Absolute voltage values at battery terminals

[klng]

Hi all,

Lets say i have a battery with a rated voltage of 3V. This means that there is a potential difference of 3 V across the +ve and -ve ends of the battery. This can mean 3V/0V, 5V/2V, 6V/3V and the list of possible combinations goes on.

My question:
Is there any way of knowing the absolute voltage values at the +ve and -ve terminals of the battery? Can we safely take it that the -ve terminal is at ground potential (i.e. at 0V)?

 

[f95toli]

There is -by definition- no such thing as "absolute voltage".
Voltage is just a measure of potential energy and as you know there is no such thing as "absolute" potential energy: the value of the gravitational potential energy of a ball on a table depends on if you calculate it with respect to the floor or the moon.
The same thing is true for voltage.
The point is that it is the difference that matters.

 

[Naty1]

Can we safely take it that the -ve terminal is at ground potential (i.e. at 0V)?

absolutely NOT: you can ground the positive terminal or the negative terminal...it's all relative. further, if neither is grounded via a connection it's likely the voltage will vary over time relative to ground while maintaining the 3 volt difference your note. Grounding is a potentially complex and interesting subject rarely dealt with in undergraduate studies.

I can't think of any circuits that are normally positive ground...some boat dc (battery) circuits were made that way until maybe the 19030's or early 1940's but it was discovered they were more susceptible to corrosion so as far as I know all such marine circuits are negative ground today.

Are car alternators ALL negative ground now?? Some used to be positive...I do not know what the current stabndard is. GM,Detroit Diesel and Ford have been negative ground for a long time...

 

[klng]

Thank you both for the wonderful and insightful replies.