RatherBeFishin
Member
- Location
- New York
- Occupation
- Electrical apprentice
Sounds silly but does anyone have a way they remember how to disconnect a car battery and reconnect the new one? Always have a brain fart in the moment and forget..
Car battery disconnect/reconnect
- Thread starter RatherBeFishin
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Think about why. The idea is to prevent making fireworks.
To the positive terminal, everything (all of the metal) around it is "hot."
To the negative terminal, only the positive terminal is hot; easier to avoid contact.
Thus, only apply the wrench to the positive terminal when the negative is not connected.
To the positive terminal, everything (all of the metal) around it is "hot."
To the negative terminal, only the positive terminal is hot; easier to avoid contact.
Thus, only apply the wrench to the positive terminal when the negative is not connected.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
To convert that rationale to a simple procedure, for those who would rather memorize than have to think:
To generalize this to more than just negative ground vehicle systems, substitute "grounded conductor" for "negative".
But in a split voltage system, like +12, 0, -12, you must not disconnect the neutral conductor, whether grounded or not, before both hots. Same reason you do not disconnect the neutral on an energized 120-0-120 circuit.
When disconnecting, disconnect the negative first. '
When connecting, connect the negative last.
To generalize this to more than just negative ground vehicle systems, substitute "grounded conductor" for "negative".
But in a split voltage system, like +12, 0, -12, you must not disconnect the neutral conductor, whether grounded or not, before both hots. Same reason you do not disconnect the neutral on an energized 120-0-120 circuit.
drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
- Occupation
- Registered Professional Engineer
Disconnect the grounded battery terminal first, and reconnect it last.
This recommendation works on both positive- and negative-grounded systems.
Put a wrench on the grounded terminal, accidentally make contact with the car body and the other end of the wrench, nothing happens.
Do the same with the hot battery terminal and you'll have a red-hot wrench in your hand. A car battery can push a kiloAmp or two at room temperature.
12-0-12 split voltage system? Where has such a thing been implemented?
This recommendation works on both positive- and negative-grounded systems.
Put a wrench on the grounded terminal, accidentally make contact with the car body and the other end of the wrench, nothing happens.
Do the same with the hot battery terminal and you'll have a red-hot wrench in your hand. A car battery can push a kiloAmp or two at room temperature.
12-0-12 split voltage system? Where has such a thing been implemented?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Very common in electronics. Not so much in vehicles.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
And a lot of sparks! It's a mistake that many people make, but most who do will not repeat. Yow!
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
Like 40 years ago I was operating a 955 Cat loader. 24v system. I got the bright idea to hook up a radio to it using only one battery. Worked fine. But them I thought why not a fan too. By the end of the day one battery was almost dead and the other one was way overcharging.
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