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Backward battery terminals

Hi , today i was trying to help someone needs a jumper cable from my car so i hooked the car battery correctly and gave him the other side of the cable to connect it with his dead battery i thought he knew what he was doing unfortunately he hooked the cable backward , so now the batteries connected in series + to - , my car stopped immediately (it was running before this procedure) then i removed the cable tried to start my car again the car started fine i checked everything (radio, AC, power windows … Etc) all are working fine , i tested the alternator found it charging as normal also measured the battery and it seems fine , so what could possibly happen to shutdown the car suddenly by itself !!! Anybody has experience with that kind of situations ??

My car is : 2010 Mazda 3


The batteries weren’t in series, they were a dead short! You are lucky nothing blew. You also had voltage go essentially to zero, so I suspect that’s what caused your car to stall.


Thanks Nybo but why the voltage went to zero at my car ?

The powertrain control module will shut down if the battery voltage drops below 10 volts to prevent circuit damage from operating with a low voltage supply. Connecting the other battery in reverse polarity momentarily lowered your battery’s voltage to a low value. The other car may have suffered damage from the reverse polarity.

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Most of the battery current was going from your car to the other battery, so none left to fire your spark plugs. On a newer car like your 2010 you are lucky the ECM wasn’t damaged. Since this occurred with your car’s engine running, it’s possible you may have damaged an alternator diode, so keep an eye on the battery (charging) warning light on the dashboard for anything unusual. Otherwise, continue driving your car as before unless or until a symptom develops. Hopefully all that happened is the voltage delivered to your car lowered to the point the ignition system couldn’t deliver a spark, and no voltage or current spikes occurred, or over-current problems that could blow fuses and fusible links. Given the expensive problems that can occur with modern cars, in anything other than a serious emergency situation, I don’t recommend either giving or getting jump starts myself. Instead, keep some tow truck company contact information in your cell phone, and offer to call a tow truck for the driver with the problem when this happens. If they want to get a jump start from the tow truck, they have that option then.


Thanks george one more thing i want to understand if we connected the two batteries in series + to - the voltage should increase right ? So why my voltage dropped ?? Thats my mystery

That sort of connection is one in which an electrical engineer would say “who knows what will happen”? Your car’s + 12 is connected to the other car’s ground, so it is more like shorting out your own battery with a screwdriver. They’ll be a lot of current flowing, and probably some sparks, but beyond that it depends on how much resistance is in the jumper cables, the jumper cable connections, and the battery internal resistances.

Your car

  • 12 - ground
    | |
  • ground + 12

Other car

A series connection would be

+12 - +12 - ground

That’s not what you had.

Thanks again George i appreciate your help :+1::+1::+1:

A 12 volt care battery consists of 6 2.1 volt cells hooked up in series. With the connection of the first cable, the batteries were in series but with no load on the combined 12 cells but with the connection of the second, you had a dead short across the combines 24 volt battery combination.

I hope that made sense.

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