Motor Vehicle Maintenance & Repair Asked by FreakyDan on July 12, 2021
Sorry if this question is long, I am trying to include all the details in case that helps.
My 2008 Honda Civic would not start after I had changed the brakes (left the trunk open and a door cracked so the interior lights were one). Every time I tried to start it turned over less and all signs pointed to it needing a jump start.
I used a 2002 Buick Century to jump start the car. The issue with the Century was that its battery it didn’t have clear indication of positive or negative (Turns out is had a slight outline for positive and negative on the top that thin dust hid, please don’t be like me and actually check) so I assumed the bright red label near one post and a black label near the other signified the positive and negative.
Needless to say my assumption was wrong. The batteries began to short and melted the protective plastic layer on the jumper cables (If this happens to you, make sure you only touch the clamps and avoid the cables, because while preventing potential further damage is good, second degree burns are no fun).
One I had the cables off and treated my burns I went to check the warning lights of each car to see if I broke anything. To my relief the Century started fine with no warning lights, and to my amazement so did the Civic.
Why did the failed jump start get the Civic to start? The battery didn’t have enough charge to start the car, and hooking the jumper cables up backwards should have only drained the battery further.
Update: Ultimately the issue was the starter. If the car had been started recently (within the last few hours), then it wouldn’t work. The time spent getting the other car moved over and attempting a jump-start allowed the starter to cool off enough. Replacing the starter resolved this issue. (Thankfully this didn’t stump me for over 3 years, I just forgot to update the post)
Are you sure you had the polarity reversed? I've had jumper cables melt their insulation while connected with the correct polarity. If the receiving battery is empty enough, the initial charge current can be huge, and cheap (thin) jumper cables can't always cope.
Answered by Hobbes on July 12, 2021
I would call it C.P.R starting. It gave your battery the kick it needed. As for the battery. Positive is always bigger around then the --
Answered by Danny on July 12, 2021
If you hook up two unequally charged batteries in series in a circuit, the stronger battery will charge the weaker one. That's effectively what you did with your short circuit across the two batteries.
So while you nearly melted everything, you also gave some charge to the weaker battery, just not in a particularly safe manner.
Answered by Separatrix on July 12, 2021
You connected the batteries in series. The resulting circuit (Battery A -> Battery B -> Battery A) draws an insane amount of current. The cables act like a resistor in this configuration, which generate heat. That heat is what melted the cable insulation. If you leave it in this configuration for long enough, the batteries can actually explode or catch fire.
Since the harness in each car is connected to its own battery, you didn't get a voltage spike in either vehicle. That would most likely have fried multiple electronic components, or at least a couple of voltage regulators. If the car was turned off at the time, there wouldn't be any current draw (open circuit) other than, potentially, for your accessory plugs, alarm or other things that are on when the car is off.
Now, as to why this allowed you to start the car: Any potential difference across the terminals of a dead battery will cause the battery to attempt to draw current, which will charge it. The battery will continue to draw current until it's internal voltage equals that of the applied voltage, at which point it acts like an open circuit. The same thing happens when you charge a capacitor: initially, it draws a lot of current, but, as it charges, it draws less and less until it becomes an open circuit.
For fun, you can connect a battery to an LED, resistor and a capacitor in series. As soon as you connect the battery, the LED will glow, then fade out quickly.
Answered by 3Dave on July 12, 2021
The thin corrosion on terminals sometimes functions as diodes –lets current out but not in. your circuit allowed high current flow that may have broken the corrosive barrier. This concern is why it’s smart to clean the terminals and posts as a first, easy maybe fix.
Answered by TomO on July 12, 2021
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