Car been garaged for 7+ years. What's the liklihood it could be made a runner?

TrickyWicky

Strider590 wrote: »

No, because the current is flowing from battery to battery, the starter still only draws it's usual current.... Therefore the earth lead is subjected to nothing more than a normal start cycle.

Yeah.. but when the leads are disconnected and the engine is turning an alternator the dead battery is still going to sap power from the alternator via the earth strap.. Didn't think about that did you?

TrickyWicky

Strider590 wrote: »

Let me quickly punch that in.......

You'd need a cable of no more than 2m length, with a cross sectional area of 25mm². Anything less would get very warm indeed......

Apart from huge, do you have any idea how big that is?

Yes I do (and yes, they're huge). I think the question is do you? I suspect you don't though because a good set of jump leads is just that - These are actually mine that are in my boot now and have been used to jump start 6L range rovers as mentioned previously:



From Halfords
They also offer 35mm2 leads.

[Deleted User]

I like that shape of Fiesta so it would be worth getting it back on the road. Don't see many these days. :cool:

mikey72

Strider590 wrote: »

Ok, how much current can a car battery + starter motor draw?

The starter alone could be anything upto 200Amps (maybe more), but let's say 90Amp.
The discharged battery can draw as much current as you can supply.

So with the 2 factors we are probably looking at a minimum 150Amp.

Let me quickly punch that in.......

You'd need a cable of no more than 2m length, with a cross sectional area of 25mm². Anything less would get very warm indeed......

Apart from huge, do you have any idea how big that is?
Pull the front panel off your 9kw electric shower, look at the supply cable on that and double it's thickness.

You worked that out on 240v ac didn't you, try it again with 12v dc, and use the voltage drop calculated with 12v dc, so pure resistance, no impedence.

Sgt_Pepper_2

TrickyWicky wrote: »

Yes I do (and yes, they're huge). I think the question is do you? I suspect you don't though because a good set of jump leads is just that - These are actually mine that are in my boot now and have been used to jump start 6L range rovers as mentioned previously:



From Halfords
They also offer 35mm2 leads.

Are Range Rover doing a special 6L engine just for you?

TrickyWicky

Sgt_Pepper wrote: »

Are Range Rover doing a special 6L engine just for you?

Erm no.. whatever you're getting at I've no idea.

I've had to jumpstart a couple of customers who had them though.

Joe_Horner

Strider590 wrote: »

Ok, how much current can a car battery + starter motor draw?

The starter alone could be anything upto 200Amps (maybe more), but let's say 90Amp.

Probably about right, assuming it doesn't stall, but say 100A cos it's a nice round figure.

Strider590 wrote: »

The discharged battery can draw as much current as you can supply. So with the 2 factors we are probably looking at a minimum 150Amp.

Not quite. The current the battery will take will depend on the difference between the voltage measured across the battery terminals (without the leads connected) and the voltage connected to it (say 13.7v assuming no drop across the leads for now) , and the internal resistance of the battery.

A battery that's been standing sulphating to itself for 7 years will probably be providing virtually no voltage but will have a VERY high internal resistance. So it'll draw almost no current, at least initially. That's why if you connect a battery that's been dead for a while to a "simple" charger (ie: transformer with a rectifier) it'll show no charging for quite a while.

Strider590 wrote: »

Let me quickly punch that in.......

You'd need a cable of no more than 2m length, with a cross sectional area of 25mm². Anything less would get very warm indeed......

Apart from huge, do you have any idea how big that is?
Pull the front panel off your 9kw electric shower, look at the supply cable on that and double it's thickness.

With 100A flowing through 4m of jump lead (2x 2M cables), and limiting total voltage drop to 0.5v ( a reasonable figure - the starter will still get over 12v from the "jump" battery) would need 6AWG cable (about 13mm²) assuming the leads are copper. The leads will produce 50 watts of heat between them (25W each) which will probably make them feel warm, but won't be a problem.

25mm² copper leads would give a total drop of about 0.3v, generating 30W (15W per lead) of heat which they should be able to cope with all day


eta: as for the 35mm² ones that Tricky linked to, they'll drop a mere 0.5v and generate 25W of heat in each lead over a 4.5 meter length - might have to invest in a pair of them myself!

mikey72

Joe_Horner wrote: »

Probably about right, assuming it doesn't stall, but say 100A cos it's a nice round figure.



Not quite. The current the battery will take will depend on the difference between the voltage measured across the battery terminals (without the leads connected) and the voltage connected to it (say 13.7v assuming no drop across the leads for now) , and the internal resistance of the battery.

A battery that's been standing sulphating to itself for 7 years will probably be providing virtually no voltage but will have a VERY high internal resistance. So it'll draw almost no current, at least initially. That's why if you connect a battery that's been dead for a while to a "simple" charger (ie: transformer with a rectifier) it'll show no charging for quite a while.





With 100A flowing through 4m of jump lead (2x 2M cables), and limiting total voltage drop to 0.5v ( a reasonable figure - the starter will still get over 12v from the "jump" battery) would need 6AWG cable (about 13mm²) assuming the leads are copper. The leads will produce 50 watts of heat between them (25W each) which will probably make them feel warm, but won't be a problem.

25mm² copper leads would give a total drop of about 0.3v, generating 30W (15W per lead) of heat which they should be able to cope with all day


eta: as for the 35mm² ones that Tricky linked to, they'll drop a mere 0.5v and generate 25W of heat in each lead over a 4.5 meter length - might have to invest in a pair of them myself!

Even 10mm cable at 1.910 ohms/km will drop 0.382 v at 100A over 2m, so only 38W, and are rated for 100% duty cycle.

Phil_L_2

I think you will find that 100A is rather on the low side. I have seen test results peaking at over 1000A and cranking at 400A to 600A.

bungle4by4

oh my god, the pedants have taken over this thread. for gods sake, did you get the car going yet? where abouts are you?