Hello. So I was just wondering if I can charge an external power source (like a 18650 battery) when I connect the Arduino UNO in a USB port. So the scenario will be where the batteries will charge when I connect the arduino to a USB port such as a laptop. Can it do that?
You need a power supply for the charging. Some regulators for charging, and maybe for discharging as well to be safe.
What pin on the Arduino would you connect to? What would your charging circuit look like? The 5V regulator output on the Arduino is of very limited capability, but is irrelevant when you're plugged into USB. When you're connected via USB, you have a 500 mA fuse limiting what you draw from the USB connection; your whole circuit must draw less than that 500 mA, including the Arduino, etc.
So I would say your charging time would be much longer than using a separate power source; if long charge time is not an issue for you, then Others may be able to guide you more.
What a real Uno looks like:
You may have a clone, in which case, well, all bets are off.
There is no need for the Uno, as it has nothing to do. Instead, use a 5V USB LiPo charger module and battery overdischarge protection circuit.
Adafruit sells them, among many other suppliers.
So tell us, is this battery part of a project you're building, or is @jremington on target?
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