A practical source (any linear electric or electronic circuit or device which generates a voltage) may be represented as an ideal voltage source in series with an impedance. This impedance is termed the Internal resistance of the source.
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Batteries
Internal resistance is a concept that helps model the electrical consequences of the complex chemical reactions inside a battery. When a current is flowing through a cell, the measured e.m.f. (voltage output) is lower than when there is no current delivered by the cell.
The internal resistance of a battery can not be measured using the "resistance" or "ohms" setting on a conventional multimeter, since it requires a current to be observed. However, it can be calculated from current and voltage data measured from a test circuit containing the battery and a load resistor RL. Since both the internal resistance and load resistor are in series with the ideal voltage source, Kirchhoff's Laws and Ohm's law give 
. This equation can be solved for internal resistance:
where
- RB is the internal resistance of the battery
- VB is the battery voltage without a load L
- IL is the current supplied by the battery with this load L
- RL is the resistance of this load L.
Internal resistance increases with the age of a battery, but for most commercial batteries the internal resistance is on the order of 1 ohm.
It should be noted that the above only applies to ideal batteries under ideal load conditions and does not directly relate to real world internal resistance of batteries due to the chemical nature of the cells.
Battery chemistry
The internal resistance of batteries depends on the battery chemistry used. This table shows typical internal resistance results for various types of cells.
| Battery chemistry | RB (in milliohms) when brand new |
RB (in milliohms) after 1000 charge-discharge cycles |
|---|---|---|
| Alkaline[1] | 150-300 | N/A |
| Lead-acid[2] | 50 | |
| Nickel-cadmium (NiCd)[3] | 75 | 75 |
| Nickel metal hydride (NiMH)[3] | ? | 650 |
| PEVE Power-dense EV-95 (NiMH)[citation needed] | <7 | 10 |
| Lithium-ion (Li-ion)[3] | 320 | 340 |
References
- ^ "Battery Internal Resistance". Energizer Technical Bulletin. Energizer Battery. http://data.energizer.com/PDFs/BatteryIR.pdf.
- ^ Křivák, P.; Bača, P.. "In situ study of the internal resistance in a lead-acid battery cells". 6th Advanced Batteries and Accumulators. http://www.aba-brno.cz/aba2005-html/article/pdf/52.pdf.
- ^ a b c Buchmann, Isidor. "Choosing a battery that will last". Isidor Buchmann (CEO of Cadex Electronics Inc.). http://www.buchmann.ca/Article9-Page1.asp.
External links
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