Datasheet

Measuring Battery

If you're running off of a battery, chances are you wanna know what the voltage is at! That

way you can tell when the battery needs recharging. Lipoly batteries are 'maxed out' at 4.2V

and stick around 3.7V for much of the battery life, then slowly sink down to 3.2V or so before

the protection circuitry cuts it off. By measuring the voltage you can quickly tell when you're

heading below 3.7V

To make this easy we stuck a double-100K resistor divider on the BAT pin, and connected it

to D9 (a.k.a analog #7 A7). You can read this pin's voltage, then double it, to get the battery

voltage.

This voltage will 'float' at 4.2V when no battery is plugged in, due to the lipoly charger output,

so its not a good way to detect if a battery is plugged in or not (there is no simple way to

detect if a battery is plugged in)

#define VBATPIN A7

float measuredvbat = analogRead(VBATPIN);

measuredvbat *= 2; // we divided by 2, so multiply back

measuredvbat *= 3.3; // Multiply by 3.3V, our reference voltage

measuredvbat /= 1024; // convert to voltage

Serial.print("VBat: " ); Serial.println(measuredvbat);