So, it's fairly popular consensus online that it's red/positive clips to the positive/red posts, then the black/negative clip to the frame

One site mentions sparks, but you'll still get a spark when connecting black to frame. If positive is the potential shouldn't it be keep disconnected as long as possible?

Why do I connect the positive first?

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    I don't know the actual justification, but to me it makes sense to always attach positive first because it is easy to accidentally touch the lead to something else while installing. If the negative lead were already attached, then touching the positive lead to just about anything on the car that is metal would short out the battery. – Paul Feb 20 '15 at 23:21
  • @Paul ... please put that as the answer, because you are spot on. I almost wrote it out, then realized you'd already done it. This mainly applies to the second car being attached to the jumper cables. Once the first is attached and then if you had the negative attached first, touching the positive cable to any part of the ground would cause the short. There are a lot more ground points than there are positive battery points on a vehicle. – Pᴀᴜʟsᴛᴇʀ2 Feb 21 '15 at 1:25
  • @Pᴀᴜʟsᴛᴇʀ2 Maybe they should but fewer ground points on vehicles. – user15009 May 10 '16 at 20:18
  • @nocomprende - Unless the car is completely made out of a non-conductive polymer, I'm thinking that really isn't an option. I believe even carbon fiber will conduct electricity. – Pᴀᴜʟsᴛᴇʀ2 May 10 '16 at 20:26
  • @Pᴀᴜʟsᴛᴇʀ2 I am thinking that if cars were made out of something really lightweight, like bubble wrap, they would be safer. Non-flammable, of course. The fuel economy would go up too. And, maybe the fuel should not be explosive. So much room for improvement... – user15009 May 10 '16 at 20:29
up vote 8 down vote accepted

Conventional lead acid batteries produce hydrogen gas as a byproduct of the charging process. This gas tends to collect in and around the battery. As you are aware making the final connection can generate a substantial spark. By making the frame connection the point where the spark occurs it is far enough from the hydrogen gas to avoid an explosion.

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    I wish I could find the studies, but the battery and battery charging manufacturers have demonstrated that the conditions required for a lead acid battery to create a concentration of >4% Hydrogen gas in the vicinity of the battery are so ridiculous as to almost never occur in practice. This was important in dealing with overzealous OSHA regulators who wanted to impose scientifically un-backed regulations everywhere a battery was being charged. I read the study when I was a member of the Battery Council International, but I can't find it on their site. Might be paywalled. – Paul Feb 22 '15 at 18:05
  • There are other flammable and explosive material around the engine. If your spark did happen to create a small flame or explosion, you probably don't want this to occur near the battery. – Nelson Feb 23 '15 at 8:18

I don't know the actual justification, but to me it makes sense to always attach positive first because it is easy to accidentally touch the lead to something else while installing. If the negative lead were already attached, then touching the positive lead to just about anything on the car that is metal would short out the battery.

  • Can you explain how the opposite would not be true? eg, both positive connected and touching "just about anything" with the negative lead? – QueueHammer Feb 21 '15 at 23:41
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    @QueueHammer The negative terminal of the battery is connected to the frame of the vehicle. Nearly every electrical component uses the frame as ground. Touching the negative lead to anything grounded will be the same, electrically, as touching the lead to the negative terminal (but note that not every component is capable of carrying the current required to start a vehicle). – Paul Feb 22 '15 at 0:03
  • It's a safety issue to avoid shorting the battery terminals through the frame. I was trying to determine if the voltage regulator might get fried if you connect the positive last, but that doesn't make sense because when you crank the car, you're applying the positive last. The grounds always have connection to the negative terminal. – Bill N Feb 24 '15 at 23:07

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