Battery
Battery refers to the part of a cup, tank, or other container or composite container containing an electrolyte solution and a metal electrode to generate an electric current,
and a device that converts chemical energy into electrical energy. There are positive and negative poles. With the advancement of science and technology,
batteries generally refer to small devices that can generate electricity. Such as solar cells. The performance parameters of the battery mainly include electromotive
force, capacity, specific energy and resistance. Using the battery as an energy source, you can get a stable voltage, stable current, long-term stable power
supply, little external influence of the current, and the battery has a simple structure, easy to carry, easy to charge and discharge operation, not affected by
the external climate and temperature, stable and reliable performance, in all aspects of modern social life play a great role.
Performance parameter
Electromotive force Electromotive force is the difference between the balance electrode potentials of the two electrodes, taking the lead-acid battery as an example, E= diameter
+ 0-diameter -0+RT/F*In (αH2SO4/αH2O).
Where: E - electromotive force
Diameter +0 - positive standard electrode potential, its value is 1.690V
With the diameter of -0 - negative standard electrode potential, its value is -0.356V
R - Universal gas constant with a value of 8.314J/ (mol*K)
T - Temperature, which is related to the temperature of the battery
F - Faraday's constant, which has a value of 96485C/mol
αH2SO4 - Activity of sulfuric acid, dependent on sulfuric acid concentration
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- (-0.356) =2.046V, so the nominal voltage of the
battery is 2V. The electromotive force of lead-acid battery is related to temperature and sulfuric acid concentration.
Rated capacity The minimum capacity that a battery should be able to discharge under the conditions specified in the design (such as temperature, discharge rate, termination
voltage, etc.) is expressed in amperes per hour and is represented by symbol C. The capacity is greatly affected by the discharge rate, so the discharge rate is
often marked with an Arabic numeral in the lower right corner of the letter C, such as C20=50, indicating that the capacity at the rate of 20 hours is 50 A · hour.
The theoretical capacity of the battery can be precisely calculated according to the amount of electrode active substance in the battery reaction formula and the
electrochemical equivalent of the active substance calculated according to Faraday's law. Due to the possible side reactions 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 the battery at room temperature, also known as the nominal voltage. It is a reference when selecting different types of batteries.
The actual operating voltage of the battery is equal to the difference of the balance electrode potential of the positive and negative electrodes with different operating
conditions. It is only related to the type of electrode active substance, and has nothing to do with the number of active substances. Battery voltage is essentially
DC voltage, but under certain special conditions, the phase transition of metal crystals or some phase forming films caused by electrode reaction will cause
small fluctuations in voltage, which is called noise. The amplitude of the fluctuation is small but the frequency range is 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 voltage. The open circuit voltage of the battery is equal to the difference between the positive
electrode potential of the battery and the negative electrode potential when the battery is open (that is, when no current passes through the poles). The open circuit
voltage of the battery is expressed by V open, that is, V open = diameter +- diameter -, where the diameter + and diameter - are respectively the positive and
negative electrode potentials of the battery. 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 a balanced electrode potential, but a 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 of the current passing through the inside of the battery. It includes ohm internal resistance and polarization
internal resistance, and the polarization internal resistance includes electrochemical polarization internal resistance and concentration polarization internal resistance.
Due to the existence of 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 a constant, and changes over time (gradually becoming larger) during charging and discharging, because the composition of the
active substance, the concentration of the electrolyte, and the temperature are constantly changing. Ohm internal resistance obeys Ohm's law, the polarization
internal resistance increases with the increase of current density, but the relationship is not linear. It usually increases with increasing current density.
The internal resistance is an important indicator to determine the performance of the battery, which directly affects the working voltage, working current, output
energy and power of the battery, for the battery, the smaller the internal resistance, the better.
impedance The battery has a large electrode-electrolyte interface area, so the battery can be equivalent to a series circuit of a large capacitor and a small resistance and inductance.
But 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
the specific measurement state.
Charge and discharge rate Sometimes both rate and magnification are expressed. The hour rate is the charge and discharge rate expressed by the charge and discharge time, which is numerically
equal to the number of hours obtained by dividing the rated capacity of the battery (amperage) by the specified charge and discharge current (amperage). The multiplier
is another representation of the charge and discharge rate, and its value is the reciprocal of the time rate. The discharge rate of a galvanic cell is expressed as the time it
takes to discharge through a fixed resistance to the termination voltage. The discharge rate has great influence on the battery performance.
Life span Storage life refers to the maximum time allowed for storage between the time the battery is made and the start of use, expressed in years. The total life period, including
the storage period and the use period, is called the life of the battery. The storage battery life is divided into dry storage life and wet storage life. Cycle life is the maximum
number of charge and discharge cycles that the battery can achieve under the specified conditions. When the cycle life is specified, the system of charge and discharge
cycle test must be specified at the same time, including charge and discharge rate, discharge depth and ambient temperature range.
Self-discharge rate The rate at which a battery loses its capacity during storage. Expressed as a percentage of the capacity lost by self-discharge per unit storage time to the
pre-storage capacity.
Relevant calculation Where E is the electromotive force, r is the internal resistance of the power supply, internal voltage U inside =Ir, E= inside U + outside U
Scope of application: Any circuit
Energy conversion in closed circuits:
E=U+Ir
EI=UI+I^2R
P release =EI
P output =UI
Pure resistance in the circuit
P output =I^2R
=E^2R/ (R+r) ^2
=E^2/ (R^2+2r+r^2/R)
P output maximizes when r=R, P output =E^2/4r (mean inequality)
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