The performance of a battery assembly mainly refers to its current-voltage characteristics, that is, the input and output characteristics of the battery assembly. The ability to convert the sun’s light energy into electrical energy is reflected in the input and output characteristics of the battery module. The curve shown in Figure 1 reflects the relationship between the output voltage, current and output power of the battery assembly when sunlight is irradiated on the battery assembly, so this curve is also called the output characteristic curve of the battery assembly. If I is used to represent current and U is used to represent voltage, this curve can also be referred to as the I-U characteristic curve of the battery assembly. There are three important points on the I-U characteristic curve of the battery assembly, namely, peak power, open circuit voltage and short circuit current.
The performance parameters of battery modules mainly include: short-circuit current, open-circuit voltage, peak current, peak voltage, peak power, fill factor, conversion efficiency, etc.
(1) Short-circuit current (ISC): When the positive and negative poles of the battery assembly are short-circuited to make U=0, the current at this time is the short-circuit current of the battery assembly. The unit of the short-circuit current is A (ampere), and the short-circuit current changes with the change of light intensity.
(2) Open circuit voltage (Uoc): When the positive and negative poles of the battery module are not connected to the load, the voltage between the positive and negative poles of the module is the open circuit voltage, and the unit of the open circuit voltage is V (volt). The open circuit voltage of the battery assembly changes with the increase or decrease of the number of battery slices in series. Generally, the open circuit voltage of the battery assembly with 36 battery slices in series is about 21V.
(3) Peak current (Im): the peak current is also called the maximum working current or the best working current. The peak current refers to the working current when the battery module outputs the maximum power. The unit of the peak current is A (ampere).
(4) Bee value voltage (Um): The peak voltage is also called the maximum working voltage or the best working voltage. The peak voltage refers to the working voltage when the solar cells output the maximum power, and the unit of the peak voltage is V. The bee value voltage of the module changes with the increase or decrease of the number of cells in series. Generally, the bee value voltage of a module with 36 cells in series is 17~17.5V.
(5) Peak power (Pm): Peak power is also called maximum output power or optimal output power. Peak power refers to the maximum output power of battery modules under normal working or test conditions, that is, the product of peak current and peak voltage: Pm=Im×Um. The unit of peak power is Wp (peak watts). The peak power of the battery module depends on the solar irradiance, the solar spectral distribution and the working temperature of the module, so the measurement of the battery module should be carried out under standard conditions. The measurement standard is the 101 standard of the European Commission, and its conditions are: irradiance, 1000w/m2: spectrum AM1.5; test temperature 25℃.
(6) Fill factor (FF): Fill factor, also called curve factor, refers to the ratio of the maximum power of the battery assembly to the product of the open circuit voltage and short circuit current: FF=Pm/Isc × Uoc. The fill factor is an important parameter to evaluate the output characteristics of the cells used in the battery module. The higher its value, the more rectangular the output characteristics of the solar cells used, and the higher the photoelectric conversion efficiency of the cells. The fill factor coefficient of the battery module is generally 0.5 to 0.8, and it can also be expressed as a percentage.
(7) Conversion efficiency (η): Conversion efficiency refers to the ratio of the maximum output power of the battery module when exposed to light to the solar energy power irradiated on the module. which is:
η=Pm (peak power of the battery module)/A (effective area of the battery module × Pin (incident light power per unit area), where Pin=1000W/m2=100mW/cm2.