In order to better use the battery in the photovoltaic system, its performance parameters should be considered when selecting the battery. The main parameters of the battery are as follows.
(1) Electromotive force in the battery
The internal electromotive force of the battery determines the output energy of the battery. For batteries of the same material, under the same conditions, the higher the electromotive force in the body, the more energy is stored, and the more energy can be output during discharge. In theory, the electromotive force of a battery is equal to the difference between the equilibrium potentials of the two electrodes that make up the battery. To improve the internal electromotive force of the battery, it is usually necessary to use active materials with higher positive electrode potential and negative electrode potential.
(2) Open circuit voltage and working voltage
The voltage between the two electrodes of the battery in the open circuit state is called the open circuit voltage. The open circuit voltage of the battery is equal to the difference between its positive potential and negative potential, and its magnitude is equal to the internal electromotive force of the battery. After the external circuit of the battery is connected to the load, the voltage between the two electrodes of the battery is called the working voltage, sometimes also called the load voltage or the discharge voltage. The working voltage at the beginning of the battery discharge is called the initial working voltage. The working voltage of the battery will be lower than the open circuit voltage, mainly due to the existence of the internal resistance of the battery itself, so the internal resistance of the battery has an impact on the discharge of the battery. In actual use, it is also necessary to improve the stability of measuring the working voltage, that is, the stability of the discharge voltage of the battery during the discharge time with the depth of discharge. The stability of the working voltage is mainly related to the stability of the reaction of the active substances inside the battery. In the design, it is required that the internal active material reaction of the battery can maintain a continuous and stable change during the discharge process, and the discharge voltage will also change continuously and smoothly. The curve of the working voltage of the battery changing with the discharge time is called the discharge curve, and its value and smoothness depend on the discharge conditions. When discharging at high rate and low temperature, the working voltage of the battery will decrease, and the level of stability will also decrease.
(3) Capacity of the battery
The capacity of the battery is an important indicator for choosing a battery. The capacity of the battery refers to the total amount of electricity that the battery can release from full charge to discharge under a certain discharge condition. The commonly used unit is ampere-hour, referred to as ampere-hour (Ah). According to different measurement conditions, the capacity of the battery can be divided into theoretical capacity, rated capacity, actual capacity and nominal capacity.
The theoretical capacity is the theoretical value obtained by calculating the complete reaction according to the number of active substances in the battery and Faraday’s law. The theoretical capacity is the highest value of the ideal charge and discharge of the battery. In order to compare different types and series of batteries, it is usually possible to use the specific capacity, which can be compared by the theoretical power that the battery can give per unit volume or unit mass, and the unit is A.h/kg or A.h/L.
The actual capacity refers to the total amount of electricity that the battery can actually output under a certain discharge condition. It is numerically equal to the product of discharge current and discharge time, and its value is less than the theoretical capacity. Because in the actual battery, the active material cannot be fully utilized, and the materials that do not participate in the reaction in the battery, such as conductive parts, also consume electric energy.
The rated capacity, also known as the guaranteed capacity, is defined according to the standards promulgated by the state or relevant departments, and mainly guarantees the minimum capacity that the battery should release under the specified discharge conditions.
Nominal capacity (or nominal capacity) is an approximation used to identify the capacity of the battery, and only indicates the capacity range of the battery rather than the exact value. Because the capacity of the battery cannot be determined without specifying the discharge conditions.
The actual capacity of the battery is mainly related to the nature, quantity and utilization of the positive and negative active materials of the battery, as well as the self-loss of the battery. The electrode design of the battery has a great influence on its actual capacity, including the electrode aspect ratio, thickness, porosity, and the form of the conductive grid. In the design and manufacture of actual batteries, the capacities of the positive and negative electrodes are generally unequal, so the capacity of the battery is limited by the electrode with the smaller capacity. For the selected battery, to ensure its actual capacity, it is necessary to improve the utilization rate of active materials, mainly considering the influence of discharge conditions, including discharge rate, discharge form, termination voltage and ambient temperature. The discharge rate is abbreviated as the discharge rate, and is usually expressed as the time rate and the magnification rate. The hour rate is the discharge rate expressed by the discharge time, that is, the time it takes to discharge a certain current to a specified end voltage. Multiplier means that the value of the battery discharge current is a multiple of the rated capacity value. Termination voltage refers to the lowest working voltage when the battery is discharged when the voltage drops to the point where it is no longer suitable to continue discharging. Generally, when discharging at high rate and low temperature, the termination voltage can be specified lower.
(4) Internal resistance of battery
When the current passes through the interior of the battery, it is subjected to various resistances, which reduce the output voltage of the battery. These resistances are expressed as the internal resistance of the battery. The internal resistance of the battery is a comprehensive parameter, which is the sum of the internal resistances of all batteries such as active materials, electrolytes, diaphragms, and electrode joints. The internal resistance of the battery is not constant, because the composition of the active material, the concentration of the electrolyte and the temperature are constantly changing, so the internal resistance of the battery also changes with time during the discharge process.
The internal resistance of the battery is different in batteries of different specifications and models, and also in different discharge modes. In general, the internal resistance of the high-power battery is small, and the internal resistance of the battery is small when it is discharged at a low rate; however, the internal resistance of the battery increases significantly when it is discharged at a high rate.
