ChatterBank1 min ago
Jump Start Not Battery To Battery
Been reading about jump starting and it says positive to positive and negative to body part not the negative terminal of dead battery. I'm sure years ago it was battery to battery, have things changed or have I been doing it wrong? Not had to jump start a car for years, just checking just in case neighbour asks for jump start. TIA.
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When I was first taught how to jump start (50-ish years ago when cars had no sensitive electronics to speak of apart from a few diodes) the correct procedure was to make the final connection from the good battery -ve terminal to an easily accessible exposed area of the engine block away from the bad battery. That way you couldn't complete the circuit with your body when leaning over the earthed bodywork to reach the battery terminal.
most of their reasons assume a dead battery, or perhaps dangers of leaning over and touching another earth. I'm talking about a flat battery. But yes with modern cars there are no doubt other reasons to use a car body part but logically if you trace the negative lead it is simply bolted to the body so it should be fine unless the battery is really dead, ie unchargeable. I had a 7 series that that had 2 jump points under the bonnet and the battery was i the boot.
I think it is really about preventing a spark near the battery as there could be fumes. You should connect to your own positive terminal first, then positive to the terminal of the battery you are jumping and finally negative to your car followed by negative to the chassis of the car you are jumping - away from the battery. When you disconnect in reverse order so you remove the negative from the chassis first in case there is any spark, that way it won't be near the battery.
the remote earth is just like connecting to the battery via a longer lead so I can't see the difference. Eg trace the negative lead it to the car body then imagine that it's joined to the car in the same place that you are going to use, QED, makes no difference It's still going through the flat battery.
I think the sparks close to the battery is a possible explanation. Another reason is to minimise the resistance in the jump-lead circuit. A starter motor draws 600 amps plus which gives it a resistance of around 1/200 ohms. An extra resistance of 1/00 ohm in the circuit path can cause a volt drop serious enough to hamper starting. There is usually not very good access to the starter motor itself to connect a jump lead to, so you really have to connect the positive to the battery lead. The starter motor is bolted firmly to the engine block, so that is the best place to connect the negative jump lead if you can find a good, clean place to connect. Connecting to the bodywork relies on the earth-link from the block to the body being very clean and uncorroded.
to make the circuit the current runs from the positive all around the car to the negative, the only connection to the negative battery terminal is via the lead so no matter where you connect the jump lead it is basically extending the battery lead itself, QED even putting the lead directly on the starter motor does not make the circuit any shorter it would still run through the block to the body to the lead to the battery to make the circuit.
this diagram helps:
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no matter where you put the lead you are just extending the negative terminal.
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no matter where you put the lead you are just extending the negative terminal.
If you are using jump leads the car battery is almost irrelevant. It is probably necessary to provide power for the ancilliary circuits (coil etc) to enable a start but, obviously, doesn't have the capacity to provide enough power to start the car. What you need is a really good connection between the jump leads and the starter motor.
Imagine a badly corroded earth link from the block to the body. A negative jump lead straight to the block will allow a start; a connection to anywhere else will require the starting current to flow through the earth link and will be greatly diminished by the resistance. It's all very well arguing that all the negatives are connected together but dirty/corroded connections will mean that those connections have an extra resistance in the circuit.
Imagine a badly corroded earth link from the block to the body. A negative jump lead straight to the block will allow a start; a connection to anywhere else will require the starting current to flow through the earth link and will be greatly diminished by the resistance. It's all very well arguing that all the negatives are connected together but dirty/corroded connections will mean that those connections have an extra resistance in the circuit.
