I'm embarrassed to admit, but I haven't done a lot of hands-on with microcontroller-based systems. So my knowledge here is lacking, but say if I have an Arduino powered by a supply that's plugged to the main and say I'm controlling external circuits through it's GPIO. The external circuits are powered by a battery. So what you usually do is have a shared ground for the GPIO and the battery powered external circuits, right? It's because the battery is an isolated voltage source.

What I mean by an "Isolated" voltage source here is a voltage source not "clamped" to a ground, that from the ground of another system (the power supply plugged to mains) is higher or lower. The battery, through electrochemistry, simply maintains the voltage it's meant to across it. We can think of it's ground being pulled up or down to the other ground it is shorted with.

So then, assuming the "shared ground" of the Arduino is the same as ground of the power supply plugged to mains, when you connect battery's ground to it, everything still works. Going further, if we have say another evaluation board or any closed system that has multiple power rails (one that has 12V, 5V and -/+3.3V), we can use a battery to power these rails, if it is connected to ground.

Is there anything wrong with this logic?

Further more, maybe we can create a 12V by having a 7V battery (though no exact 7V exists) in series with the positive terminal of the 5V?

 

hi ren,
Do not think in terms of 'ground' , consider it as a Common 0V connection or reference line.

You could connect the negative terminal of your 7v battery to the +5v power connection, the positive terminal of the battery would be at +12v with reference to the 5v common line.
E