I am using an Arduino WiFi Rev 2. Would it be possible to use two 9V batteries in parallel to power the board via the power port? I want to increase the overall current to increase battery life thus using two batteries in parallel instead of one.
Two times nothing is still nothing 
Using batteries parallel can cause unwanted chemical reactions.
The Arduino Wifi Rev2 is not designed for battery operation.
For example the MKR Wifi has a connector for a Li-ion battery.
An option is to use a large 12V battery and use a DC/DC-converter for the Arduino board.
I Am really shocked that you would have to ask a forum that question. Why not just try two batteries and see if it works for you?
Paul
You can't because they will cross charge, even if they are not rechargeable batteries. You could connect each battery through a diode to what you want to drive to avoid this, but you will loose 0.7V using this technique.
So long as the batteries are identical this will increase the available current and capacity by a factor of two.
However what you should be doing is determining what the current and capacity requirements of the WiFi rev 2 is, and then selecting a power source that can provide that, i.e. not trying to guess that doubling a 9V battery is enough - i.e. do the maths.
Could you elaborate on this, please? Is it something specific for the Arduino WiFi Rev 2? How about the regular Uno R3?
P.S. this topic has made it into google results already, lol.
The 9V batteries that Paul ghosts are fine for a short classroom demonstration, but the mAh rating of the 9V battery is so low that it would have to be replaced daily. Not very practical for a working project. A better solution would be to use a 12V battery (like a 4S LiPo) and a DC-DC buck converter to get 5V for the Arduino board.
Batteries in parallel is risky because no two batteries are identical. One would have slightly more potential then the other, and depending on the chemistry of the battery you could either have wasted energy in heat or, unlikely, a catastrophic meltdown.
Trucks and trailers do often put lead-acid batteries in parallel, but lead-acid batteries are more tolerant of a small imbalance.
As Shakespeare said “ahhh there’s the rub”
Two in my truck and two in my 5th wheel trailer. You always get a voltage equal to the lowest voltage of the two batteries, but twice the current of one. The also charge to the lowest voltage.
Paul
Oh, so it was about the practical side.
I thought there may be some kind of incompatibility that may damage the device, lol. The only possibility i could think of was if those are some fancy rechargeable batteries with built-in boost converter, whose working frequency would cause interferences
It is the fact that changing batteries every day, while possible and expensive, is not what any sane person would call a practical solution.
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