Jump starting a car

This isn’t so much a question about car repair but about another question I saw on here recently about turning a battery upside down to jump start another. What kind of idiot leaves all the accessories and headlights on to jump start a car? No wonder it wouldn’t work with cables

I am a bit dubious about the answer to that question. They said the things got burned out because the batteries (jumped car’s battery and the one used to jump it) got connected in series, causing 24 volts to be applied to the devices in the dead car.

Is that really what happened?

The description said the tow driver turned a free battery upside down and sat it on the jumped car’s battery. If the two batteries were oriented so that the positive terminals touched each other and the negative terminals touched each other, that would be just like a proper jump with cables.

No 24 volts there.

If the two batteries were oriented the other way, so that the positive terminal of the free battery touched the negative terminal of the car’s battery and the negative terminal of the free battery touched the positive terminal of the car’s battery, that would not be a series connection, since the car’s devices would be connected to both terminals of both batteries. One battery would be trying to push current through the car’s circuits in one direction and the other battery would be trying to push current trough the car’s circuits in the other direction. And the batteries would be trying to push current through each other, too. I’m not sure what the result of all of that would be, but I’m pretty sure it would not result in 24 volts being applied to the devices in the car. If the car’s battery were completely dead, or very weak, it would be very much like hooking up a single 12 volt battery backwards.

Are headlights sensitive to polarity? The old-style sealed beam headlights would not be, but I don’t know anything about the modern ones. Might a reversed 12 volts blow modern headlights? I doubt reversed 12 volts would blow the diode bridge in the alternator. Perhaps the alternator was dead before the jump (that could be the reason the car’s battery was dead).

About the only way I can imagine getting the two batteries in series would be if the connection from the car battery’s negative terminal to the frame were broken and the free battery were placed so its positive terminal touched the negative terminal of the car’s battery and the negative terminal of the free battery touched the frame. That isn’t what the submitter of the question said was done, but that might have looked pretty close to what she did say was done.

The only other explanation I can think of is that the free battery was of voltage higher than 12 volts. I don’t know anything about higher voltage batteries. Might a tow truck be carrying a higher voltage battery? Is the spacing of the terminals on higher voltage batteries the same as the spacing for a 12 volt battery?

That the free battery was a higher voltage battery seems unlikely to me, but it is the only other thing I can think of that makes much sense.

Something blew out the car’s headlights and alternator (unless the alternator was dead before the jump). But I’m pretty sure that putting two 12 volt batteries together with the terminals reversed, as the answer describes, won’t result in sending 24 volts into the car.

If the two batteries were oriented so that the positive terminals touched each other and the negative terminals touched each other, that would be just like a proper jump with cables. With the addition excitement of a possible splash of battery acid.

Holy smokes! Who would ever do such a thing? Forget about the accessories being turned on . . . . who would ever turn a battery upside-down and put it on another battery? MacGyver might get away with it, but that’s TV. DON’T EVEN THINK ABOUT IT! Rocketman

My though exactly; what a bone head move to even attempt to directly connect the battery terminals. If one or both battery explode that guy is dead, and it won’t be an open casket at the funeral.

77Honda is absolutely correct. I’m amazed more people didn’t jump on the Car Talk guys about this one. 24 volts simply isn’t possible without disconnecting one of the cars battery cables and attaching that cable to the donor battery.

Your 100% correct 77Honda. No way they could get the 24VDC without disconnecting cables. I’m glad others jumped on this topic as this article only appeared in my newspaper today and this is the first time Tom and Ray have let me down. Time for them to take Basic Electronics 101 again, haa haa.

Why the headlight blew out is a interesting subject. Maybe the alternator was still able to produce voltage but it had an internal regulator that got destroyed. Thus the alternator output jumped up in voltage enough to blow out the headlights after that “jumper” battery was removed (regulator damage already done). I would hope that the on board computers, radio and some of the other higher tech electronics would have simple reverse voltage protection, which would normally be in the form of a cheap 5 cent diode in series with the + lead on those devices to allow current flow in only the proper direction. A good protection practice for the person who accidentally connection the jumper cables up wrong too!

Picture the connections in a simple flashlight with two “D” size batteries. There is a connection between the positive terminal of the lower battery to the negative terminal of the upper battery. Between the upper battery positive terminal and the lower battery negative terminal, you will have 3 volts. If you connect these two terminals together, (positive to negative)you have a shorted 3 volt battery stack.
That’s exactly what you get when you jumper two car batteries backwards, except that the you get a shorted 24 volt stack. All of this is assuming that both batteries are equaly charged.

However, when jumpering a weak battery backwards with a good battery. the good battery will force enough current into the weak one to reverse polarize it and deliver power in the wrong polarity to the car. This is how damage can be done.

Remember early last year when we had the long thread about turning the lights on before attempting a start from a low charge battery,something about warming the battery up. That thread went on for days.

Sorry markmast - Your not correct! In your flash light you have two series connected 1.5V batteries. The touching between them is equivalent a cable or wire connecting them in series to give you the 3 volts. In the case of this example, of the 12V batteries, there is no condition of the batteries being series connected. Laying the second battery on top the first only provides for a parallel connection. The battery cables would have required removal from one of the batteries and connected to the other battery to possibly get a series connection.

Think about your flashlight batteries again with the the leads only attached to one of the dry cells. Those leads are not removed and they are connected to a voltmeter. Now if you could connect the other battery, lets assume the end connections actually go slightly over the edge, there is no way you can series connect them without removing the leads from the original dry cell. You would need to remove one of those voltmeter leads! In the case used in the article the car battery wasn’t disconnected.

So sorry, a series connection is not possible as described in the newspaper article. Only a parallel connection which could be “current” improved +/+ and -/- as in a normal battery jumper or voltage opposed +/- and +/- which could cause really problems as in connecting up jumper cables backwards.