Special attention to factors when designing battery capacity

Special attention to factors when designing battery capacity
  1. Total radiation and total sunshine hours

When the Tycorun Battery is used for power supply, the total solar radiation or total sunshine hours provided by the meteorological station are indispensable data for determining the capacity of the battery.

  1. Temperature factor

(1) Reasons for temperature effects Although the solar radiation intensity is high in summer, the excess power generation of the square array can completely make up for the reduced electrical energy due to temperature, not to mention that some standard components have already considered the impact of temperature rise in summer, so in general The effect of temperature on the capacity of lead-acid batteries can be ignored at temperature. However, when the temperature is low (such as <0°C), due to the increase in the viscosity and temperature of the sulfuric acid electrolyte, the lead-acid battery is difficult to diffuse, the resistance increases, and it is easy to form dense lead sulfate, resulting in the electrochemical reaction inside the active material. It is difficult to carry out, and the discharge capacity of the lead-acid battery decreases. Therefore, when designing a solar photovoltaic power generation system, it is still necessary to consider the influence of temperature.

(2) The relationship between the rated capacity and temperature of the battery decreases with the temperature, the capacity at 0°C drops to about 90% of the rated capacity; the capacity at -10°C drops to about 80% to 90% of the nominal capacity; At a temperature of 120°C, it drops to about 80% of the rated capacity. Battery manufacturers generally provide relevant battery temperature-discharge rate-capacity correction curves (Figure 1). The battery capacity correction coefficient corresponding to the temperature can be found on the curve, and the above-mentioned preliminary calculation result of the battery capacity can be corrected by dividing it by the battery capacity correction coefficient during design.

Figure 1

(3) The relationship between the rated capacity of the battery and the temperature The rated capacity of the battery is usually specified at 25°C and a specified discharge rate. The best working temperature of the battery is 25℃. If the battery operating temperature is not at this temperature, the actual capacity calculation formula is as follows:
Ce=Ct/1+k(T-250)
In the formula, C1——measured capacity;
Ce——nominal capacity when the ambient temperature is 25℃;
K——Temperature coefficient (The value of K is: 10h rate discharge, K=0.006℃-1; 3h rate discharge
Electricity, K=0.008℃-1; 1h rate discharge, K=0.01℃-1);
T——actual ambient temperature.
It can be seen from the formula that when the ambient temperature is higher than 25°C, the actual release capacity Ct of the battery is greater than the design rated capacity Ce; and when the ambient temperature is lower than 25°C, its actual release capacity Ct is lower than the design rated capacity Ce It can also be seen from the value of the temperature coefficient K that the greater the discharge rate, the greater the effect of temperature on the capacity.

(4) The influence of low temperature on the maximum depth of discharge of the battery The influence of low temperature on the maximum depth of discharge of the battery must also be considered when designing the battery capacity. The electrolyte in lead-acid batteries may freeze at low temperatures. When the battery is discharged, the negative lead and positive lead dioxide become lead sulfate and water. As the battery continues to discharge, the water generated in the battery will continue to increase, and the sulfuric acid electrolyte will be diluted by more and more water generated, causing the condensation point of the battery electrolyte to continue to rise. In cold climates, if the battery is discharged too much, the electrolyte can condense and damage the battery. Therefore, even if a deep-cycle battery is used in the design of the photovoltaic system, the maximum depth of discharge value should not exceed 80%. Figure 2

Figure 2
  1. The effect of discharge rate

(1) Average discharge rate and definition The concept of average discharge rate commonly used in battery technology will be used in the design. The average discharge rate formula is as follows:
Average discharge rate = maximum continuous rainy days × load working time / maximum discharge depth correction factor
According to the above formula, the actual average discharge rate of the photovoltaic system can be calculated, and the capacity of the battery can be corrected according to the battery capacity of the type of battery provided by the battery manufacturer at different discharge rates.

(2) The relationship between capacity and discharge rate In order to set uniform conditions, a number of discharge time rates are set for batteries with certain structural characteristics and uses, such as 20h rate, 10h rate, and 5h rate, which are written as C20, C10, and C5 respectively. , where C represents the capacity of the battery, followed by a number indicating the number of hours the battery of this type is discharged to the set voltage with a certain strength of current. So the capacity is divided by the rated discharge current to obtain the number of discharge hours (that is, the discharge rate), which refers to the current value required by the battery to discharge its rated capacity within a specified time. The relationship between the discharge rate and the capacity can be expressed by the following formula:
Discharge hours = capacity / rated discharge current

     It can be seen from the above formula that a battery with a capacity of 10A·h and a discharge rate of 2h should have a rated discharge current of 10A·h/2h=5A, which means that when the battery is discharged at a rate of 20h, the rated capacity is 60A·h, In this way, the discharge current is 60/20=3(A).

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