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4 Ways to Power Your Arduino:
Outer power supply, or battery, going into the 2.1mm x 5.5mm DC power jack (suggested 7~12V info)
Outside power supply, or battery, going into the "VIN" and "GND" pins on the board, through jumpers (suggested 7~12V information)
Outer, managed 5V power going straight into the "5V" and "GND" pins; this might in fact come from another Arduino's 5V and GND pins.
I would investigate getting one of those Lithium-Ion cell packs that put out an ostensible 7.2V and interfacing that to the DC power jack - organizations like Tenergy make trustworthy ones.
Check the power necessities of your Arduino project against the power limit of the cell packs to work out generally how much run time you can anticipate while utilizing them to drive it.
Standard Engineer's reaction: It depends!
A solitary Lithium-Ion battery's terminal voltage will go from 4.2V completely energized, down to as low as 2.7V totally level, yet regularly no lower than 3.0V - it relies upon the specs of the cell, and of the Cell Protection Module that is essential for the cell (prepares for working at too low or high a voltage, or drawing a lot of current).
Consequently that lets us know that we want to focus on the Vcc supply voltage that the microcontroller (and all the other things on the Arduino Uno) will dependably run at.
The central point of contention here is, What speed (MHz) do you need/need to run the Arduino at? Since you can't run the Arduino's microcontroller at max MHz right down to least Vcc. That ATmega328P on an Arduino Uno has a 'Safe Operating Area' of what speed in MHz it can run at, at different Vcc supply voltages:
That is from page-303 of the ATmega328/328P (and its sisters 168A/168PA, 88A/88PA, and 48A/48PA), every one of its 650 pages.
So in the event that you supply the Arduino Uno with >4.5V, you're great as far as possible up to 20Mhz! (The Uno is planned with a 16MHz precious stone).
However, assuming your Vcc is given straightforwardly from the Lithium-Ion battery (for example with just a Lithium-Ion battery associated with Vcc, no voltage controller, you put the battery +ve into the +5V pin of the Power-Header of the Arduino Uno), then, at that point, it could go as low as 2.7V. At 2.7V, the '328P is possibly ensured to work dependably on the off chance that you clock it at 10MHz! Truly this is a - 40 to +85ºC rating. At 'room temperature', you could very well have the option to run at a higher MHz even at that low 2.7V Vcc.
So you have two options!
Arrange the clock pre-scale register with the goal that your Uno runs constantly at 10MHz, and along these lines be ensured to experience difficulty free activity even as your single Li-Ion battery gets right down to level, or…
You really want a lift switch-mode controller that can take the Li-Ion battery's 2.7-4.2V territory and lift it up to a consistent 5.0V, and that will allow you to run at 16MHz constantly.
Assuming you choose to involve 2 cells in series, that gets you a consolidated battery terminal voltage scope of 5.4V to 8.4V. Since that is a lot to supply the '328P straightforwardly, you would place that into the Uno's 'Vin' terminal, where its installed 'direct' 5V voltage controller will then, at that point, supply the '328P with a decent steady 5.0V.
On the off chance that you're not drawing a lot of force for Other Things associated with your Uno, then the Uno's ready 5V 'straight' controller will cheerfully adapt to that without getting excessively hot. Assuming you ARE controlling "critical" different peripherals (a series of LEDs, a ravenous GPS module, and so on), then, at that point, that could be an issue for heat scattering in the Uno's ready 5V controller sooner.
ETHER WAY, you're actually squandering a LOT of battery power (for example run time) - all that power (P = V x I) between 5.0V to anything the battery's terminal voltage is (between full and level) must be squandered as hotness with a 'direct' voltage controller. So in a 2-cell-in-series course of action, I'd utilize a 'buck' change mode controller to direct that 5.4-8.4V from the battery back to 5.0V with normally 90+% effectiveness and not much hotness.