Power Management

Battery + USB Power

We wanted to make the Feather easy to power both when connected to a computer as well as via battery. There's two ways to power a Feather. You can connect with a MicroUSB cable (just plug into the jack) and the Feather will regulate the 5V USB down to 3.3V. You can also connect a 4.2/3.7V Lithium Polymer (Lipo/Lipoly) or Lithium Ion (LiIon) battery to the JST jack. This will let the Feather run on a rechargable battery. When the USB power is powered, it will automatically switch over to USB for power, as well as start charging the battery (if attached) at 200mA. This happens 'hotswap' style so you can always keep the Lipoly connected as a 'backup' power that will only get used when USB power is lost.

The JST connector polarity is matched to Adafruit LiPoly batteries. Using wrong polarity batteries can destroy your Feather

The above shows the Micro USB jack (left), Lipoly JST jack (top left), as well as the 3.3V regulator and changeover diode (just to the right of the JST jack) and the Lipoly charging circuitry (to the right of the Reset button). There's also a CHG LED, which will light up while the battery is charging. This LED might also flicker if the battery is not connected.

The charge LED is automatically driven by the Lipoly charger circuit. It will try to detect a battery and is expecting one to be attached. If there isn't one it may flicker once in a while when you use power because it's trying to charge a (non-existant) battery. It's not harmful, and its totally normal!

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 600mA peak AP2112K-33. While you can get 600mA from it, you can't do it continuously from 5V as it will overheat the regulator. It's fine for, say, powering the attached 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.

Download: file
#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);

ENable pin

If you'd like to turn off the 3.3V regulator, you can do that with the EN(able) pin. Simply tie this pin to Ground and it will disable the 3V regulator. The BAT and USB pins will still be powered

Alternative Power Options

The two primary ways for powering a feather are a 3.7/4.2V LiPo battery plugged into the JST port or a USB power cable.

If you need other ways to power the Feather, here's what we recommend:

Here's what you cannot do:

  • Do not use alkaline or NiMH batteries and connect to the battery port - this will destroy the LiPoly charger and there's no way to disable the charger
  • Do not use 7.4V RC batteries on the battery port - this will destroy the board

The Feather is not designed for external power supplies - this is a design decision to make the board compact and low cost. It is not recommended, but technically possible:

  • Connect an external 3.3V power supply to the 3V and GND pins. Not recommended, this may cause unexpected behavior and the EN pin will no longer. Also this doesn't provide power on BAT or USB and some Feathers/Wings use those pins for high current usages. You may end up damaging your Feather.
  • Connect an external 5V power supply to the USB and GND pins. Not recommended, this may cause unexpected behavior when plugging in the USB port because you will be back-powering the USB port, which could confuse or damage your computer.

Power Usage & Saving with WiFi

WiFi is a very power-hungry protocol. During transmit and SSID association, you'll see high power usages. For example, here is an MQTT demo running where it connects to the WPA SSID and then sents a packet every 5 seconds or so:

You can see the chip launch at about 1.5 seconds, then turn on the WiFi and at about 2s make the SSID connection and MQTT connection. The average current is about 100ms afterwards, and a packet spikes up to ~130mA at the 7 second mark.

100mA is still quite a bit, you can very easily reduce this by letting the WINC1500 manage its own power:

Download: file
WiFi.setSleepMode(M2M_PS_H_AUTOMATIC, 1); // go into power save mode when possible!

When this line is added, it lets the WINC1500 know that when nothings going on, shut down unneeded parts. You dont have to manage the power modes, and the power will drop down nearly instantly to about 22mA average (there's still spikes during transmit of course)

If you're using the Arduino WiFi101 library, call this instead to enable automatic sleep:

Download: file
WiFi.lowPowerMode();

Note that 10mA or so is for the ATSAMD chip, so that means you've got ~12mA for the WiFi module.

If you want ultra-low power you can manage the WINC1500 module your own with

Download: file
WiFi.setSleepMode(M2M_PS_MANUAL, 1);

And then when you want it to go to sleep call:

Download: file
WiFi.requestSleep(sleeptimeinmilliseconds)

With this mode, you can get much much lower power when you call the requestSleepmode (basically 1-2mA) and still have an active live WiFi connection...but, when not actively sleeping the power usage seems higher (see that spikey part between seconds 3 and 8.5)

A mix of the two may give you the best performance. And don't forget that the SAMD21 is going to draw 10mA so put the main chip to sleep too if you want to get to very low power sleep modes!

This guide was first published on Jan 26, 2016. It was last updated on Jan 26, 2016. This page (Power Management) was last updated on Nov 20, 2019.