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I'm a total newbie at this, and I'm trying to make my Arduino-mini work on batteries.

Is it 100% safe if my arduino (or other components) receive like the minimum current to the point it's about to shutdown, but keeps working ? Can this damage anything in the long run ?

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The amount of current the Arduino will draw is based on its circuitry and your added components, not the power source. (Although if the power source cannot provide at least that amount, it may not operate correctly.) In other words, the Arduino draws whatever current it needs based on the supplied voltage.

(In theory, to maintain equal power, it would draw more current as voltage drops. In reality, however, the current is limited by what the (dying) battery can provide.)

In a perfect world, when the battery dips below \$V_{min}\$ for the device (7V for the Arduino Mini*1), it would stop working immediately. Of course, there are gray areas, so you may find it works rather below the stated specifications.

To answer the question, nothing is 100% "safe." Some devices may not be as tolerant of low-voltage scenarios*2. Generally speaking, however, the Arduino Mini should survive such low power issues. Consider the multitude of battery-powered devices in your home: remote controls, MP3 players, mobile phones... When the batteries start to run out, these devices don't suffer irreparable damage, they just stop working until you recharge them or find some fresh batteries.

Note that insufficient power may alter timing, sensor accuracy, and so forth, but if the device is operating "correctly" to your definition, then it should be fine.

*1:The Arduino Mini has a LP2985 5V linear regulator on board and it appears to have a low dropout of ~300mV so in theory the board would continue to get 5V even when the batteries are approaching <5.3V; that's at its max current of 150mA, dropout is lower at lower currents.

*2:Power supplies, for example, start to "de-rate" when the input power is insufficient to maintain a load. They might try to compensate or go into a shutdown mode, but lacking such protections, they can self-destruct.

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