How is the battery and load matched?

The capacity of the battery is very important to ensure continuous power supply. Within a year, the power generation of solar cell modules varies greatly from month to month. In the month when the power generation of the solar cell module cannot meet the electricity demand, it must be supplemented by the power of the battery; In months when electricity demand exceeds, the excess electric energy is stored by batteries. Therefore, the shortage and excess value of power generation of off-grid solar photovoltaic power generation system is one of the bases for determining battery capacity. Similarly, during continuous rainy days, the load power must also be obtained from the battery. Therefore, the power consumption during this period is also one of the factors to determine the battery capacity. The capacity of the battery is determined by the following factors.

① The number of working days of the battery alone. Under special climatic conditions, the battery is allowed to discharge up to 20% of the normal rated capacity.

② The daily discharge of the battery. For occasions with stable daily load and low requirements, the daily discharge cycle depth can be limited to 80% of the rated capacity of the battery.

③ The battery must have sufficient capacity to ensure that the battery will not lose water due to overcharging. Generally, when selecting the battery capacity, as long as the battery capacity is greater than 25 times the peak current of the solar cell module, the battery will not cause water loss during charging.

④ Battery self-discharge rate. With the increase of battery service time and battery temperature, the self-discharge rate will increase. For new batteries, the self-discharge rate is usually less than 5% of the rated capacity, but for old batteries with poor quality, the self-discharge rate can be increased to 10% ~ 15% per month.

The selection and design of batteries for off-grid solar photovoltaic power generation systems includes the design calculation of battery capacity and the series-parallel design of battery packs. The basic steps for calculating battery capacity are as follows.

① The preliminary battery capacity can be obtained by multiplying the electricity consumption required by the daily load by the number of self-sufficiency days determined according to the actual situation.

② Divide the calculated battery capacity by the allowable maximum discharge depth of the battery (because the battery cannot be completely discharged in the number of self-sufficiency days, it needs to be divided by the maximum discharge depth) to obtain the required battery capacity. The selection of the maximum discharge depth needs to refer to the performance parameters of the battery used in the off-grid solar photovoltaic power generation system. Detailed information about the maximum discharge depth of the battery can be obtained from the battery supplier. Usually, if a deep cycle battery is used, 80% depth of discharge (DOD) is recommended; If a shallow cycle battery is used, 50% DOD is recommended.