A practical electrical power source which is a linear electric circuit may, according to Thévenin’s theorem, be represented as an ideal voltage source in series with an impedance. This resistance is termed the internal resistance of the source. When the power source delivers current, the measured e.m.f. (voltage output) is lower than the no-load voltage; the difference is the voltage (the product of current and resistance) drop caused by the internal resistance. The concept of internal resistance applies to all kinds of electrical sources and is useful for analyzing many types of electrical circuits. Internal resistance can be caused by a number of outcomes, though a possible cause is by interior chemical installment. When thermal energy is applied to provide the current, that applied energy is most of the power source’s energy which produces the chemicals. The load current is delivered in a lap and returns to the battery (voltage source) and then performs resistance.
When current flows through the cell there is an IR voltage drop across the internal resistance of the cell which decreases the terminal voltage of the cell during discharge and increases the voltage needed to charge the cell thus reducing its effective capacity as well as decreasing its charge/discharge efficiency. Higher discharge rates give rise to higher internal voltage drops which explains the lower voltage discharge curves at high C rates.
The internal impedance is affected by the physical characteristics of the electrolyte, the smaller the granular size of the electrolyte material the lower the impedance. The grain size is controlled by the cell manufacturer in a milling process.
Spiral construction of the electrodes is often used to maximize the surface area and thus reduce internal impedance. This reduces heat generation and permits faster charge and discharge rates.
The internal resistance of a galvanic cell is temperature dependent, decreasing as the temperature rises due to the increase in electron mobility.
The internal resistance of most cell chemistries also tends to increase significantly towards the end of the discharge cycle as the active chemicals are converted to their discharged state and hence are effectively used up. This is principally responsible for the rapid drop off in cell voltage at the end of the discharge cycle.
The internal resistance also influences the effective capacity of a cell. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that for high discharge rates the lower the available capacity of the cell. Conversely, if it is discharged over a prolonged period, the Amp-hour capacity is higher. This is important because some manufacturers specify the capacity of their batteries at very low discharge rates which makes them look a lot better than they really are.
Internal Impedance on WiKi: http://en.wikipedia.org/wiki/Internal_resistance
Neware has protable BVIR product for battery ACIR test, and our BTS4000 is capable of performing the battery DCIR test.
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