Jumping a dead battery

May 5, 2005 4:08 PM

This question spurred another: When jumping a dead battery, do you attach the cables to the positive node of the dead battery or the live battery first?

Every person I have helped jump a dead battery has had differing opinions and still insisted that their way was THE WAY to do it. I have noticed that car manuals have even given conflicting information, as does Google. So which is it? Does it even matter? Once completely attached, do you have to rev the engine of the running car, as my brother so religiously believes?

posted by rhapsodie to travel & transportation (8 comments total)

Order is, I've been told:

Positive good
Positive bad
Negative good
Negative engine block

For some reason it's stuck like a little sing-song in my head and I always remember it (true or not).

Also the engine block won't always provide a good contact point, and you may need to do negative bad battery terminal, but that's not desirable.
posted by wolftrouble at 4:16 PM on May 5, 2005

Doesn't matter which order you attach the first three clips, as long as the last clip goes to a metal point on the engine or frame that's away from the battery of that car. That's because the last clip is the only one that can make a spark, and the spark can set off hydrogen that may be around the battery.

I've always revved the running car modestly (you don't have to run it up to redline or anything) but it's probably not absolutely necessary.
posted by spacewrench at 4:17 PM on May 5, 2005

There really is no "wrong" way to do it, as long as you're going positive-to-positive and negative-to-negative, but the least bad method would be the following:

1) Make the positive connection to the donor car's battery
2) Connect the other end of the positive cable to the dead car's battery
3) Make the negative connection on the donor car
4) Finally, connect the other end of the negative cable to a solid, grounded, metal part of the dead car (the frame, engine block, etc.)

The reason you don't connect the batteries directly terminal-for-terminal is because sparks inevitably occur when making the final connection, and sparks + batteries = explosions.

(wow, my first post!)
posted by porntips guzzardo at 4:29 PM on May 5, 2005

Positive to positive then negative to ground. Why ground instead of negative? Batteries give off hydrogen gas. In very rare instances the spark of attaching that last connection could ignite hydrogen being liberated from the battery. I have watched lots of folks violate this rule without penalty. Theoretically though, you could pay big time. Don't get the positive - negative thing mixed up. That leads to melted cables, which I have seen. You get a feel for how much energy is stored in a car battery when you watch a half inch cable melt like butter when connected improperly. The ground is just any part of the engine block physically removed from where any gas could be escaping from the battery.
posted by caddis at 6:13 PM on May 5, 2005

Positive to positive and negative to ground, just like everybody else on this thread has said. All other answers are wrong, and potentially dangerous.

If you're concerned about getting this wrong and have an unreliable car, there are jumper cables with a little box in the middle that refuses to let power flow until you have everything hooked up correctly.
posted by mosch at 6:29 PM on May 5, 2005

On engine revving:

It's a good idea to do this only if the "dead" vehicle requires a longer-than-normal amount of cranking to start. 2500 or so RPM is plenty. An example would be if someone tried to start a car that was completely out of fuel and wore down the battery in the process.

The reason for doing this is because the jumper vehicle's alternator (or generator, as they're calling them once again these days) can provide much more current flow and a tad bit more voltage at cruising speed, compared to when it's idling. Less strain on the diodes inside the generator this way.

And yes, P>P, N>G is correct.
posted by peewee at 10:26 PM on May 5, 2005

I've used wolftrouble's method many more times than I care to remember. The situation that's most dangerous is clipping both terminals on the running vehicle, which allows one to arc-weld with the clips on the other end, something that is all to easy to do in the cold with heavy mits on.

Also, I've always had someone slightly rev the engine, because you don't want two dead vehicles at 20 below. A boost at low temperatures can stall another car running at idle.
posted by bonehead at 6:01 AM on May 6, 2005

There's one IMPORTANT THING TO AVOID: Don't crank the dead vehicle with the cables still connected! That's a good way to destroy the donor vehicle's voltage regulator. Especially if you're revving the donor's engine a little bit, suddenly placing hundreds of amps of drain on the system will vastly overload the regulator's current capacity.

The goal of a jump-start is to recharge the dead battery enough that it can start the car all on its own. The battery can supply the massive current to crank the starter, the alternator can't. During a start, the voltage across the terminals of a 12-volt battery will sag to 4 or 5 volts because of the massive current draw. If this happened while the alternator was spinning at its normal speed, the regulator would fry.

Winter starting: All battery chemistries are affected by temperature, lead-acid in particular. If the dead vehicle is equipped with a battery warmer blanket, find a way to power it. (If the donor vehicle has an inverter, so much the better!) Getting the dead battery up to operating temperature (above 50F) will make all the difference in the world, in terms of allowing it to accept and deliver enough power to start the engine.

In reality, we always crank the dead vehicle with the cables still connected, because waiting for the battery to charge would take too long. In this case, the important factor is that the donor's battery supplies most of the current, rather than the donor's alternator. This means making sure the donor's battery is warmed up, because if it too is frozen, the alternator is the only part making itself useful, and it'll be overtaxed and possibly fry.
posted by Myself at 12:24 PM on May 7, 2005