A few cold facts about off-grid storage batteries
An off-grid photovoltaic (PV) system is comprised of various components, including the photovoltaic array, solar controller, inverter, off-grid storage batteries, and the load. The PV array captures solar energy and converts it into electricity, which charges the battery bank through the controller. The inverter then delivers the stored power to the load. With the addition of a battery between the PV array and the inverter, the current flow direction and equipment selection undergo significant changes.
What Is Off-Grid Energy Storage?
An off-grid PV energy storage system combines the PV power generation unit, energy storage system, and inverter into a single, integrated, and controllable unit. This system allows the PV modules to directly charge off-grid storage batteries, ensuring that the energy needs of the connected load are met. A common example of an off-grid energy storage system is the uninterruptible power supply (UPS), which is widely used in areas with frequent power outages, unreliable power grids, or no grid connection at all. These systems are also used where there are high demands for stable and guaranteed power supply.
Off-grid solar systems are ideal for remote locations such as communication towers, border posts, islands, or field operations, where there is increasing demand for these solutions.
Does Solar Power Need to Pass Through the Off-Grid Storage Batteries Before Reaching the Load?
Typically, power is stored in the battery and then discharged, which results in certain energy losses and may decrease battery lifespan. But is it possible for the inverter to supply power directly to the load without charging and discharging the battery? This can indeed be achieved, but not by the inverter itself—it is automatically handled by the system's circuitry.
From a circuit design perspective, current can only flow in one direction at a time. The battery is either in a charging or discharging state, but not both at once. When the solar power is greater than the load power, the battery charges, and the entire load power is provided by the photovoltaic system. Conversely, when the solar power is less than the load power, the battery discharges to supply the necessary power, with no direct PV input to the load.
How to Calculate the Charging Current for Off-Grid Energy Storage Batteries?
The maximum charging current for a battery is determined by several factors: the inverter's maximum charging current, the size of the PV module, and the maximum charging current allowed by the battery itself.
The charging current is calculated using the following formula:
Charging Current = (PV Module Power × MPPT Efficiency) / Battery Voltage
For example, if the PV module power is 5.4 kW, the MPPT efficiency of the controller is 0.96, and the battery voltage is 48V, the maximum charging current would be:
Charging Current = (5400 × 0.96) / 48 = 108A
If the inverter's maximum charging current is 100A, the charging current will be limited to 100A. For a lead-acid battery, the charging current is typically limited to 0.2C. This means for a 12V 200Ah battery, the maximum charging current would be:
Maximum Charging Current = 200 × 0.2 = 40A
Therefore, to meet the 100A requirement, three 200Ah batteries would need to be connected in parallel. Alternatively, one can choose lithium batteries with a 48V 100A rating.
How to Calculate the Discharge Current?
The maximum discharge current is also influenced by the inverter's discharge capabilities, the load size, and the battery's maximum discharge capacity.
The discharge current is calculated with the formula:
Discharge Current = Load Power / (Battery Voltage × Inverter Efficiency)
For example, if the load is 3 kW, the battery voltage is 48V, and the inverter efficiency is 0.96, the discharge current would be:
Discharge Current = 3000 / (48 × 0.96) = 60A
It's important to note that the discharge and charging capabilities of some battery types, such as lead-carbon batteries, can differ. Certain lead-carbon batteries can handle discharge currents as high as 1C.
How to Design the Cables for Off-Grid Energy Storage Batteries?
Off-grid inverters have overload capabilities. For instance, a 3kW off-grid inverter can support a 1kW motor startup, with the maximum instantaneous power surge reaching up to 6kW. Some might assume that this surge requires power from outside the inverter, but the millisecond energy is actually provided by the inverter itself, using capacitors and inductors.
Since both charging and discharging share the same cable, it's crucial to design cables that can handle the maximum current for either operation. For example, in a 5kW inverter system with a 4kW PV array, a 3kW load, and a 48V 600Ah battery, if the inverter does not support simultaneous PV and utility charging, the cable should be selected based on the 80A maximum current. The appropriate cable size would be 16 square millimeters.
However, if the system allows simultaneous charging from both PV and utility, the current could rise to 120A, so the cable should be sized to handle this increased current, requiring 25 square millimeters.
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