of the energy storage battery = load power / energy storage battery voltage * inverter efficiency, such as the load power is 3KW, the energy storage battery voltage is 48V, the inverter If the efficiency of the device is 0.96, the maximum discharge current at this time = 3000/(48*0.96) = 60A. It should be noted that the charge and discharge capacity of the energy storage battery may be different. For some lead-carbon batteries, the discharge current can reach 1C. When the solar energy storage system is running normally, if there is light, the current of the energy storage battery may not be calculated according to the above formula, and the current of the energy storage battery is less, because it is possible that the photovoltaic and the energy storage battery can supply power to the load at the same time.

## 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 energy storage battery, and the off-grid efficiency is the highest; when the photovoltaic power generation and the load are not used in the same time period, such as photovoltaic power generation during the day, the load is used at night. At this time, photovoltaic power generation must first enter the energy storage battery and then enter the load, and the off-grid efficiency is low. The cable of the energy storage battery should be designed according to the maximum charging and discharging current of the battery. The same inverter has different currents in different applications and needs to be calculated differently.

IMPROVE BATTERY established in 2014, is a modern enterprise engaged in research &development, production and export of lithium batteries, China Lithium Battery Supplier. Its products cover lead to lithium energy storage batteries, wall mounted home energy storage batteries, cabinet type energy storage batteries, low-speed power batteries, etc., series