Estimate Amp Capacity of Jumper Leads

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MikeAqua

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#218071 24-Jul-2017 15:42

I have an unlabelled set of jumper leads.

I need to know what amperage they are capable of conducting. The cable insulation continues onto the clips so I can't measure conductor diameter.

They are for the Pajero which (according to my multimeter) is drawing about 450Amps at start up. I've never had to jump-start it so I don't know if the leads will be adequate if we need them.

However, I can measure the full-length resistance of one of the leads with a multimeter.

Is there a way to calculate how many amps the jumper leads will conduct based on their resistance?

Mike

ubergeeknz

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#1828511 24-Jul-2017 15:44

Need to measure the diameter of the conductors, also know what material they are (a lot of cheap leads are just strands of copper plated aluminium, not copper)

Just plain resistance won't tell you.

MikeAqua

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#1828517 24-Jul-2017 16:04

ubergeeknz:

Need to measure the diameter of the conductors, also know what material they are (a lot of cheap leads are just strands of copper plated aluminium, not copper)

Just plain resistance won't tell you.

Bugger. I was hoping volts divided by ohms might give me amps.

I'd have to damage then to find that diameter and materials.

May as well keep them for the boat and buy some new ones for the Paj. SCA do a 750A set.

Mike

Rikkitic

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#1828524 24-Jul-2017 16:12

I'm no engineer (not even close) so this may well be wrong, but here goes anyway:

Ohm's law R=E/I. Voltage is 12 so if I=450 amps, resistance would have to be .02666. If it is higher, you won't be able to pull 450 amps through the cable. So measure/calculate total cable resistance and see how close it comes to .02666. If it is more, it won't like the load. If it is less, you are sweet.

Plesse igmore amd axxept applogies in adbance fir anu typos

MikeAqua

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#1828543 24-Jul-2017 16:39

That what I thought but @ubergeeknz 's post suggests not.

Mike

MikeB4

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#1828545 24-Jul-2017 16:43

Carry a portable booster that is what I did for off road and the boat. Only ever needed it a couple of times to get others out of trouble

RunningMan

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#1828547 24-Jul-2017 16:46

Rikkitic:
I'm no engineer (not even close) so this may well be wrong, but here goes anyway:

Ohm's law R=E/I. Voltage is 12 so if I=450 amps, resistance would have to be .02666. If it is higher, you won't be able to pull 450 amps through the cable. So measure/calculate total cable resistance and see how close it comes to .02666. If it is more, it won't like the load. If it is less, you are sweet.

If that was the case then the voltage drop in the leads would be 12v.

You might get away with a volt or 2 drop when jump starting, but any more than that and you will have trouble. Redo the calculation with v=2. R will be so low it's not measurable on a normal multimeter

mclean

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#1828555 24-Jul-2017 16:59

The simple thing you want to do is complicated by a lot of variables:

1 The resistance will be so low that you will struggle to measure it with a multimeter.

2 The important thing is the voltage drop when the engine is cranking. But this also depends on the length of the cables, how good the clips are, and the voltage drop of the battery itself. All these things mean you'll get nothing like 450A through most jump leads.

3 In any case you can't measure that sort of current through most multimeters - unless you measure the voltage across a calibrated shunt.

Your best bet might be measure the OD of the insulation and compare it with leads of the same length at Repco or Supercheap. That will give you some idea. Or persuade a battery supplier to hook the leads up to your battery and test the output with a battery tester set to your battery CCA. Or you could hook up your battery to the Pajero through the leads and try them (provided you know what you are doing - at your own risk, etc)

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RunningMan

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#1828619 24-Jul-2017 18:47

Putting aside the question, in all reality, even the smallest leads will probably get the pajero going if you get stuck. Connect them up and leave it for half a minute or so to transfer some charge to the dead battery before trying to crank it. They'll heat up - if they start getting too warm, stop and let them cool. Of course, as long as they are in good condition - corroded leads or clips won't ever work...

richms

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#1828654 24-Jul-2017 20:03

Usually use a clamp meter to measure starting current of a vehicle.

