Power supplies
You have a lot of power supply options here! We bring out the BAT pin, which is tied
to the lipoly JST connector, as well as USB which is the +5V from USB if connected.
We also have the 3V pin which has the output from the 3.3V regulator. We use a
500mA peak AP2112. While you can get 500mA from it, you can't do it continuously
from 5V as it will overheat the regulator. It's fine for, say, powering an ESP8266 WiFi
chip or XBee radio though, since the current draw is 'spiky' & sporadic.
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.
#define VBATPIN A9
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);
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