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In the process of designing independent ​off grid photovoltaic system, the essential part is the design of the battery, including the capacity design of the battery and the series and parallel design of the battery group.

Battery design method for independent off grid photovoltaic system

  • Xindun
  • November 20, 2020

In the process of designing independent off grid photovoltaic system, the essential part is the design of the battery, including the capacity design of the battery and the series and parallel design of the battery group. The design process of the battery in the independent system is introduced in detail, and the example is used to calculate the storage of the independent photovoltaic system. I hope you can learn the battery design method of independent photovoltaic system through this article.


Battery design method for independent off grid photovoltaic system


First, we need to introduce an indispensable parameter: the number of self given days, that is, the number of days that the system can work normally without any external energy. This parameter allows the system designer to choose the capacity of the battery to use.

Generally speaking, the determination of the number of self - given days is related to two factors: the requirement of the load to the power supply, the weather conditions for the installation of the photovoltaic system, that is, the maximum number of days of continuous rain and rain. The maximum number of continuous and rainy days in the installation of the photovoltaic system is usually used as the number of days used in the system design, but the requirements of the load on the power supply are also taken into consideration. For photovoltaic applications whose load is not very strict with the power supply, we usually take the design from 3~5 days. For photovoltaic applications with very strict load requirements, we usually take 7~14 days in design. The so-called system of not strict load usually means that the user can adjust the load demand to adapt to the bad weather inconvenience, and the strict system refers to the use of electrical load is more important, for example, used in communications, navigation or important health facilities such as hospitals, clinics, and so on. In addition to the installation of the photovoltaic system, if in very remote areas, it is necessary to design larger battery capacity, because the maintenance personnel to arrive at the site takes a long time.

The batteries used in the photovoltaic system are nickel hydrogen, nickel cadmium batteries and lead-acid batteries. In the larger system, lead-acid batteries are usually used in consideration of technical maturity and cost. The batteries involved in the following contents do not specify that they are all lead-acid batteries.


The design of the battery includes the design of the battery capacity and the series and parallel design of the battery pack. That will be introduced in detail.

1. Basic formula

1. The basic method of calculating the capacity of the battery.

The first step is to get the initial battery capacity by multiplying the daily load need and the self determined days determined according to the actual situation.

The second step is to divide the battery capacity in the first step by dividing the maximum allowable discharge depth of the battery. Because the battery can not discharge completely in the number of days, it needs to be divided into the maximum discharge depth and the required battery capacity. The selection of the maximum discharge depth needs to refer to the performance parameters of the battery selected in the photovoltaic system, and the detailed data on the maximum discharge depth of the battery can be obtained from the battery supplier.

Normally, if you are using a deep loop battery, it is recommended to use 80% discharge depth (DOD); if the battery is used for shallow circulation, 50% (DOD) is recommended.


2. The basic method for determining the series and parallel battery of the battery.

Each battery has its nominal voltage. In order to achieve the nominal voltage of the load operation, we connect the battery in series to supply the load. The number of batteries in need of series is equal to the nominal voltage of the load divided by the nominal voltage of the battery.

In order to illustrate the application of the above basic formula, we use a small AC photovoltaic application system as an example. It is assumed that the power consumption of the AC load of the PV system is 10KWh/ days. If the PV system is used in the PV system, the efficiency of the inverter is 90% and the input voltage is 24V, then the required DC load demand is 462.9. 6Ah/ days. (10000 Wh 24 V = 462.96 Ah). We assume that this is a system with a load that is not very strict for the power supply. The user can be more flexible in adjusting the electricity according to the weather conditions. We choose a 5 day self supply number, and use a deep cycle battery, and the discharge depth is 80%. So:

Storage battery capacity =5 days * 462.96Ah/0.8=2893.51Ah.

If 2V/400Ah single battery is used, then the number of batteries to be connected is =24V/2V=12 (a).

The number of batteries that need to be parallel is =2893.51/400=7.23

We take integer 8., so the system needs to use 2V/400Ah\'s battery number: 12 series x 8 parallel = 96 (a).

The following is an example of a pure DC system: a rural hut photovoltaic power supply system. The cabin is only used on the weekend. It can use low cost shallow recirculating battery to reduce the system cost. The load of the village hut is 90 Ah/ day, the system voltage is 24V., the number of self supplied days is 2 days, and the maximum discharge depth allowed by the battery is 50%. So, then:

Storage battery capacity =2 days * 90Ah/0.5=360Ah.

If the battery of 12V/100Ah is chosen, then the battery 2 should be connected in series (4 parallel) = 8.

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