Views: 0 Author: Site Editor Publish Time: 2026-04-30 Origin: Site
In various DC power supply systems such as photovoltaic power supply, emergency backup power supply, and industrial energy storage, the storage battery serves as a core energy storage component. The rationality of its capacity design directly determines the reliability, stability, and economy of the system power supply. A reasonable capacity design can not only meet the long-term stable power supply needs of the load, avoid power outages caused by insufficient capacity, but also prevent cost waste and resource idleness due to excessive capacity redundancy. Combining the working principle of storage batteries, influencing factors, and engineering practice, this paper detailedly analyzes the design logic, calculation method, and application points of battery capacity, providing professional reference for engineering designers.
Battery capacity refers to the total electric quantity that a fully charged storage battery can release under specified discharge conditions (including discharge current, ambient temperature, cut-off voltage, etc.), usually denoted by the symbol "C" and measured in ampere-hours (A·h). Its core calculation formula is: Capacity (A·h) = Discharge Current (A) × Continuous Discharge Time (h). This indicator is the core parameter for measuring the discharge capacity of the storage battery and the quality of the product, as well as the core basis for battery selection and capacity design.
It should be clarified that the actual capacity of a storage battery is not a fixed value, but a dynamic parameter affected by many factors. Different from the rated capacity marked by the manufacturer (the capacity value measured and marked under standard conditions), the working capacity in practical applications needs to be corrected and calculated in combination with specific usage scenarios, environmental conditions, and load characteristics, which is also the core difficulty in capacity design.
The performance of battery capacity is restricted by many factors such as product structure and service conditions. In the design process, the following core factors should be focused on to ensure that the calculation results are in line with the actual application scenarios.
As the core medium of the electrochemical reaction of the storage battery, the electrolyte density directly affects the reaction efficiency and capacity performance. When the electrolyte density increases appropriately, the battery electromotive force increases, the number of active substances participating in the electrochemical reaction increases, and the battery capacity increases accordingly; however, when the density is too high, the viscosity of the electrolyte will increase significantly, leading to a slowdown in ion diffusion speed, an increase in internal resistance, and an intensification of the plate sulfation trend, which will instead reduce the battery capacity and shorten the service life. Therefore, in the design process, the optimal electrolyte density range should be selected according to the battery type (such as lead-acid batteries, lithium-ion batteries) to balance capacity and service life.
Temperature is an important environmental factor affecting battery capacity, and its mechanism of action is mainly reflected in the fluidity of the electrolyte and the utilization rate of active substances. When the ambient temperature decreases, the viscosity of the electrolyte increases, the difficulty of penetrating into the electrode plate increases, the active substances cannot fully participate in the reaction, and the internal resistance of the battery increases, leading to a significant decrease in capacity; on the contrary, an increase in temperature (within a reasonable range) can improve the fluidity and reaction activity of the electrolyte, which is conducive to capacity performance, but excessive temperature will accelerate electrode plate aging and electrolyte decomposition, and also damage the battery life. In the design of cold areas, additional temperature correction factors should be considered to avoid insufficient capacity caused by low temperature.
In addition to the above factors, the product structure of the storage battery (such as electrode plate area, thickness, active material porosity), discharge current, cycle times, and self-discharge rate also affect the capacity performance. For example, the larger the discharge current, the easier the pores of the active material on the electrode plate surface are blocked by lead sulfate, resulting in the internal active material being unable to participate in the reaction, and the capacity will be significantly reduced; the difference in the active material structure between deep-cycle and shallow-cycle batteries also determines their different capacity characteristics and applicable scenarios.
The core logic of battery capacity design is: based on load demand and usage scenarios, calculate the minimum capacity that meets the requirements of power supply reliability, while taking into account economy and service life. Its basic steps can be divided into three stages: parameter determination, capacity calculation, and correction adjustment, which are detailed as follows.
Before design, three core parameters should be clarified to provide a basis for capacity calculation:
Daily Average Load (A·h/day): Calculate the average daily power consumption according to the working current and working time of all loads in the system, that is, the sum of the daily power consumption of all loads, with the unit of A·h/day.
Self-sufficiency Days (days): Refers to the maximum number of days that the storage battery needs to continuously supply power to the load when there is no external charging (such as no light in the photovoltaic system, power grid interruption). It should be determined according to the actual needs of the user (such as emergency power supply duration, frequency of rainy days), usually ranging from 1 to 7 days.
Depth of Discharge (DOD): Refers to the percentage of the maximum electric quantity that the storage battery is allowed to release accounting for its rated capacity. Its core function is to protect the storage battery and avoid electrode plate damage and service life shortening caused by over-discharge. The selection of discharge depth should be combined with the battery type: for deep-cycle batteries (suitable for long-term cycle scenarios such as photovoltaics and energy storage), the recommended DOD is 80%; for shallow-cycle batteries (suitable for emergency backup and short-term discharge scenarios), the recommended DOD is 50%, and specific reference should be made to the battery product manual.
