Portable Battery Charger.

[saintpeter48]

I have just bought a portable battery charger, to jump start my car if battery fails. In the instructions it say connect the positive (red) clamp to the positive terminal on battery and connect the negative (black) clamp to a metal part of the car, why is this, why can't I connect it to the negative clamp to the negative battery terminal, i'm confused and not very mechanically minded, as you can tell!

[Tuvok]

Possibly to do with earthing or serial/parallel connections?

[tonyav]

This explains it.
http://electronics.stackexchange.com/questions/74956/why-do-we-connect-a-battery-to-ground-when-jumping-a-car

[KARL]

Normally making the connection you suggest or the one according to the instructions you have would be one and the same because the negative battery post is normally connected directly to the chassis/body. The connection would have to be to clean, naked metal (no paint, etc.).

[gingejbee]

It's now recommended that the negative clamp should be attached to a metal part of the car (rather than the battery itself) because it's much safer...accidental arcing could cause lots of damage to the electrics of one or both cars.

[ToraToraTora]

you can use the neg terminal on the battery, if you look at the lead on that it will be bolted to the car chassis somehow, most cars are negative earth these days.

[gingejbee]

TTT...you can do that, but it is not recommended for several (mainly safety) reasons.

[EDDIE51]

Just a point, a portable battery charger will recharge your battery but it does not produce enough current to 'jump start' a car.
To do that you need 'jump leads' and another car or battery.

[tonyav]

I think he means something like this, EDDIE.
http://www.amazon.co.uk/dp/B00IDCMLFE

[gingejbee]

EDDIE: I assume that the OP meant portable jump starter!

[douglas9401]

If your starter pack is connected to the flat battery it's trying to charge it AND start the car.
If it's connected as instructed it bypasses the flat battery and powers the car directly giving enough to start the vehicle.

[TheChair]

"If it's connected as instructed it bypasses the flat battery and powers the car directly giving enough to start the vehicle."
Nonsense. As has been pointed out, the car chassis is connected to the negative terminal of the battery. Connecting the jump starter to the chassis in no way bypasses the battery.

[douglas9401]

"During jump-starting, we connect the boosting battery to ground rather than to the dead battery's - terminal for the simple reason that this provides a more direct return path to the good battery which is powering the dead car: the return current does not have to travel through the dead battery's minus terminal hookup cable and then to the jumper cable, but can go directly from the chassis ground to the jumper cable."

http://electronics.stackexchange.com/questions/74956/why-do-we-connect-a-battery-to-ground-when-jumping-a-car

[douglas9401]

None taken.

[TheChair]

I suggest you read your quote again. Nowhere does it say that it bypasses the battery or anything about it trying to recharge the battery. It says it uses a shorter path. If you draw yourself a circuit diagram, you will see that in both cases the battery is connected in parallel with the jump starter.

[douglas9401]

Peremptory. That's a good word.

[TheChair]

Here's a diagram of the two alternative connection methods.
http://i59.tinypic.com/24xgfgh.png