With the development of battery technology and the rapid decline in cost, lithium batteries have become the mainstream choice in household energy storage systems. Compared with lead-acid batteries, lithium batteries have the advantages of high efficiency, long cycle life, accurate battery data, and high consistency. The capacity of the lithium battery is directly related to whether the power supply is sufficient, and a variety of situations and factors need to be considered in the design process. The following is a specific method for designing battery capacity in a home energy storage system
Four common misunderstandings in lithium battery capacity design
1. Choose battery capacity only according to load power and power consumption
In the design of lithium battery capacity, the load condition is the most important reference factor. However, the charging and discharging capacity of the battery, the maximum power of the energy storage machine, and the period of time for the load cannot be ignored.
2. Theoretical and actual capacity of lithium batteries
Usually, the battery manual is marked on the theoretical capacity of the battery, which is the maximum amount of power that the battery can release when the battery goes from SOC100% to SOC0% in an ideal state.
In actual applications, considering the battery life, it is not allowed to discharge to SOC0%, and the protection power will be set.
3. The larger the choice of lithium battery capacity, the better
In practical applications, the battery usage rate should be considered. If the capacity of the photovoltaic system is small, or the power consumption of the load is large, the lithium battery cannot be fully charged, which will cause waste.
4. Lithium battery capacity design perfectly fits
Due to process loss, the discharge capacity of the lithium battery is less than the battery capacity, and the load power consumption is less than the discharge capacity of the lithium battery. Ignoring the efficiency loss is likely to cause insufficient battery power.
Lithium battery capacity design in different application scenarios
Mainly introduce the design ideas of battery capacity in three common application scenarios: self-consumption (high electricity costs or no subsidies), peak and valley electricity prices, and backup power supply (the grid is unstable or has important loads).
1. Spontaneous use
Due to higher electricity prices or lower subsidies for photovoltaic grid connection (no subsidies), photovoltaic energy storage systems are installed to reduce electricity bills.
Assuming that the power grid is stable, no off-grid operation is considered. Photovoltaics is only to reduce the power consumption of the grid, and generally, there is sufficient sunlight during the day.
Ideally, the photovoltaic + energy storage system can completely cover household electricity. But this situation is difficult to achieve. Therefore, we comprehensively consider the input cost and power consumption, and we can choose to choose the battery capacity according to the average daily power consumption (kWh) of the household (the default photovoltaic system has sufficient energy).
If you can accurately collect electricity usage rules, combined with energy storage management settings, you can maximize the utilization rate of the home energy storage system.
2. Peak and valley electricity prices
During the day, the electricity consumption is small (the photovoltaic system can basically cover it). During the peak period of electricity consumption, it is necessary to ensure that at least half of the electricity is powered by the battery to reduce the electricity bill.
Calculate the maximum demand value of battery capacity based on the total power consumption during the peak period. Then find an optimal battery power in this interval according to the capacity of the photovoltaic system and the benefit of investment.
3. Areas with unstable power grids-standby power
It is mainly used in unstable power grids or in situations with important loads.
Application requirements: Under normal power grid conditions, lithium batteries are charged first; when the grid is out of power, lithium batteries + photovoltaics ensure normal operation of important loads
When choosing the battery capacity, what needs to be considered is that the lithium battery alone supplies the required power when off-grid (assuming a power outage at night and no PV).
Among them, the total power consumption when off-grid and the estimated time off-grid are the most critical parameters. If there are other important loads in the system, you need to list them all (as in the example below) and finally determine the required battery capacity based on the maximum electrical load power and power consumption during the longest continuous power outage period throughout the day.
Two important factors in the design of lithium battery capacity
1. Photovoltaic system capacity
Assuming that the batteries are all charged by photovoltaics, the maximum power that the energy storage machine can charge the lithium battery is 5000W, and the number of hours of sunshine per day is 4h.
(1)In the mode of using a lithium battery as a backup power source, the average need for a battery with an effective capacity of 800Ah to be fully charged in an ideal state: 800Ah/100A/4h=2 days.
(2)In the mode of self-generation and self-use, it is assumed that the system charges the battery at an average of 3000W within 4 hours a day. A battery with an effective capacity of 800Ah is fully charged (without discharging): 800Ah*50V/3000=13 days.
2. Redundant design of lithium battery
Because photovoltaic power generation has instability, line loss, invalid discharge, battery aging, etc., which cause efficiency loss, a certain margin needs to be reserved when designing the capacity of the lithium battery.
The design of the remaining lithium battery is relatively free, and the designer can make a comprehensive judgment based on the actual situation of his own system design.
When designing the capacity of the lithium battery, we must combine the ideal state and the actual situation to design. If you want to know more about the home energy storage system after reading the above content, please contact us for detailed information and solutions.
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