Wilco van Esch

The EV3 programmable brick should be powered with 6 1.5 V AA batteries for a total voltage of 9 V.

When total voltage drops below 5 V, you will get strange issues such as the programmable brick spontaneously shutting down.

Connecting to Wi-Fi can quickly drop the voltage.

If you use ev3dev, you can make it easier to learn when the EV3 is dipping below 5 V. The following is an example using ev3dev with Python.

In a shell or script, use PowerSupply() to access the power_supply class:

ps = PowerSupply()

Then get the battery's voltage at the time of measuring through:

ps.measured_volts

Now you can have the EV3 give you a warning if the voltage drops below a threshold you specify. For example:

while True:
  if ps.measured_volts < 5:
    ALL_LEDS = [Leds.LEFT, Leds.RIGHT]
    for l in ALL_LEDS:
      Leds.set_color(l, Leds.RED)
    Sound.play("Feed me. Feed me. Feed me.")

Read more about the available methods in the ev3dev documentation.