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If you go to: http://www.battery-chargers.com/charging_instructions.htm

...Under "Operating Instructions" part "B: Charging battery outside of vehicle", it says you have to attach an extra jumper cable to the negative battery post, that is then clamped to the charger's actual negative cable. See below:

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Both my charger's instructions and these look identical on this point (Indeed, they might be the same instruction sheet).

Why is this extra cable needed?

(Not like I don't have one, just rather curious...)

Thanks!

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3 Answers 3

up vote 12 down vote accepted

So that you can connect/disconnect while not standing over the battery. The batteries contain sulfuric acid which is not a very pleasant substance to splash. Even the fumes are dangerous.

While connecting inside a vehicle, connect the positive first and the negative on the vehicle body while not facing the battery. The negative can be connected to any exposed metal part on the vehicle (and at a distance from the battery). Similarly while disconnecting, disconnect the negative first and then the positive cable. The reason is that the vehicle body is connected to the negative terminal. If the positive cable slips and touches any other vehicle body part, it will cause a short.

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Should connect to both battery posts when boosting, though, as a lot of current will surge through. –  tyblu Feb 4 '11 at 3:48
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That makes sense. Thanks! Also, @tyblu -- I'm not sure that's correct. The booster cables I have specifically say to ground to the car body when boosting a negatively-grounded vehicle. –  aendrew Feb 4 '11 at 4:58
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Also, when charging outside the vehicle, always switch the charger on at the mains after connecting the battery and switch off the charger before disconnecting the battery. The sparks produced when disconnecting a powered charger can ignite the hydrogen gas produced by the charging battery with very unpleasant results. –  MikeJ-UK Feb 4 '11 at 9:25
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Aendrew - I suspect that the reasoning is the same - They want you to not be standing over the battery. Tyblu is pointing out that whatever you connect to might limit the current, either through a poor connection to the metal through paint or corrosion, or by a poor conduction path through more corrosion-the solid lead battery post won't do that. Plus, when boosting, you can connect to a known good battery on the boosting car and be much less likely to cause an explosion or boil-over. –  Kevin Vermeer Feb 4 '11 at 14:12

Lead acid batteries, like those used in vehicles, create hydrogen gas from the sulphuric acid. If the gas concentration is high enough and you get a spark, it explodes, the acid goes everywhere including in your eyes.

You almost always get a spark when completing the circuit, sometime even if the charger if off. By making the last connection away from the battery this lessens the chance of an explosion. If the battery is still in the vehicle, attach the positive to the battery terminal then attach the negative to a ground point away from the battery. Many people use a point on the engine block. With the battery removed, you don't have a ground point to connect to, so make the last connection away from the battery by using an extra ground cable.

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...and sparks + hydrogen gas = fire. –  Kevin Vermeer Feb 4 '11 at 14:13

Just a guess, but perhaps it forms a makeshift switch? Disconnect to stop charging.

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It's a thought, but why is it needed for charging outside the vehicle but not inside it? You'd think it'd be more useful inside the vehicle -- not the other way around! –  aendrew Feb 4 '11 at 1:52
    
in the vehicle you connect the negative to a ground point on the vehicle, not directly to the battery terminal. –  Jim C Feb 4 '11 at 13:57

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