The main performance of the battery includes electromotive force, rated capacity, rated voltage, open circuit voltage, internal resistance, charge and discharge rate, impedance, lifetime and self-discharge rate.
Electromotive force
The electromotive force is the difference between the equilibrium electrode potentials of the two electrodes. Taking a lead-acid battery as an example, E=Ф+0-Ф-0+RT/F*In(αH2SO4/αH2O).
Of which: E-electromotive force
Ф+0—positive standard electrode potential, which is 1.690
Ф-0—Negative standard electrode potential, the value is -0.356
R—general gas constant, which is 8.314
T—temperature, related to the temperature at which the battery is placed
F—Faraday constant, which is 96500
αH2SO4—the activity of sulfuric acid, related to the concentration of sulfuric acid
αH2O—the activity of water, related to the concentration of sulfuric acid
As can be seen from the above formula, the standard electromotive force of the lead-acid battery is 1.690-(-0.0.356)=2.046V, so the nominal voltage of the battery is 2V. The electromotive force of a lead-acid battery is related to temperature and sulfuric acid concentration.
Rated Capacity
Under the conditions specified by the design (such as temperature, discharge rate, termination voltage, etc.), the minimum capacity that the battery should be able to discharge, in ampere-hours, is indicated by the symbol C. The capacity is greatly affected by the discharge rate, so the discharge rate is often indicated by Arabic numerals in the lower right corner of the letter C, such as C20 = 50, indicating a capacity of 50 amps per hour at a 20 o'clock rate. The theoretical capacity of the battery can be accurately determined from the amount of the electrode active material in the battery reaction formula and the electrochemical equivalent of the active material calculated according to Faraday's law. Due to the side reactions that may occur in the battery and the special needs of the design, the actual capacity of the battery is often lower than the theoretical capacity.
Rated voltage
The typical operating voltage of a battery at room temperature, also known as the nominal voltage. It is a reference when using different types of batteries. The actual operating voltage of the battery will vary depending on the conditions of use. The open circuit voltage of the battery is equal to the difference between the balanced electrode potentials of the positive and negative electrodes. It is only related to the type of electrode active material, regardless of the amount of active substance. The battery voltage is essentially a DC voltage, but under certain special conditions, the phase change of the metal crystal or some phase-forming film caused by the electrode reaction causes a slight fluctuation of the voltage. This phenomenon is called noise. The amplitude of the fluctuation is small but the frequency range is very wide, so it can be distinguished from the self-excited noise in the circuit.
Open circuit voltage
The terminal voltage of the battery in the open state is called the open circuit voltage. The open circuit voltage of the battery is equal to the difference between the positive electrode potential of the battery and the electrode potential of the negative electrode when the battery is open (ie, when no current flows through the two poles). The open circuit voltage of the battery is expressed by V, that is, V on = Ф + - Ф -, where Ф +, Ф - are the positive and negative electrode potentials of the battery, respectively. The open circuit voltage of a battery is generally less than its electromotive force. This is because the electrode potential established by the two poles of the battery in the electrolyte solution is usually not the equilibrium electrode potential but the stable electrode potential. Generally, it can be approximated that the open circuit voltage of the battery is the electromotive force of the battery.
Internal resistance
The internal resistance of the battery refers to the resistance that is received when current passes through the inside of the battery. It includes ohmic internal resistance and polarization internal resistance, and polarization internal resistance includes electrochemical polarization internal resistance and concentration polarization internal resistance. Due to the internal resistance, the operating voltage of the battery is always less than the electromotive force or open circuit voltage of the battery. The internal resistance of the battery is not constant and changes with time (gradually large) during charging and discharging, because the composition of the active material, the concentration and temperature of the electrolyte are constantly changing. The ohmic internal resistance follows Ohm's law, and the polarization internal resistance increases with increasing current density, but is not linear. It often increases as the current density increases.
Internal resistance is an important indicator to determine the performance of the battery. It directly affects the operating voltage, operating current, output energy and power of the battery. For batteries, the internal resistance is as small as possible.
Charge and discharge rate
Sometimes there are two representations of rate and magnification. The time rate is the charge and discharge rate expressed by the charge and discharge time, and is numerically equal to the number of hours obtained by dividing the rated capacity (A·h) of the battery by the predetermined charge and discharge current (A). Magnification is another representation of the rate of charge and discharge, the value of which is the reciprocal of the rate of time. The discharge rate of the primary battery is expressed as the time to discharge to the termination voltage via a certain fixed resistance. The discharge rate has a large impact on battery performance.
impedance
The battery has a large electrode-electrolyte interface area, so the battery can be equivalent to a series circuit of a large capacitance and a small resistance and inductance. However, the actual situation is much more complicated, especially the impedance of the battery varies with time and DC level, and the measured impedance is only valid for a specific measurement state.
life
Storage life refers to the maximum time allowed to store between the time the battery is manufactured and the time it is used. The total term including the storage period and the usage period is called the expiration date of the battery. The life of the storage battery is divided into dry storage life and wet storage life. The cycle life is the maximum number of charge and discharge cycles that the battery can reach under the specified conditions. The system of charge and discharge cycle test must be specified at the specified cycle life, including charge and discharge rate, discharge depth and ambient temperature range.
Self-discharge rate
The rate at which the battery loses its capacity during storage. The capacity lost by self-discharge in the unit storage time is expressed as a percentage of the capacity before storage.
Battery related calculation
Where E is the electromotive force, r is the internal resistance of the power supply, the internal voltage U is =Ir, E=U is +U
Scope of application: pure resistance circuit
Energy conversion in a closed circuit:
E=U+Ir
EI=UI+I^2R
P release = EI
P output = UI
Pure resistance circuit
P output = I^2R
=E^2R/(R+r)^2
=E^2/(R^2+2r+r^2/R)
P output is maximum when r=R, P output=E^2/4r (mean inequality)
This article is taken from the battery forum : club.battery.com.cn
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