Batteries Not Part of A System
Batteries that are part of a system, such as computer batteries, can have their properties checked and logged in operation to assist in determining remaining charge. A real battery can be modeled as an ideal battery with a specified EMF, in series with an internal resistance. As a battery discharges, the EMF may drop or the internal resistance increase; in many cases the EMF remains more or less constant during most of the discharge, with the voltage drop across the internal resistance determining the voltage supplied. Determining the charge remaining in many battery types not connected to a system that monitors battery use is not reliably possible with a voltmeter. In battery types where EMF remains approximately constant during discharge, but resistance increases, voltage across battery terminals is not a good indicator of capacity. A meter such as an equivalent series resistance meter (ESR meter) normally used for measuring the ESR of electrolytic capacitors can be used to evaluate internal resistance. ESR meters fitted with protective diodes cannot be used, a battery will simply destroy the diodes and damage itself. An ESR meter known not to have diode protection will give a reading of internal resistance for a rechargeable or non-rechargeable battery of any size down to the smallest button cells which gives an indication of the state of charge. To use it, measurements on fully charged and fully discharged batteries of the same type can be used to determine resistances associated with those states.
The cost of an ESR meter makes it uneconomic for measuring battery voltages as its only function, but a meter used for cheeking capacitors can take on the additional duty.
Read more about this topic: Battery Indicator
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