Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Battery boosting debate

Help me solve an on-going debate between me & a colleague at work…

At the Hotel where I do Valet we will, when able, help guests with dead batteries.

My friend usually handles it and he simply runs the positive terminal from the booster to the dead battery, and does the same thing with the negative terminal.

I always point out that the negative terminal needs to be hooked up to a good ground on the boostee vehicle, not to the negative terminal.

Whenever I point this out he gets very defensive and basically ignores my advice.

Who is right?

I was taught that you want a ground away from the battery b/c when you complete the connection you often get sparks. Batteries can produce explosive gases and you want the sparks away from the source.

Tell your friend to keep doing it that way if he wants to blow himself and a guest’s car up.

First question is, does the ground cable from the battery make good contact with both the battery and the chassis? If it does, then it makes no difference where you hook the jumper, the terminal or the chassis. If it does not, then you wont get a full circuit anyway, and while it again makes no difference, the battery will not charge after you get the engine running by most likely bump starting it. Because with no ground, the thing shouldnt crank. So the short Answer is maks neekes.

since you conn. the dead battery first, there will be no sparks. This is the order of connecting a jumper. Red to dead pos. Red to live pos. black to dead neg. black to live neg. With order you avoid sparks on the jumped bat. So, if you need to feel safer attach the black jumper to the chassis on the helper car. Disconnect in opposite order.

I think he is talking about a battery booster, not jump-starting the vehicle by connecting it to another vehicle.

Does the booster have an on/off switch? If you connect the cables before you turn the booster on, it won’t matter if you hook the negative cable to the battery or a grounded part of the car body. There won’t be any spark.

Just to clarify, I’m talking about hooking two cars up.


In that case, you are technically right. However, with today’s car batteries being sealed quite well, the chances of an explosion are very remote, and your colleague might go on to live a long life without winning a Darwin Award. However, to err on the side of caution, I suggest you stay at a safe distance whenever your colleague has jumper cables in hand.

If you can’t convince your colleague to do it the right way, the least you can do is see to your own safety and refuse to assist.

“I always point out that the negative terminal needs to be hooked up to a good ground on the boostee vehicle, not to the negative terminal.”

You got that right! Tell your friend that if he wants to get sulphuric acid in his eyes some day, to just keep on doing it his way. Doing it your way is safer, but creates a little more resistance in the jump circuit. If you absolutely have to connect the neg clip

to the neg battery post, blow across the top of the dead battery to remove any hydrogen gas & then turn away as you make the connection. Even if there’s no hydrogen gas present, the huge surge of current going from a strong booster to a completely dead battery can heat the dead battery to the point of explosion–rare, but when it happens… Also turn away when you make the initial disconnection. The final connection makes a spark; the
initial disconnection makes a spark, too.

Personally, if the neg connection on a ground far from the battery doesn’t succeed, I’d tell the car owner to call a tow truck.

More info:

You are both right. The negative cable can be connected to the negative side of the battery or the egnine block. Using the engine as a ground can avoid sparks that can cause the battery to explode.

Hydrogen gas is the lightest gas in the universe. It rises. Blow across the top all you want, unless the battery is constantly outgassing there will be no gas there to start with. And if you follow the correct order of conn. as outlined above, the bad battery will not explode because there can be no spark. By the way, a good battery does not put out much gas, and that is why the battery fluid, sulfuric acid and water, does not have to be topped up very often. If either battery is going to have gasout, it will be the bad one.

“I always point out that the negative terminal needs to be hooked up to a good ground on the boostee vehicle, not to the negative terminal.”

Finding that “good ground” can be difficult in today’s plastic world…The REASON for this is to prevent a hydrogen gas explosion ignited by a spark near the battery. But hydrogen gas, being VERY light, QUICKLY dissipates into the air so the chances of it accumulating to an explosive level are remote. In order for a battery to produce enough hydrogen to become explosive, the battery would have to be in a heavy overcharge condition and gassing vigorously, like on a battery charger that had been left on too long or set at to high a rate. HERE is where battery explosions occur.

I have the habit of bending over and blowing across the top of the battery just before making that last connection to dispel any accumulated gas…If you DO get a big arc, INSTANTLY break the connection as you have connected the cables backwards! It is wise to make the connections with the donor car not running. Start it only after the connections have been properly made…


You are being good people by lending a helping hand but are taking a risk that you might ruin somebody’s vehicle electronics if you should in error choose a negative terminal on the donor car thinking that it is positive. Hopefully your employer will cover for you if this happens. Always connecting the negative cable to metal on the dead car reduces the chance of this possibility but does not eliminate it.

Live car to dead car using battery terminal connection possibilities

    • to + and - to - = OK.
    • to - and - to + = blow electronics.
    • to + and + to - = blow electronics.

Live car to dead car always using metal on dead car for negative cable.

    • to + and - to metal = OK.
    • to - and - to metal = harmless short circuit.
    • to - and + to metal = harmless short circuit.
    • to + and + to metal = blow electronics.

Using metal for the negative connection on the dead car gets my vote.

You’re right.

If you jump cars on a regular basis and especially if you’re near your friend when he does it the wrong way, I’d suggest keeping a pair of safety glasses around to wear. The chances of an explosion are pretty small, but it’s your eyesight at risk if it ever does happen. In fact, your employer really should have this as a required policy.

Metal is - when you conn. to it you conn. to the neg terminal of the battery. The polarity of the batt. is marked on the bat. and hard to miss unless you really try.

It’s more important that you connect the dead car first and the good car last. You want the spark of the final connection to be at the good car, because it’s battery is less likely to be venting hydrogen.

Your friend is wrong. I used to jump a car that way many years ago because that was the way it was done. I hooked up a battery one day to have both battery posts fly about 100 feet up in the air. I had to spray my eyes out with a garden hose. I lost a good shirt and a nice pair of jeans to the acid. I came out very lucky. I’ve jump started vehicles differently ever since. Show your friend these posts and maybe he will come to his senses. Again…you are right and he is wrong.

I once gave someone a jump start who had the red battery cable hooked up to the negative terminal and the black battery cable hooked up to the positive terminal. I even asked him to confirm which was positive and which was negative. As soon as I saw that arc, I disconnected the cables and found the + and - markers on the battery. Now I don’t take the battery terminal cables for granted and always confirm the terminal polarity by finding the symbols on the battery.

I think if they are going to continue providing this service, they should get their employer to buy a battery booster so they don’t need a donor car. I believe many boosters also have a safety feature that detects when you hook up the cables to the wrong terminals.

Just an example about battery explosions. We had a dead Subaru into the shop one day just before lunch. After pushing it in I connected the large roll-around battery charger (which had a shaky connection on the negative cable clamp).
About 10 minutes later while up in the office and debating with some other guys where lunch was going to be that day we heard a loud boom that sounded like one of those fireworks concussion bombs. Some of the overhead fluorescent light fixtures were still swinging on their chains.

Going out the shop we saw no smoke or anything. After some looking around I discovered the strong gusty wind that was blowing through the shop had apparently caused the cable to move a bit, which in turn caused a small spark at the cable/clamp connection.
The boom was that hydrogen gas blowing up.

Odds are the customer needed a new battery anyway and this removed all doubt. Just lucky that no one was standing there when it happened.