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Topic: Batteries discharging (Read 1 time)previous topic - next topic

kevinsa5

Feb 06, 2011, 09:14 pm
Alright first off, I do understand the concept of a complete circuit and generally how electricity works. Just thought I'd get that out of the way.

What I don't understand is how a battery 'knows' the difference between its own negative and another negative. For instance, when I put the + of one AA to the - of a different battery, why doesn't it discharge? The only thing I can come up with is that the inside of a battery is comparable to a capacitor in that one side can't accept electrons without the other side donating them because they'd repel each other. Again, I know it has to have a complete circuit, so I'd appreciate it if that phrase was left out of any and all replies  XD

retrolefty

#1
Feb 06, 2011, 09:19 pm
Quote
Again, I know it has to have a complete circuit, so I'd appreciate it if that phrase was left out of any and all replies

Well that restriction removes all valid circuit analysis and answers to questions you might have.

Without complete circuits you are restricting topics to electrostatic properties which is a small part of the physics of the electronics field.

Lefty

kevinsa5

#2
Feb 06, 2011, 09:21 pm
d'oh. Okay, I guess my question is now "why isn't the + from battery A to the - of battery B a complete circuit?"

retrolefty

#3
Feb 06, 2011, 09:23 pmLast Edit: Feb 06, 2011, 09:26 pm by retrolefty Reason: 1

d'oh. Okay, I guess my question is now "why isn't the + from battery A to the - of battery B a complete circuit?"

Because there isn't a complete  path between battery A and B for current to flow. But of course that's not the answer you will except, is it?

A complete circuit would also require a conductive path from -A to +B. It's the same reason you can't just wire from the +A to one lead of a lamp and expect the lamp to conduct current.

kevinsa5

#4
Feb 06, 2011, 09:29 pmLast Edit: Feb 06, 2011, 09:32 pm by kevinsa5 Reason: 1
Quote
Because there isn't a complete  path between battery A and B for current to flow.

Precisely the answer I was trying to prevent with my original wording

So is it a hardware limitation or something else? I'm really hoping it's not simply "that's just the way it is"

Edit:
Quote
A complete circuit would also require a conductive path from -A to +B. It's the same reason you can't just wire from the +A to one lead of a lamp and expect the lamp to conduct current.

But what IS that reason?

retrolefty

#5
Feb 06, 2011, 09:38 pmLast Edit: Feb 06, 2011, 09:40 pm by retrolefty Reason: 1

Quote
Because there isn't a complete  path between battery A and B for current to flow.

Precisely the answer I was trying to prevent with my original wording

So is it a hardware limitation or something else? I'm really hoping it's not simply "that's just the way it is"

Electrostatic charge is a real force in electronics. A good example is the gate terminal of a insulated gate MOSFET transistor. Because it's insulated from the other terminals there is no steady DC current flow needed to turn on the device. However to charge that gate terminal still requires a 'complete circuit' in that the applied positive gate voltage must have it's negative terminal wired to the source terminal. This creates the 'complete circuit' so that the positive voltage potential is felt between the gate and source of the mosfet and a charge voltage can then be developed.

It's the same thing as why a bird can land on an energized 20,000volt power wire and not feel a thing, because there is no complete current path back to the power wires return potential.  Connecting just the + of one battery to the - on another battery just does not create a potential difference between them. The + terminal on a battery means absolutely nothing without referencing the same battery's - terminal.

Lefty

kevinsa5

#6
Feb 07, 2011, 01:29 am
Alright. I don't like it, but I guess that doesn't really matter.

On a slightly related note, if I have several separate units (heat pumps, say) connected to arduino inputs and they all have separate power supplies, connecting all the grounds together will not be bad at all and will in fact be necessary to measure voltage differences between each unit?

retrolefty

#7
Feb 07, 2011, 02:07 am
Quote
connecting all the grounds together will not be bad at all and will in fact be necessary to measure voltage differences between each unit?

Yes, necessary unless some kind of galvainc isolation (opto-isolators, transformer coupling, etc) is used between the different 'circuits'.

kevinsa5

#8
Feb 07, 2011, 02:47 am
right, thanks lefty!

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