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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, 2015 at 23:21
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    @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, 2015 at 1:25
  • @Pᴀᴜʟsᴛᴇʀ2 Maybe they should but fewer ground points on vehicles.
    – user15009
    May 10, 2016 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, 2016 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, 2016 at 20:29

4 Answers 4

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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, 2015 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, 2015 at 8:18
  • @Paul Actual experience wins: seen a battery that had just come of charge, and was gassing, used to jump star a car : one spark and the resulting explosion caused 3 cars needing to be re-sprayed and the worker needed his eyes flushed very rapidly so he suffered no damage.
    – Solar Mike
    May 19, 2019 at 8:11
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    @SolarMike Since what you describe was not in a laboratory, it is impossible for me to believe you over the the various experts I dealt with while working in the lead acid battery manufacturing industry.
    – Paul
    May 19, 2019 at 14:59
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    @SolarMike Please post when you can establish a causal relationship between charging and hydrogen gas concentration high enough to cause an explosion. Give the number of other causes of fires and explosions in a shop environment (including turning charging cables into plasma), I find no reason to conclude the cause was related to hydrogen outside of the battery.
    – Paul
    May 19, 2019 at 16:00
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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.

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  • 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, 2015 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, 2015 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, 2015 at 23:07
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Simple, the car is always connected to the negative terminal of the attached battery meaning if you connect a charged battery's negative to amother car first, the whole car chassis will be connected to that charged battery's negative. This means that if you accidentally touch the positive terminal to any part of the car except the positive terminal, there's a high chance of a shirt circuit, arc and ultimately a weld, a second dead battery and possible burn marks on the person. Now vice versa, if you connect the positive terminal first, you can touch the negative terminal of the charged battery to any part of the "dead" car without shorting the charged battery. This is much safer.

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Positive has greatest potential. Connect negative fisrt, positive can arc and fuse. Connect positive first, negative having less potential won't arc. The higher the voltage, the greater the chance of arcing and fusion. On a car if negative first and you are touching any metal part of car, when attaching positive there is possibility of arcing through you. Your body becomes part of the circuit. Think of positive like the Marines, first in, last out. Be safe, don't die of stupidity.

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    Potential does not matter, it's potential difference, also called voltage. Connecting the first cable never causes a spark, since there is no return path for the current. With the second cable, one creates the return path, and it gives sparks - no matter which cable is connected first / last.
    – sweber
    May 19, 2019 at 10:00

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