Batteries
Batteries can be approximately modeled as a voltage source in series with a resistance. Internal resistance of a battery is dependent on the specific battery's size, chemical properties, age, temperature and the discharge current. Measurement of the internal resistance of a battery is a guide to its condition, but may not apply at other than the test conditions. Internal resistance depends upon temperature; for example, a fresh Energizer E91 AA alkaline primary battery drops from about 0.9 ohms at -40 °C to about 0.1 ohms at 40 °C.
The internal resistance of a battery can be calculated from its open circuit voltage, voltage on-load, and the load current. An equivalent series resistance (ESR) meter as used to measure the ESR of capacitors, essentially an AC milliohmmeter, can be used.
Internal resistance increases with the age of a battery, but for most battery types ranges from a fraction of an ohm to a few ohms.
In use the useful voltage produced by a disposable battery decreases until it drops so far that the battery must be discarded. This is largely due to an increase in internal resistance rather than a drop in the voltage of the equivalent source.
Read more about this topic: Internal Resistance
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