Battery + USB Power
We wanted to make our Feather boards 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 USB 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 rechargeable 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). This happens 'hot-swap' style so you can always keep the LiPoly connected as a 'backup' power that will only get used when USB power is lost.
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 next to the USB jack, which will light up while the battery is charging. This LED might also flicker if the battery is not connected, it's 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 500mA peak regulator. 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 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.
#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:
- For permanent installations, a 5V 1A USB wall adapter will let you plug in a USB cable for reliable power
- For mobile use, where you don't want a LiPoly, use a USB battery pack!
- If you have a higher voltage power supply, use a 5V buck converter and wire it to a USB cable's 5V and GND input
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
- 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 work. 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:
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:
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
WiFi.setSleepMode(M2M_PS_MANUAL, 1);
And then when you want it to go to sleep call:
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!
Page last edited August 23, 2024
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