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[kevinp58]

I always put both leads from battery to the battery ie pos to pos and neg to neg that should give you good contact, and have the donar vehicle running

[Entwood]

I always put both leads from battery to the battery ie pos to pos and neg to neg that should give you good contact, and have the donar vehicle running

Not the best way as much of the "donor" energy is now used trying to "recharge" the flat battery, effectively it is a resistance in the circuit.

Better to connect just the positive to the battery and the negative to a good 0v (earth ) on the engine ... usually a lifting bracket is a good point. Bolt heads are not usually good as they are often covered in an oily film as best ..

[Webby the Bear]

I always put both leads from battery to the battery ie pos to pos and neg to neg that should give you good contact, and have the donar vehicle running

Not the best way as much of the "donor" energy is now used trying to "recharge" the flat battery, effectively it is a resistance in the circuit.

Better to connect just the positive to the battery and the negative to a good 0v (earth ) on the engine ... usually a lifting bracket is a good point. Bolt heads are not usually good as they are often covered in an oily film as best ..

they told us at school lifting eyes are best... but then theyre not always accessable 

[kevinp58]

I always put both leads from battery to the battery ie pos to pos and neg to neg that should give you good contact, and have the donar vehicle running

Not the best way as much of the "donor" energy is now used trying to "recharge" the flat battery, effectively it is a resistance in the circuit.

Better to connect just the positive to the battery and the negative to a good 0v (earth ) on the engine ... usually a lifting bracket is a good point. Bolt heads are not usually good as they are often covered in an oily film as best ..

Yes but if the battery is that discharged then atleast it will charge it up quicker and then give it a chance

[Entwood]

I always put both leads from battery to the battery ie pos to pos and neg to neg that should give you good contact, and have the donar vehicle running

Not the best way as much of the "donor" energy is now used trying to "recharge" the flat battery, effectively it is a resistance in the circuit.

Better to connect just the positive to the battery and the negative to a good 0v (earth ) on the engine ... usually a lifting bracket is a good point. Bolt heads are not usually good as they are often covered in an oily film as best ..

Yes but if the battery is that discharged then atleast it will charge it up quicker and then give it a chance

But you don't want the battery to be charged ..yet .. you want all the available energy to go to start the engine ... then the alternator can charge the battery far more efficiently.. 

and the amount of charge put in is negligible anyway .. it is far more of a drain on the good system than a charge on the bad !!

[Kevin Wood]

I disagree. You want to get some charge into the battery on the disabled vehicle because there is probably too much resistance in the jump leads to power the starter motor directly, especially if the leads are a bit cheap. You need some contribution of current from the flat battery too, and that means charging it a little first. That means connecting as close as possible to the battery, really.

Get a car with a properly flat battery (as I had last night  ) and you'll find it needs the jump leads connected for a few minutes with the donor engine running before it will turn over.

The main reason for the recommendation to make the final negative connection to a lifting eye or similar is that there will be a spark when you connect it, and the battery is a potential source of hydrogen and oxygen, so could explode given a spark at its' negative terminal.

[Entwood]

I disagree. You want to get some charge into the battery on the disabled vehicle because there is probably too much resistance in the jump leads to power the starter motor directly, especially if the leads are a bit cheap. You need some contribution of current from the flat battery too, and that means charging it a little first. That means connecting as close as possible to the battery, really.

Get a car with a properly flat battery (as I had last night  ) and you'll find it needs the jump leads connected for a few minutes with the donor engine running before it will turn over.

The main reason for the recommendation to make the final negative connection to a lifting eye or similar is that there will be a spark when you connect it, and the battery is a potential source of hydrogen and oxygen, so could explode given a spark at its' negative terminal.

I agree you leave the system standing a few minutes .. but that doesn't actually "charge" the dead battery at all .. it just changes what is happening on the surface of the plates and reduces the resistance.

Put it this way ... if the "donor" alternator is rated at 120 amps ... and you connect it to your dead battery for 4 minutes (and who waits that long ? most folks don't even wait 2 minutes)  then it "puts in"  a massive .... 8 amp/hours !!! ... which is about as much use in starting an engine as tits on a bull !!!

Just my views .. nowt else ..

[kevinp58]

I disagree. You want to get some charge into the battery on the disabled vehicle because there is probably too much resistance in the jump leads to power the starter motor directly, especially if the leads are a bit cheap. You need some contribution of current from the flat battery too, and that means charging it a little first. That means connecting as close as possible to the battery, really.

Get a car with a properly flat battery (as I had last night  ) and you'll find it needs the jump leads connected for a few minutes with the donor engine running before it will turn over.

The main reason for the recommendation to make the final negative connection to a lifting eye or similar is that there will be a spark when you connect it, and the battery is a potential source of hydrogen and oxygen, so could explode given a spark at its' negative terminal.
 

[Kevin Wood]

To start a car you might need a current of the order of 300 Amps for 3 seconds. That's only 0.25 Amp hours of charge, so 8 Ah is plenty.

A typical set of jump leads simply won't pass that sort of current, hence the wait required, during which time you are charging the battery..

Batteries accept charge or lose it, depending on the direction of current flow, and that's all they do. Place a fully charged battery in parallel with an empty one and current will flow and the empty battery start charging, more so if the charged battery is itself being charged by an alternator meaning that it has upwards of 14V across it.

[Toledodude1973]

So would you agree after putting jump leads on (neg.lead to engine lift brackets)and they heat up immediately the dead has probably shorted out.

[Kevin Wood]

So would you agree after putting jump leads on (neg.lead to engine lift brackets)and they heat up immediately the dead has probably shorted out.

Yes, sounds like you've got a short at the dead car's end or maybe the starter solenoid has jammed in the engaged position?

I suppose it's possible that if the battery is completely flat in that vehicle it might take a considerable current. Enough to heat up a cheap pair of jump leads? Not sure.

I think I'd give it a charge with a mains battery charger whilst monitoring the voltage across the battery. If it's taking a good few amps of charge you should see the voltage rise quite quickly to around 12V. If it's sitting much lower than that and a gentle charge doesn't raise it you either have a short in the vehicle somewhere or an internal short in the battery (may even be only one cell meaning it levels off at 10 Volts). Charging the battery disconnected from the vehicle will reveal if it's a problem with the battery itself.

[Toledodude1973]

Thanks Kevin, a full answer as always on OOF cheers