For DC batteries ,how does the ground have higher voltage than +ve terminal?
- Thread starter Kaaud
- Start date
- Status
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
110123-0828 EST
Kaaud:
If the + (positive) terminal of a DC source is connected to earth (ground), then the - (negative) terminal will read a voltage to ground equal to the battery voltage.
If you have a 12 V battery with a mid-point tap and connect the mid-point tap to ground, then the end-point terminals will read +/- 6 V relative to ground.
At one time some cars were wired + to ground. Today I believe all production cars use a negative ground system.
.
Kaaud:
If the + (positive) terminal of a DC source is connected to earth (ground), then the - (negative) terminal will read a voltage to ground equal to the battery voltage.
If you have a 12 V battery with a mid-point tap and connect the mid-point tap to ground, then the end-point terminals will read +/- 6 V relative to ground.
At one time some cars were wired + to ground. Today I believe all production cars use a negative ground system.
.
K8MHZ
Senior Member
- Occupation
- Electrician
Assuming his meter was connected correctly, he is stating that he is seeing a reading of -26 volts between the + terminal and the earth.
Since we don't know the voltage of the system, I am going to assume that it is made up of 24 volt batteries that would read 26 volts with no load.
If the system is positive grounded, he should have read zero volts. If it is negative grounded, he should have read positive 26 volts, not negative.
So, his question is how did he come up with such a large negative value?
The only way would be if he was reading from one of the positive to negative connections on a series system that has the positive connected to the metal, or 'positive ground'. One scenario would be a 48 volt system made from 12 volt batteries. Measuring from the point where the middle battery posts tie together to the metal would read values that would change polarity depending on which grounding system was used.
So, it is possible for this to be a reading not to worry about. It could also be an indication of a problem. Without more to go on, you have my best guess.
What would help is to know more about the system. How many total volts, what is the voltage of each individual battery, is is pos or neg ground.
Since we don't know the voltage of the system, I am going to assume that it is made up of 24 volt batteries that would read 26 volts with no load.
If the system is positive grounded, he should have read zero volts. If it is negative grounded, he should have read positive 26 volts, not negative.
So, his question is how did he come up with such a large negative value?
The only way would be if he was reading from one of the positive to negative connections on a series system that has the positive connected to the metal, or 'positive ground'. One scenario would be a 48 volt system made from 12 volt batteries. Measuring from the point where the middle battery posts tie together to the metal would read values that would change polarity depending on which grounding system was used.
So, it is possible for this to be a reading not to worry about. It could also be an indication of a problem. Without more to go on, you have my best guess.
What would help is to know more about the system. How many total volts, what is the voltage of each individual battery, is is pos or neg ground.
- Status