Jumping a Car Battery

Discussion in 'The Garage' started by Shockmaster, Apr 28, 2018.

  1. Shockmaster

    Shockmaster Registered User

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    Not too long ago I had to help a neighbor jump-start her car. I pulled my car up next to her's and connected the cables to each positive and negative terminal of each battery as I usually do. My neighbor started to panic as she read on the internet that connecting to the negative terminal on a dead battery could cause a spark, which in turn would ignite hydrogen from the dead battery and cause an explosion. I told her I've jumped batteries this way before and nothing bad ever happened, but she wouldn't have it. So I had to search for the metal ground connection in her car's engine (which was put in the most ****ing stupid place by the manufacturers). I eventually found it and got her car started for her.

    Later on I searched the internet myself and found a lot of material backing what she said about not connecting the cable to the negative terminal of the dead battery. However, whenever I ask people who work in repair shops or people I know that are good with cars, they say there is nothing wrong with connecting the cable to the negative terminal of the dead battery.

    I'm curious on what other people think. Is it really unsafe to connect the jumper cable to the negative terminal of the dead battery? Or is that basically an old wives tale?
     
  2. Winger98

    Winger98 Moderator

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    I'm with you, I always just hooked each cable up to the its corresponding terminal and jumped away. I've heard it's dangerous, and I'm sure someone will put up a youtube within five minutes of my posting this, but I've never actually seen anything bad come of it. For me it's like cell phones at gas stations.
     
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  3. Nullus Reverentia

    Nullus Reverentia Hic Sunt Dracones

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    Every mechanic I've heard whose talked about this say it's a load of ****. Urban myth it sounds like.
     
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  4. GarbageGoal

    GarbageGoal Heel, you say? Sponsor

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    Even most jumper cable instructions say connect it to a bolt on the engine block or some other grounded metal part.

    I think for the most part it's fine, but if something is messed up in your electrical system you don't know about (yet) it's the easiest way to cause an issue.
     
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  5. ProstheticConscience

    ProstheticConscience Let 'er ryp.

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    Urban myth. The negative terminal on the battery is just a ground that completes the circuit anyway.

    I've had to jump-start a lot of vehicles. Used to work in RV rental. Done it probably hundreds of times. Never had any issues with connecting jumper cables to negative terminals.

    The real danger is mixing them up and reversing polarity. Then the sparks fly.
     
  6. Chonged

    Chonged Registered User

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    Just Have to Make Sure Once the Car Is Started to take the Cables Off Immediately

    the Blow Up thing Is When You are Double Charging a Battery
     
  7. New Jersey

    New Jersey yung nico

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    I have a 2013 Impreza still under Subaru warranty and a jump starter battery pack.

    One time I left a map light on all night and the battery went flat.

    Positive on first, off last. Nothing bad happened.
     
  8. Roboturner913

    Roboturner913 Jerky McJerkface

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    Attaching to the negative terminal can cause a spark. I've seen it happen. Is it super dangerous? I doubt it. I guess if you had a pretty big buildup of hydrogen gas, but you'd have to charge the battery for hours and hours for it to create enough gas for a spark to ignite.

    But I mean, it's usually super easy to find an unpainted metal surface to ground to. I usually use the hood latch.
     
  9. Plural

    Plural Registered User

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    It can cause a spark. It won't explode. Car batteries are grounded from one terminal anyways.
     
  10. Plural

    Plural Registered User

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    Yeah, the negative terminal is grounded. Mixing the cables is dangerous.
     
  11. TD Charlie

    TD Charlie Registered User

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    Red on red. Black on black. I’ve never done it any other way. I did have a battery explode when i was 16 though. That was wild.
     
  12. JMCx4

    JMCx4 Gateway to Hockey

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    The risk of generating a spark when making the final connection to the dead battery's negative terminal is very real, especially on a battery whose terminals have corrosion. And since you cannot pre-detect any possible hydrogen gas being emitted by the dead battery - and even though the emitted gas levels may be very low - the standard recommendations are to perform the jumping procedure in a well-ventilated space AND to make your final connection to a clean unpainted part of the car's frame as far away from the battery as practical. [NOTE: @Roboturner913 suggestion to use the hood latch as an electrical ground point is dubious, because it has at best a poor connection to the car chassis ground via several high resistance mechanical links.]

    @Winger98 ended his advice with: "For me it's like cell phones at gas stations." What should be added is: "It's rare, but still a safety risk that's avoidable." Better safe than sorry.
     
  13. Winger98

    Winger98 Moderator

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    Eh, if you want to go with the eliminating all risks angle, I won't argue it re: jumping a battery. The cell phone thing is a myth, though.
     
  14. Timmer44

    Timmer44 Registered User

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    A lot of misinformation here. You can't "double charge" a battery. You can draw current from the other battery, but it's not like voltage is doubled. It might do some weird things with your car computer, but wont cause a battery to blow.

    I work with batteries daily. They can vent gas when charging, which can be hazardous if not vented properly. If your battery is sitting under your hood and needs boosting, it likely hasn't been charging recently. Also, an open hood will immediately vent the area. I've never heard of one exploding during jumping.
     
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  15. Chonged

    Chonged Registered User

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    Funny, that is the only time I have heard of a car battery exploding, and explain to me how if you have two alternators pumping voltage into a battery it isn't in a sense double charging? I may have used incorrect terminology, but my point stands that you just doubled the amps.

    It isn't like one of the alternators just stops because there is already enough voltage being pumped to the battery...

    care to explain?
     
  16. Timmer44

    Timmer44 Registered User

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    Essentially, it sort of does. My alternator will only supply current while the output voltage is higher than load voltage it is connected to. The alternator (and battery) supplying the higher voltage, will supply the current. IE. You will not simply double the current flowing into the battery. Not to mention all the other components the alternator/battery will be supplying current to while a car is running.

    That all being said, you can absolutely damage batteries by supplying too much current while charging, which is what I believe you meant. The high current can short the cells inside the battery causing them to melt.

    I'm not a mechanic, so I don't know if there is any circuitry limiting alternator voltage. I'm actually a little interested to see the current draw from each vehicle when jumping a car. If I have some free time at work I'll give it a shot and measure what is actually going on.
     
  17. Chonged

    Chonged Registered User

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    Thanks for testing this.

    I am pretty sure the standard output from an alternator is 14.1 or 14.2 amps. You can test this right at the alternator, or at the battery post. I would imagine if you had two alternators pumping into one battery (even through cables as the other battery is fully charged) that it would in fact double the amps being pushed into the battery.

    let us know what your test results are.
     
  18. Timmer44

    Timmer44 Registered User

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    Standard car batteries are around 50Ah, and even if current was doubled (which I explained above, will not happen), it's below the expected tolerance of charging current of the battery.
     
  19. VickAshley

    VickAshley Registered User

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    NEVER have the car you are trying to "jump" running when you are jumping it. Both cars should be off.
     
  20. Winger98

    Winger98 Moderator

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    Good news, had to jump the wife's car this morning and nothing exploded.
     

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