What is the relationship between voltage and charge in a battery?

[Alex Hughes]

I'm trying to understand what exactly it means in terms of the voltage of a battery. I know that the voltage in a battery is just the potential difference between the two terminals. But, is the negative terminal used as a relative point and seen as 0? If that's the case does 12V mean that the positive terminal has a potential of 12J/C and the negative terminal is 0? Or can the negative terminal technically be any value and it only matters that the DIFFERENCE in potential is 12J/C. If that's the case what is the actual charge values on the negative and positive terminals of the battery. Would appreciate an in depth answer not just an equation. Thanks.

 

[Drakkith]

Voltage is always a difference in the electric potential between two points, in this case the terminals. It doesn't matter what the electric potential of each terminal actually is, the difference between their electric potentials will always be 12 volts (or whatever the voltage of the battery is).

So your negative terminal could be -12 V and your positive 0 V (0-(-12) = 12), or your negative could be +12 V and your positive +24 V (24-12 = 12).

If that's the case what is the actual charge values on the negative and positive terminals of the battery.

Hmm. A good question and one that I can't answer.

 

[CWatters]

First off batteries don't store charge like a capacitor, in a battery the energy is stored in the form of chemical potential energy so it's not really relevant to ask what the difference in charge is between +ve and -ve plates.

However...

Suppose you somehow arranged your 12V battery so that the -ve terminal was at say 2V and the +ve terminal was at 14V with respect to a sheet of metal laying on the ground. The terminal and plates of the battery have some stray capacitance with respect to that sheet of metal and that would hold a modest charge. This capacitance is usually very small so the amount of charge required to develop 2V would be tiny.

 

[Alex Hughes]

Voltage is always a difference in the electric potential between two points, in this case the terminals. It doesn't matter what the electric potential of each terminal actually is, the difference between their electric potentials will always be 12 volts (or whatever the voltage of the battery is).

So your negative terminal could be -12 V and your positive 0 V (0-(-12) = 12), or your negative could be +12 V and your positive +24 V (24-12 = 12).
Hmm. A good question and one that I can't answer.

Thank you very much, makes sense to me now

 

[Alex Hughes]

Another question, I've heard people refer to the negative terminal as the ground before. Is this technically incorrect? Aren't both the negative and positive terminal both relative to the ground, which is a point that has a potential of 0V? Also, does ground in this sense actually refer to the ground of the earth? If that is the case, is it actually at a potential of 0V? I find that hard to believe. Sorry if I'm rambling I'm just trying to make sense of it in terms of a battery. What is the ground in a battery?

 

[NFuller]

Also, does ground in this sense actually refer to the ground of the earth?

No, it is just understood to be the line held at 0V.

If that is the case, is it actually at a potential of 0V? I find that hard to believe.

Voltage, and any other potential, is invariant under addition of a constant. This means that it is the potential difference that matters, not the actual value of the voltage itself. Say you had a 9V battery in a circuit. You could say the negative side is at 0V and the positive side at 9V, or you could say that the negative side is at 1000V and the positive side at 1009V. Both lead to the same circuit behavior, one is just more sensible to work with.

 

[rcgldr]

Another question, I've heard people refer to the negative terminal as the ground before. Is this technically incorrect?

Consider a series of batteries connected in series. Any one of the positive to negative connections or either end of the chain of batteries could be connected to Earth ground. The point is the voltage at the terminals can be relative.

The terminals in a battery when connected to a circuit and delivering a current have to maintain a relative charge at the terminals that corresponds to the electric field that drives the current.

For a battery in an open circuit, I don't know the relationship between the relative charge at the terminals versus true neutral (no net charge). If one terminal of the battery is connected to a true neutral ground, the if the terminals initial charge was not neutral, then for a brief moment in time, some charge will flow into or out from the battery, but the flow stops because the internal reaction requires charge to flow out from or into the other terminal in order to maintain a current.

 

[CWatters]

In most cars the -ve terminal is connected to the chassis and this is called a negative Earth system. Light bulbs have one terminal connected to the chassis and the other is connected to the +12V terminal of the battery by a wire, fuse and switch to turn it on.

On some old cars the +ve terminal is connected to the chassis and this is called a positive Earth system. The -ve terminal produces -12V w.r.t the chassis. Light bulbs have one terminal connected to the chassis and the other is connected to the -12V terminal of the battery via wire, fuse and switch.

The chassis is referred to as "earth" even though it's only connected to the planet by four insulating rubber tyres. That's because no electrical system in the car connects to anything outside the car so the can't really tell if the chassis and planet Earth are at the same voltage.

Aside: People are sometimes lazy and use terms like 0V, ground and Earth interchangeably. They aren't always the same thing.

 

[Mister T]

If that's the case what is the actual charge values on the negative and positive terminals of the battery.

Depends on the size of the battery, I would think. For example, there a number of different sized 1.5-volt batteries, AAA, AA, C, and D. They each have a different number of milliamp-hours. For example, size AA might be 1500 mAh, which is equivalent to 5400 J. So, 5400 J divided by 1.5 J/C is 3600 C.

Certainly there isn't 3600 C of charge residing on the battery terminals, instead this is the number of coulombs of charge you can expect to get out of the battery, roughly.