The advice for jump starting a car is to always connect the negative terminal from the donor car to the engine block (or other metal area) of the dead car. I’ve been wondering about this. Why wouldn’t connecting the negative terminal of the donor battery directly to the negative terminal of the dead battery work? In fact how is the current supposed to flow through the dead battery if the donor battery is not connected across it?
My theory is that the negative terminal of the donor battery is grounded to the chasis and the reason for not connecting to the negative terminal direct, is to keep any sparking away from the battery. But then surely the advice should be “Connect to a metal part of the engine as far away from the battery as possible”. Am I close?
I am not a mechanic, but as far as I know, connecting both the positive and negative terminals will work to jump start the dead car. However, the reason this is not good practice is because it can drain the good battery, whereas connecting negative-earth cannot. As to why this latter method is effective at creating the required circuit, that is where I need to ask someone with more electrical knowledge. I’m sure they will be along shortly (no pun intended).
Connecting the cable to the negative terminal of the battery or a metal part of the car accomplishes exactly the same thing. The reason you do it is so that when it makes that little spark upon connection, you’re not making it right next to the battery.
A HS teacher of mine, whether correct or not I don’t know, told us that when you start a car the battery generates a little bit of hydrogen gas. When you spend a few minutes trying to start the car on a battery not strong enough to start it, you generate a lot of hydrogen gas. That creates an explosion risk…hence wanting to make that spark somewhere else. Of course, if that reason, you’d think you’d make the final connection…the one that sparks, on the good car. Besides, aren’t batteries totally sealed anyways.
As I said above, connecting in either spot is exactly the same. If you ever take a look under your hood (this is easier to see in older cars). The cable that comes from the negative terminal of the battery runs right to the frame of the car. Most metal on the car is electrically attached to the negative terminal of the battery.
Ah that makes sense. Still it is weird that if an explosion is the reason for connecting to the car frame then, as you say, why not connect to the donor battery last?
Yeah, that part never really made sense to me. I think it’s more about keeping the spark away from the battery…either battery. As for connecting to the recipient battery last, the only thing I can think of is that since you want to make the final negative connecting away from the battery, might as well do it on a car that doesn’t have a bunch of spinning parts.
Most of the metal on the car is connected to the negative terminal. Electrically speaking, connecting to the metal on the car is the same as connecting to the negative post. Anytime I jump a car, my Dad tells me about when he was a kid and they just brought the cars close enough together that the (metal) bumpers were touching and connected the positive terminals.
#3-4 were supposed to be done some distance from the batteries to minimize risk of explosion. It’s less likely on the healthy battery, but not unheard of, and as bad as a battery explosion is, there’s no good reason to risk it. Connecting to the good car last generally keeps you far away from the dead car’s battery (and at least not right next to the good battery) when completing the circuit.
On a typical car, the starter is the only component with a direct ground connection to the battery, since it moves such a massive amount of electrical current; (just about) everything else on the car is grounded to the chassis, and then the chassis itself is grounded to the battery. This makes wiring easier/cheaper than trying to bring a ground wire from every component on the car back to a common ground strip. And this is why running the jumper cable’s ground connection from chassis to chassis works.
Wikipedia claims that hydrogen and oxygen are generated by overcharging a battery. Other sites claim that it’s not the level of charging, but the rate of charging that causes release of hydrogen. Still other sites claim that any charging activity releases hydrogen gas. Bottom line is that if you have a truly flat battery, and you connect it to a healthy 12-volt battery (and possibly a running car stepping the supply voltage up to 14+ volts), the dead battery will be charged at a rather high rate; there’s a pretty good chance of hydrogen in the area.
The disconnect procedure I was taught is the reverse of above:
ground connection on healthy car
ground connection on (previously) dead car
connection on healthy battery
connection on (previously) dead battery
Any sparks are likely to happen when/where the circuit is interrupted, which takes place here during step #1, away from any batteries.
And it works. It was particularly useful back in the old days when cables were not connected to each other and you could only find one or one of them was melted.
Confirmation bias? Cars seem to jump faster (i.e., when the battery is so dead that you have to sit there) when the cables are connected to the negative terminal. Or is that because finding an unpainted part of the frame is difficult within range of the cables, and the thin coat interferes with the process?
Battery could explode. Seen it happen. I was about six feet away. My friend has some really ugly scars on his hand, thankfully it wasn’t his face. The inside of the hood of the car got corroded pretty bad too.
My method when jump starting someone else is + to +. then negitive to my car followed by negitive to other car engine bare metal.
H2 is created when a battery is charged at a rate higer than the battery can react. Over charging a battery causes gassing, or charging at a very high rate.
Today most batteries are sealed so the danger is not as high as it was.
Could be either way. The main thing is that a good solid connection is required. If the ground cables and their connections are good, then a good jumper cable connection to the metal of the body or block will work as well as a good connection to the terminal. In my observation, most problems come from a poor connection at one or more of the jumper cable clamps. It’s generally easier to get a good clamp connection onto a battery terminal, due to its size and shape, than onto other parts of the car.
Just any old metal part under the hood will not do when it comes to connecting the ground cable. There are metal parts under there that while grounded CANNOT carry the amount of current you’ll get when you attempt a jump. I’ve heard stories of people melting, destroying (and possibly catching on fire) expensive stuff in there.
The safest thing to do is read your manual and connect where they say. Next safest to connect directly to the engine block. Next safest after that is to connect to some really stout metal part that is **obviously **bolted the engine block.
Every time I do a jumper cable connection, if possible, I turn my whole head around so that I am facing away the battery. If you can’t do that and don’t have goggles at least cover your favorite eye with your free hand and squint a lot.
Yeah, a lot of things are easier to see and figure out in older cars. It is all right out there, and intended to be easy to work on. In fact, the owner’s manual for my '62 has instructions on how to push start it!
That was another thing my dad taught me when I was little. I think he put the fear of jesus in me. To this day I have a hard time paying attention when I make that final connection.
But I think he had a friend that had a battery explode in his face.
I’m with JoeyP here – you don’t want a spark near the battery, and a discharged battery allegedly generates more gas.
1 and 2 can be interchanged w/no difference, but 3 should be good battery negative, then 4 should be frame/engine block on car with dead battery – anywhere a spark won’t cause an explosion.
Sometimes it helps to disconnect the bad battery from the system if the car still doesn’t start.
Most times it helps to start the good car so the alternator can kick the voltage up a bit, for an extra oomph.
See this is what I’m confused about. If the dead battery is the one most likely to case an explosion then why not connect to the negative terminal of the dead battery in step three and then connect to the frame of the donor car in step 4. This would keep any spark away from the dead battery completely.
