Scott Blomquist

My friend Nick wasn’t quite sure how not to die while jump-starting his co-worker’s car. It’s pretty easy not to die, really. Here’s the skinny.

I notice that he had two main concerns. The first was how not to get electrocuted. The second was how not to explode while attaching clamps to batteries.

First: how not to get electrocuted. It turns out that you don’t really need rubber, or dryness, to avoid getting electrocuted by 12-volt batteries hooked up to 14-volt alternators. Even when wet, human skin resists 12 (or 14) volts pretty well. You know the 9-volt battery/tongue trick? Without knowing the actual values, I’m going to estimate that 12 volts from one hand to the other passes far less current than 9 volts to the tongue. For a couple of reasons: 1) your tongue’s job is to be wet–wetter than Seattle’s wussy rain gets your skin, and 2) the distance that the current has to travel along your tongue is really short compared to the distance that it would have to travel along your skin (or through your skin and along your water-filled innards), and resistance is usually directly proportionally to the length of the conductor. So, seriously, there’s no possible way you could electrocute yourself. I’d say that having non-insulated handles on the jumper cables is more of a threat to the battery (easier to accidentally short it) than to human life. Don’t believe the car-battery torture scenes you see in the movies–Mel Gibson is such an over-actor.

Second: how not to explode while attaching the clamps. The biggest reason that the jumper cable manufacturers tell you never to complete the circuit by connecting directly to the battery is that there are usually some really tiny sparks produced upon completing the circuit, even when done completely correctly. (Think slightly bigger sparks than a shock from a doorknob.) In some very unusual circumstances, the battery in a car can be caused to electrolyze the water in them into hydrogen gas and oxygen gas. This is fairly difficult to do, and is not part of the battery’s normal routine, but if you _happen_ to have a battery that built up some hydrogen and you happen to spark near the hydrogen, you could get a nice little hydrogen explosion that results in splashing sulfuric acid and lead-laced water all over you and your engine. (Think "ouch".) All it took was one jumper cable manufacturer realizing that they might get sued if this happens, and they quickly found a way to move the sparks to some other place where there is no chance of hydrogen build-up instead of some absurdly small chance.

Don’t fear the jump-start. It seems certain that you’re more likely to die in your car driving home from the jump-start.