Based on the above parameters, the basic calculation formula for battery capacity is:
C=T*P/DOD Where: C is the required battery capacity (A·h); T is the self-sufficiency days (days); P is the daily average load (A·h/day); DOD is the maximum depth of discharge (in decimal form, such as 80% recorded as 0.8).
The core logic of this formula is: first calculate the total power required by the load within the self-sufficiency days, then correct it through the discharge depth to obtain the minimum rated capacity required by the battery, ensuring that the long-term power supply demand is met without damaging the battery.
After the capacity calculation is completed, it is necessary to adjust the series-parallel combination in combination with the nominal voltage of the load and the nominal voltage of the battery to ensure that the voltage and current of the battery pack meet the system requirements:
Series Combination: When the nominal voltage of the battery is lower than the nominal voltage of the load, the series method is adopted to increase the total voltage. The calculation formula for the number of series batteries is: Number of Series Batteries = Nominal Voltage of Load ÷ Nominal Voltage of Battery. After series connection, the total voltage of the battery pack is the sum of the voltages of the single batteries, and the total capacity is consistent with the capacity of the single battery.
Parallel Combination: When the current (capacity) of the battery pack is lower than the system demand, the parallel method is adopted to increase the total capacity. The calculation formula for the number of parallel battery groups is: Number of Parallel Groups = Required Total Capacity ÷ Capacity of Single Battery. After parallel connection, the total capacity of the battery pack is the sum of the capacities of the single groups, and the total voltage is consistent with the voltage of the single battery.
To further clarify the capacity design process and calculation method, a detailed explanation is given in combination with a photovoltaic power supply system example, which is in line with the actual engineering application scenario.
Example: A photovoltaic power supply system has a load nominal voltage of 24V, a daily average load of 20A·h/day, and the self-sufficiency days are determined to be 4 days in combination with the local rainy day conditions; considering cost control, a shallow-cycle battery is selected, with a maximum depth of discharge (DOD) of 50%, and the single battery has a nominal voltage of 12V and a nominal capacity of 100A·h. Try to design the battery capacity and series-parallel combination scheme of the system.
Capacity Calculation: According to the core formula, the required battery capacity is C = 4*20/0.5= 160 (A·h).
Series Combination: The nominal voltage of the load is 24V, and the voltage of a single battery is 12V. The number of series batteries = 24V ÷ 12V = 2 (pieces). After series connection, the capacity of each group is 100A·h, and the voltage is 24V.
Parallel Combination: The required total capacity is 160A·h, and the capacity of each group is 100A·h. The number of parallel groups = 160A·h ÷ 100A·h ≈ 2 (groups). After parallel connection, the total capacity is 200A·h (redundant design to ensure power supply reliability), and the total voltage is still 24V.
Final Scheme: Select 12V/100A·h shallow-cycle batteries, adopt 2 pieces in series as one group, and a total of 2 groups in parallel, with a total number of 4 pieces. It can meet the system's needs of 4-day self-sufficiency and 24V power supply, while taking into account cost and service life.
Parameter values should be in line with reality: The self-sufficiency days should be reasonably determined according to local climate and power supply reliability requirements, avoiding power outages caused by too small values or cost waste caused by too large values; the discharge depth should strictly follow the battery product specifications, and over-discharge is strictly prohibited.
Consider ambient temperature correction: In high or low temperature environments, the calculated capacity needs to be corrected. For example, in cold areas, 10%-20% capacity redundancy can be appropriately added to offset the capacity attenuation caused by low temperature.
Match battery type: There are significant differences in capacity characteristics between deep-cycle and shallow-cycle batteries. The battery should be reasonably selected according to the system usage scenario. For example, deep-cycle batteries are preferred for photovoltaic energy storage systems, and shallow-cycle batteries can be used for emergency backup power supplies.
Reserve capacity redundancy: In engineering design, 10%-15% capacity redundancy is usually reserved to cope with capacity decline caused by factors such as load fluctuations and battery aging, and to improve the reliability of system power supply.
Battery capacity design is a core link in the design of DC power supply systems, and its essence is a process of balancing power supply reliability, economy, and battery service life. In the design process, it is necessary to first clarify the load demand and usage scenario, grasp the influence law of factors such as electrolyte density and temperature on capacity, and then realize reasonable capacity design through scientific calculation methods and series-parallel combinations. The design steps, calculation methods, and engineering examples proposed in this paper can provide practical reference for battery capacity design in scenarios such as photovoltaic systems and emergency power supplies, helping to improve system operation stability and reduce design and operation and maintenance costs. In the future, with the continuous development of battery technology, capacity design needs to be optimized in combination with the characteristics of new batteries (such as high energy density and long cycle life of lithium-ion batteries) to further adapt to the needs of various energy storage scenarios.