Feather FONA is a little special and different with how it manages battery and USB power. Whereas other Feathers can run direct from 3.3V and thus from USB or battery...
This is because the cellular module cannot run off of 3.3V, and it has thin but common spikes of an amp or two when connecting/sending data on the cellular network. We could have gone with a huge voltage regulator but instead we decided to just power the FONA module direct from the lipoly battery. If this isn't plugged in, the battery charger ends up trying to source an amp, which it cant, and the board will reset
So, yeah. Use a battery! It will keep topped up when using USB, and allow the cellular module to draw current spikes without issue
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 or when the cellular module is in action
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. Note that the regulator doesn't power the cellular module, that's directly powered from VBAT
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
Other Feather's have a resistor divider to read the battery voltage. We decided to skip this and instead let you read the battery voltage via the cellular module using the AT+CBC command, which will give you the battery voltage in millivolts
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
Note that this will not disable power to the cellular module! If you want to depower the cell module, cut the KEY trace on the bottom of the board, wire KEY to an unused pad, and toggle the pin low for 100ms to completely turn on/off the module
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 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.
If you think WiFi is power hungry, you will be surprised at how much power draw you'll need to manage with a cellular module.
Here's some power traces for common events with a cellular module:
You can put the FONA into sleep mode (with the AT+CSCLK command) which will drop the current draw but keep the cellular connection open so you can still receive an SMS and/or wakeup quickly.
Note that the quiescent current drops from 40mA to 20mA and of that 20mA, about ~12mA is the ATmega32u4. Like we said, the best way to really reduce power for long-term usage is to completely turn off the module with KEY