connecting multiple floating grounds (batteries)

[electrolust]

If I have a battery, what is its negative terminal's potential wrt earth ground?  Something arbitrary, or are batteries somehow made such that the negative terminal is close to earth ground potential?

If I have 2 batteries in parallel, of the same type (say 9V alkaline), is the construction such that the negative terminals are approximately at the same potential?  Thus allowing them to be connected in parallel?

If I have 2 batteries in parallel, of same voltage but different types (say 1.5V Li and 1.5V alkaline) and connect them in parallel, have I created a circuit where current flows from one into the other due to different negative (and therefore positive) potentials?

I guess I want to understand all that before getting to my real question.  I think the answers are

- similar batteries have chemistry that places the negative potential at similar values, but they may or may not be at some definite potential wrt earth ground -- we don't know and don't care for small batteries (maybe we care for car sized batteries)

- 2 similar batteries have the same negative potential

- 2 dissimilar batteries do not necesarily have the same negative potential

My real question is, if I have 2 independent, floating power sources (batteries or other) that are powering different devices/signals in the same circuit, and I connect the grounds only (not the +V which would create a parallel voltage situation), do I have a problem?

Say I have a signal generator with its own floating power supply, and an op amp with its own, separate floating power supply.  The grounds are connected.  The signal from the generator goes into a 20dB gain inverting CMOS op amp input.  Does this even work?  Doesn't the op amp need to sense the voltage relative to its ground connection, which will be meaningless for the signal since its reference is a completely different ground?  Or is it the case that the op amp can still sense the signal potential (aka voltage) relative to the signal's ground, since the grounds are connected?

Or maybe more simply, in a common emitter NPN configuration, where VCC comes from some power supply and the base input is generated by a different power supply.  Yes there is a "common ground" at the emitter but it's actually 2 grounds tied together.  I think this does work because AIUI the NPN transistor is a current sensing device and current will flow through the device from base to emitter even if the Vbe current flow isn't in the same circuit as the Vce current flow.

This must work because we can attach a portable music player to a mains-powered amp, proving that it does work although perhaps not as I have intuited above.

I am not sure if this works the same for FET transistors since they are voltage sensing (?) not current sensing, and what is the relative point to which the voltage is sensed, given that there are 2 (connected) grounds.

[Paul Price]

When you connect the negative connections of each battery together they are at the same voltage and you can refer to this connection as the common ground and consider this to be the 0-volt reference ground point for any measurements. Feeding an op-amp or a BJT transistor or anything else requires a return connection to complete the circuit. Usually the return or reference connection for a signal is the ground connection.

If the circuit you are working with has a ground that is connected by wire to the earth ground wire via the power supply power cord or some other metal connected to the earth, only then is your circuit ground no longer floating (referenced to the earth ground) and is truly earth-grounded.

[electrolust]

When you connect the negative connections of each battery together they are at the same voltage

But not when they are separate?  How does that work?

[rs20]

When you connect the negative connections of each battery together they are at the same voltage

But not when they are separate?  How does that work?

There is no such thing as "absolute" voltage (aside from static electricity sort of). When you measure voltage with your multimeter, you need to connect two leads to the circuit in question; (you know, the red and the black leads). If the two points you probe are in the same circuit, then you can use circuit theory to figure out what the voltage reading ought to be. If you connect to two separate circuits (e.g., two completely separate batteries), the result is completely undefined. You could put 1000 volts or zero volts across the multimeter terminals and no current would flow either way, there's no complete circuit. So the voltage is practically undefined; nothing is driving that voltage to any particular value.

So a 9V battery always has (approximately) 9V across its two terminals. This says nothing about the voltage of a single terminal, which is generally a meaningless concept.

If you try to re-state your question in terms of relative voltages, it'll rapidly become clear how much more worthwhile this way of looking at thing is, and all of the "contradictions" you're seeing will disappear.

It's worth noting that if you have a node sitting at 1000 volts (relative to a reference of your choice), and connect the negative terminal of a 9V battery to that node, the voltage at the positive terminal of that battery will be 1009 volts (relative to the same reference). The battery doesn't care; can't even tell; what the "absolute" voltage of its terminals are because that's purely an artifact of the reference you choose to measure against.

[electrolust]

Ah.  I get it now.