The design idea of ??the batteries of battery design approach is to ensure that the load can still work in the sunlight for less than the average case. We can imagine that the battery is fully charged, the light intensity is lower than the average case, the electricity generated by solar modules can not completely fill the vacancy created by the load energy consumption from the battery, so the end of the first day, The battery will be in a not fully charged state. If the second day of light intensity is still lower than the average, the battery will still have to discharge the need to supply the load and battery charge status continued to decline. The third day of the fourth day will be the same situation may occur. However, in order to avoid damage to the battery, the discharge process can only be allowed to persist for some time, until the battery state of charge reaches the specified risk. For quantitative assessment of this sunlight consecutive below average during battery design, we need to introduce an indispensable parameter: self-sufficiency in the number of days, that is, in the absence of any external energy load can still work the number of days . This parameter allows the system designer to select the size of the required battery capacity. In general, the self-sufficiency in the number of days with two factors: the load on the power supply requirements; weather conditions of the PV system installation site, consecutive rainy days. Can usually photovoltaic system installation location of the maximum continuous rainy days as a self-sufficiency in the number of days used in the system design, but also considering the load on the power supply requirements. The load on the power supply is not very demanding of photovoltaic applications, we usually taken from the design to the number of days for 3 to 5 days. Load requirements are very strict photovoltaic applications, we usually from 7 to 14 days to the number of days in the design. The so-called load requirements are not strict system usually refers to the user can slightly adjust the load needs to adapt to the inconvenience of inclement weather, and the strict system is more important electrical loads, such as commonly used in communications, navigation or important health facilities such as hospitals, clinics and so on. Moreover, you should consider the location of the installation of photovoltaic systems in remote areas must be designed larger battery capacity, the maintenance personnel to arrive at the scene to take a long time. Battery nickel-metal hydride and nickel cadmium batteries and lead-acid batteries used in PV systems, but in a larger system, taking into account the technical maturity and cost factors, commonly used lead-acid batteries. The following content related to the battery did not specify the means are lead-acid batteries. The battery design includes the design and calculation of battery capacity and a battery of series-parallel design. Firstly, the basic method to calculate the battery capacity. (1) The basic formula I. The first step daily load electricity consumption multiplied by the self-sufficiency in the number of days determined in accordance with the actual situation you can get the initial battery capacity. I. The second step, the first step is getting the battery capacity divided by the maximum allowable depth of discharge the battery. Because they can not let the battery completely discharged in the self-sufficiency in the number of days, so divided by the maximum depth of discharge, the battery capacity. The choice of the maximum depth of discharge performance parameters of the reference PV system battery, you can get detailed information about the maximum depth of discharge of the battery from the battery supplier. Under normal circumstances, if you use a deep cycle battery, recommended a 80% depth of discharge (DOD); If you are using shallow cycle batteries are recommended for 50% DOD. The basic design of the battery capacity formula below: self-sufficiency in the number of days X daily average load
Battery capacity = --------------------- (4.1)
Maximum depth of discharge
Here we introduce the method of battery series-parallel. Each battery has a nominal voltage. In order to achieve the nominal voltage of the load work, we will be battery in series to the number of battery power to the load, need to be connected in series is equal to the nominal load voltage divided by the nominal voltage of the battery.
Load nominal voltage
Series battery = -------------- (4.2)
Battery nominal voltage
To illustrate the application of the basic formula, we use a small AC photovoltaic applications as an example. Assuming that the PV system AC load power consumption the 10KWh / day, the photovoltaic system, we chose the inverter efficiency of 90%, the input voltage is 24V, then the availability of the required DC load demand / day 462.96Ah. (10,000 Wh ÷ 0.9 ÷ 24 V = 462.96 Ah). We assume that this is a load on the power requirements are not very strict system, users can be more flexible electricity adjusted according to weather conditions. We have chosen the five days the number of days of self-sufficiency, and use the deep cycle batteries, 80% depth of discharge. Then:
Battery capacity = 5 days the × 462.96Ah/0.8 2893.51Ah.
If to selected 2V/400Ah single cell, then you need to concatenate the number of battery:
Series battery number = 24V/2V = 12 (a)
Need the number of batteries connected in parallel:
Parallel battery = 2893.51/400 = 7.23
We take the integer 8. Therefore, the need to use 2V/400Ah battery number is: 12 series × 8 parallel = 96 (a).
The following are examples of a pure DC system: photovoltaic power supply system of the country house. The cabin is only used on weekends, you can use low-cost shallow-cycle batteries to reduce system costs. Load of the country house of 90 Ah / day, the system voltage of 24V. We choose to self-sufficiency days for two days, the battery's maximum allowable depth of discharge was 50%, then:
Battery capacity = 2 days the × 90Ah/0.5 = 360Ah.
If the selection 12V/100AH ??battery, then you need batteries in series 2 × 4 parallel = 8.
