battery connection screw up |
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battery connection screw up |
dt4 |
Jan 12 2020, 09:57 AM
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#1
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Senior Member Group: Members Posts: 502 Joined: 26-May 19 From: England Member No.: 23,161 Region Association: England |
I have had the battery off the car whilst stripping paint and priming etc.... but thought it would be good to turn the engine over and run it for a while to keep things in good health etc.....
I had the battery on the bench to charge it and then put it back in the car but very stupidly put the negative lead to positive terminal and positive lead to negative terminal. I didnt turn the key as I was checking other things when I realised what I had done. I noticed that there was a smell of smoke coming from the alternator area and the tin on top of the engine in that area was warm so suspect there is a connector to the alternator under there? I put the terminals on the right way around and tried to start the engine but it appeared the battery had drained? I recharged the battery overnight but wouldnt start, again the battery seemed low on charge Apart from seemingly screwing the battery, is there other damage likely to have occured cheers David |
DickSteinkamp |
Jan 13 2020, 10:13 PM
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#2
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Senior Member Group: Members Posts: 500 Joined: 27-February 17 From: Bellingham, WA Member No.: 20,876 Region Association: None |
Electricity is somewhat of a mystery to me so bear with me.
I don't understand how switching the + and - leads can cause a dead short and burn up wiring. I can understand how polarity sensitive devices (like a diode) might be harmed but why wiring? Also, there is so very little that gets power with the key in the off position. How would the radio (for example) be damaged? |
Superhawk996 |
Jan 14 2020, 06:07 AM
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#3
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914 Guru Group: Members Posts: 5,748 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch |
Electricity is somewhat of a mystery to me so bear with me. I don't understand how switching the + and - leads can cause a dead short and burn up wiring. I can understand how polarity sensitive devices (like a diode) might be harmed but why wiring? Also, there is so very little that gets power with the key in the off position. How would the radio (for example) be damaged? Most basic answers: 1) Since chassis is assumed to be ground, it can basically source infinete electrons (in theory) and those electrons moving = current. The chassis ground strap between the body and the transmission for example is about 1" wide and is pretty thinck braided wire that can easily handle several hundred amps drawn though it upon cranking and to run the coil and alternator. When the connections are switched (+ for negative) now all the positive wires become ground. Current goes looking for a ground path where ever it can find it - electricity always tries to follow the path of least resistance. If that path happens to be a small gauge + wire that was only sized for 1A but is now trying to support 10A of current draw being drawn through it, poof it melts. 1a) IF, every circuit had an equally sized positive and negative (ground) wires, this situation wouldn't occur. However, that isn't the way cars are wired. Often there is only a positive wire that feeds the circuit and the ground path is provided by "chassis ground". Now you have an asymetric sizing between positive and negative current paths. Positive is limited by wire size. Chassis ground is very large. 2) Electronic componenets like diodes. A diode is only designed to flow current in 1 direction. Diodes have a characteristic called Peak Inverse Voltage (PIV). A diode will resist reverse current flow up to the PIV but beyond that it will "leak". The rate of leakage depends on how much higher the PIV is and whether or not the diode overheats and fails open. As it continues to "leak", allowing current to flow backward, that situation becomes a basis for situation #1. Small gauge + wires can melt since they weren't sized for that current. Really big and well heat sinked diodes (i.e. the alternator) can have some substantial leakage before the overheat and eventually fail open. What circuits flow current in reverse up until the diode opens is up to the electrons trying to find their way home is somewhat of a guess (IMG:style_emoticons/default/sad.gif) You are correct that if a circuit was completely turned off and has no circuit path, it would be unafffected, unless there are diodes in that circuit that fails, and, creates a path where otherwise it would not have existed when connected properly. |
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