The internal resistance of the battery includes ohmic internal resistance, polarization internal resistance and diaphragm resistance, the sum of which is the internal resistance of the battery. Due to the existence of internal resistance, the output voltage of the battery when discharging is lower than the open-circuit voltage of the battery, and the input voltage must be higher than the open-circuit voltage of the battery when charging. Ohmic resistance obeys Ohm’s law; polarization resistance does not obey Ohm’s law, it increases with the increase of current density, showing a nonlinear relationship.
a. Ohmic resistance Ohmic resistance is mainly reflected in the conductive parts inside the battery, which is composed of electrode material, electrolyte, resistance of diaphragm and contact resistance of various parts. It is related to the size, structure of the battery, the forming method of the electrode (such as the paste-type electrode and the tubular electrode of the lead-acid battery) and the tightness of the assembly.
b. Polarization internal resistance Polarization internal resistance refers to the internal resistance caused by polarization when the positive and negative electrodes undergo electrochemical reactions. It is related to the characteristics of the active material, the structure of the electrode and its manufacturing process, especially the battery It is related to the operating conditions, such as discharge current and temperature, which have a great influence. When a large current is applied, both electrochemical polarization and concentration polarization increase, which may cause passivation of the negative electrode. In addition, the decrease in temperature has an adverse effect on polarization and ion diffusion, so that the internal resistance of the battery increases under low temperature conditions.
c. Diaphragm resistance Diaphragm resistance is an important parameter for the performance of the diaphragm in the battery, and it is also one of the main factors affecting the low-temperature performance of the battery in the high-rate discharge phase. The diaphragm material itself is an insulator. The internal resistance of the diaphragm does not refer to the resistance of the diaphragm material itself, but the resistance of the diaphragm to the current passing through the electrolyte in the pores of the diaphragm. Therefore, the resistance is actually the porosity, pore size and tortuosity of the pores of the diaphragm. resistance to migration. The microporous structure of the separator is filled with electrolyte, and ions in the electrolyte migrate through the pores to conduct electricity. Therefore, in battery design, it is hoped that the smaller the diaphragm resistance, the better.
(5) The energy of the battery
The energy of the battery refers to the electrical energy that the battery can give under a certain discharge condition, usually expressed in watt-hours (w.h), which also indicates the ability of the battery to discharge. As mentioned above, batteries rely on chemical reactions to release electrical energy, so the energy released under different discharge conditions will be very different. Therefore, considering the energy of batteries should be divided into theoretical energy and actual energy. The theoretical energy WT can be expressed as the product of the theoretical capacity (CT) and the electromotive force (E),
The actual energy of the battery is the product of the actual capacity CR and the average working voltage UR under certain discharge conditions, namely
In the process of use, the more commonly used is the specific energy. Because the energy density of different batteries can be compared more intuitively through the specific energy. The specific energy is the electric energy that the battery can output per unit mass or unit volume. The former refers to the theoretically output energy when the unit battery reaction material is completely discharged; the latter refers to the actual energy that the unit battery reaction material can output. The actual specific energy of the battery is much smaller than the theoretical specific energy. Usually, the relationship between the actual specific energy W and the theoretical specific energy wi is as follows:
In the formula, KU is the voltage efficiency, which is the ratio of the working voltage to the electromotive force; KR is the reaction efficiency, that is, the ratio of the active material participating in the reaction to the total amount; Km is the mass efficiency, that is, the active material participating in the reaction. The ratio of the total material in the battery .
(6) Battery power and specific power
The power of the battery refers to the amount of energy given by the battery per unit time under certain discharge conditions, and the unit is w (watts) or kW (kilowatts). The power that a unit mass battery can give is called specific power, and the unit is w/kg or kw/kg. Specific power is also one of the main indicators to measure battery performance. The greater the specific power of the battery, the greater the discharge current it can withstand. The specific energy and specific power performance of the battery are important parameters in the selection of batteries for photovoltaic power generation systems. Because the battery must be matched with the photovoltaic cell array and the electrical load device. Factors such as mass, volume, specific energy, temperature range used and price should also be considered when determining the type of battery to choose.
(7) The charging efficiency of the battery
Battery charging efficiency, also known as output efficiency, is one of the important indicators to measure the performance of the battery, and it represents the energy loss of the battery during the charging and discharging process. The actual battery is not an ideal energy storage device, and there must be a certain amount of energy consumption during the working process, which is usually expressed by the capacity output efficiency and the energy output efficiency.
Capacity output efficiency refers to the ratio of the output capacity of the battery when it is discharged to the input capacity when it is charged. The main factor affecting the output efficiency of battery capacity is various side reactions inside the battery. For example, when the battery is charged, part of the electricity is consumed in the decomposition of water; Caking, diaphragm shrinkage, etc. can also reduce power output.
Energy output efficiency, also known as electrical energy efficiency, refers to the ratio of the energy output when the battery is discharged to the energy input when charging. The main reason that affects the energy output efficiency is the internal resistance of the battery. The internal resistance increases the charging voltage, the discharging voltage decreases, and this part of the energy consumed by the internal resistance is released in the form of heat.