Appendix D. Power Sources
Monitoring Battery Voltage
The battery voltage can be monitored using an Arduino analog input, but you can't directly connect the battery to an input pin because a fully charged battery can exceed the maximum voltage that the Arduino chip can tolerate.
Another factor to be aware of is that the default voltage reference for
analogRead is the 5 volt output from the regulator on the Arduino board.
This regulator requires more than 6 volts to produce a stable 5 volt output. When the voltage
difference between the regulator input and output (referred to in the regulator datasheet as
the dropout voltage) is less than a volt, the output voltage will drop below the required 5 volt level. Because this voltage is used as the default ...
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