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Take care when jump-starting your battery, or pay the price

By Peter DeMarco
November 24, 2011
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Sears has an impressive, if somewhat ludicrous, video in its automotive centers showing a DieHard battery getting blasted by a high-powered rifle. The battery remains in one piece, doesn’t leak, and, yes, still works when hooked up to a pickup truck.

The average driver doesn’t need a bulletproof battery. But you can’t watch the video, also on YouTube, without appreciating how vastly better modern car batteries are than the ones your father had to mess with.

Back then, batteries often leaked, got caked with corrosion, and needed to be opened and filled with extra water about once a year. Modern batteries are for the most part sealed tight and maintenance-free.

“You needed to know a lot about your car battery back then. Today there’s not much to know. Just get the right one for your car,’’ said Jeff Short, whose family has run Royal Battery in Malden for nearly 50 years.

Still, even modern batteries aren’t foolproof. If you make a mistake while jump-starting a car, you could do everything from short-circuiting your vehicle’s onboard computers to blowing up the battery, potentially causing serious injury. We began our review of car batteries last time, but there’s still much to cover.

Keeping order

We’re all taught that when jump-starting a car, you must connect the booster cables in the right sequence. Connect the positive cables to the positive terminals on each battery, and then connect the negative cable to the negative terminal on the working battery.

For the last step, attach the negative cable’s other clamp to any unpainted metal on the dead car’s frame, as far from its battery as the cable allows.

But why is the order so important? I asked Greg St. Aubin, manager of after-market training for the battery maker ACDelco and a longtime technician, to explain.

The average car these days, he began, has 20 or more onboard computers. By crossing wires - connecting a positive cable to a negative terminal, or vice versa - you’re going to cause an electrical short that could instantly fry some of those computers.

“You may not find out the damage until your radio doesn’t work or your power windows don’t work,’’ St. Aubin said. “The damage could be expensive. You might have blown a $100 microprocessor, but to get it diagnosed it may cost you another $200 because the technician doesn’t know what shorted out.’’

On the more extreme end, crossing wires could even explode a battery, St. Aubin said.

Personally speaking, I’ve never crossed up my negative and positive connections. But I have taken the shortcut of putting a positive and negative connection on the working battery first, then walking over and putting the remaining jumper cables on the disabled car. Is that bad?

“You know what usually happens when you do that,’’ he said. “As you’re walking over and trying to open the hood, the ends of the booster cables accidentally touch. By touching them together, they’ll short out, and that can mess up the charging system on the car that’s giving the jump.’’

Final connection

You’re jump-starting a dead battery and are down to the last connection. Why go hunting for some piece of the car frame to connect to when the dead battery has a nice, clean negative terminal beckoning you to use it?

Not so long ago it was acceptable practice to latch onto that negative terminal. I used to do it myself. But not anymore. Everyone from Short to Sears to the Battery Council International, the industry’s biggest trade group, strictly warns against using the dead battery’s negative terminal. But why?

Typically, a battery goes dead after you’ve left the car lights on overnight. But a battery also can fail because of a malfunction in your car’s charging system related to the alternator, or maybe an onboard computer, St. Aubin said.

When a malfunctioning system “over-charges’’ the battery, sulfuric acid inside the battery gets boiled into vapors. Over time, those vapors build up inside the battery and begin to seep out through small vents without you knowing it.

When jumping a battery, it’s not uncommon to get a spark when making the last connection. If you hook up the last booster cable to the dead battery’s negative terminal, the spark may be close enough to ignite the escaping vapors and blow up the battery.

“If the charging system is charging at the proper rate,’’ he said, the danger “is extremely minimal. There’s virtually no vapor coming out.

“The problem is, if it is overcharging, you’re not going to know it. Your headlights don’t go too bright; your radio doesn’t go too loud. You don’t know it’s overcharging until it doesn’t start.’’

So play it safe, and make your connection to the car frame as far from the battery as you can.

Safety, maintenance tips

Wear goggles when jump-starting a battery and remove ties, scarves, and jewelry, especially wedding rings, St. Aubin said.

“Gold is the best conductor known to man,’’ he said. “If you short-circuit a ring on a battery cable, it will toast a nice circle around your finger faster than you can blink.’’

Corrosion isn’t as big an issue with modern batteries, but if your battery terminals or connections are corroded, a simple mixture of baking soda and water will neutralize the acid. Apply the mixture and use a stiff brush to remove the residue.

Most battery shops have contraptions that can tell exactly how much life is left in your battery, be it 90 percent or 9 percent, so stop in for a free test if your battery’s getting up there in age.

“The big push right now is preventative maintenance instead of waiting until you break down by the side of the road,’’ St. Aubin said. “If you take your car into the shop and they test the battery and you’ve got 10 or 15 percent left, you know to replace it before you run into a problem come winter.’’

Update on Prius

In my last column I included a warning from the Battery Council not to jump-start a hybrid or electric car because their batteries differ chemically from standard batteries. But Prius owners say they can, in fact, jump their vehicles like any other car because the Prius has a normal car battery in addition to its hybrid battery.

Officially, Toyota says it’s fine to jump-start the vehicle’s “auxiliary’’ battery. But don’t expect the favor in return, as the small Prius battery isn’t powerful enough to jump-start a standard, 12-volt car battery.

“The attempt would likely discharge the Prius battery so much as to possibly render the Prius inoperable and in need of a jump!’’ Toyota e-mailed me. “We don’t advise this.’’

Peter DeMarco lives in Somerville and can be reached at He also updates a Facebook page, “WhotaughtYOUtodrive?’’

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