High power consumption during power down mode I try to use power down mode to extend battery life, but no matter what sleep library I use, the power consumption never goes below 3.4-3.6 mA. I'm using a chinese pro mini board with VCC input (but using the RAW results the same).

avr/power sketch:

#include &lt;avr/power.h&gt;
#include &lt;avr/sleep.h&gt;

void setup()
{          
  Serial.begin(9600);      
  byte i;

  // Ensure no floating pins
  for(i=0; i&lt;20 ; i++)
  {
    pinMode(i, OUTPUT);
    digitalWrite(i, LOW);
  }

  // Power-down board
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);

  sleep_enable();

  // Disable ADC
  ADCSRA &amp;= ~(1 &lt;&lt; ADEN);

  // Power down functions
  PRR = 0xFF;

  // Enter sleep mode
  sleep_mode();
}

void loop()
{
}


rocketscream/Low-Power sketch:

#include &lt;LowPower.h&gt;
void setup() {
}

void loop() {
  LowPower.powerDown(SLEEP_8S, ADC_OFF, BOD_OFF);
}
I tried your first sketch on my "bare-bones" board.



With the sketch exactly as written, it used 197 µA.

I wondered why it was that high, so I commented out this line:

  Serial.begin(9600);


That reduced consumption to 122 nA.

See Power saving techniques for microprocessors.

You need to get rid of any "power" LEDs, and disconnect any voltage regulators. If you want really low consumption you need to have minimal hardware.

Of course, if you need the voltage regulator, well so be it. But don't expect 100 nA consumption if you use it.