Generally you hook them up, start the donor car and let it run for a while to put some juice into the flat car. Dont start the flat car with the donor car running as the sudden load and then dump of it will make the charging system overvolt the donor car. The full starting current doesnt go thru the cables.

Also the CCA of a car is its cold current when friction is worse and the battery has to run the motor and also keep the voltage high for the injection and ignition. Warm day and you never see anywhere near those currents and batteries perform much better than when cold.

Richard rich.ms

cadman

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#1828903 25-Jul-2017 10:45

richms:

Usually use a clamp meter to measure starting current of a vehicle.

Generally you hook them up, start the donor car and let it run for a while to put some juice into the flat car. Dont start the flat car with the donor car running as the sudden load and then dump of it will make the charging system overvolt the donor car. The full starting current doesnt go thru the cables.

Also the CCA of a car is its cold current when friction is worse and the battery has to run the motor and also keep the voltage high for the injection and ignition. Warm day and you never see anywhere near those currents and batteries perform much better than when cold.

CCA being cold cranking amps refers to the battery temperature (0°C) not the engine. The rated CCA is what it can deliver for 30s before the voltage drops below 1.2V/cell.

Connection order is critical - dead +ve, donor +ve, donor -ve, dead engine block. Disconnect in the reverse order - no need to stop either vehicle to disconnect.

You always start the dead vehicle with the donor still connected and running at about 1/3rd max engine speed or you'll be waiting a long time to charge a very dead battery to get enough energy in it to start the vehicle.

MikeAqua

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#1828945 25-Jul-2017 11:10

richms:

Usually use a clamp meter to measure starting current of a vehicle.

That what I did. ~450A.

Mike

MikeAqua

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#1828975 25-Jul-2017 11:17

MikeB4:

Carry a portable booster that is what I did for off road and the boat. Only ever needed it a couple of times to get others out of trouble

I just see that as another thing to charge. The day I need it I will probably have forgotten to charge the thing. Also quite difficult to find one that will start a big diesel.

The boat is a different scenario. Outboards are small. I can actually hand start them if I need to (PITA but possible) and jumper leads there would be for using the 'house' battery as a donor if both the starter batteries (linkable by a parallel switch) are dead.

Mike

tripper1000

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#1829197 25-Jul-2017 16:24

RunningMan:

Putting aside the question, in all reality, even the smallest leads will probably get the pajero going if you get stuck. Connect them up and leave it for half a minute or so to transfer some charge to the dead battery before trying to crank it. They'll heat up - if they start getting too warm, stop and let them cool. Of course, as long as they are in good condition - corroded leads or clips won't ever work...

What he said above.

Too many people clip the leads on and immediately crank the dead car - even good leads can struggle to simultaneously recharge the dead battery and start the dead motor. If you give the dead battery some time to recharge, it assists in starting the dead vehicle, rather than detracting. Using this method, you can start big vehicles using small donor vehicles and/or small jumper leads. You may need to be patient and give it 4 or 5 minutes in extreme cases.

In addition to above advice, the donor car should have the motor running faster than approx. 1500 RPM otherwise the alternator isn't charging the dead battery at it's best speed.

To answer the ohms question more precisely, you need to measure milliohms to use formulas to calculate the leads amp capacity. Even Lab grade 2 wire DMM's won't be sensitive enough - you need a 4 wire ohm-meter or a high current milliohmmeter.

Link to 4 wire explanation:

Edit: Link added.

Aredwood

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#1830410 26-Jul-2017 00:50

cadman:

You always start the dead vehicle with the donor still connected and running at about 1/3rd max engine speed or you'll be waiting a long time to charge a very dead battery to get enough energy in it to start the vehicle.

Be careful running the donor car's engine any faster than idle when jump starting a car. Reason - you can easily destroy the alternator or blow the alternator fuse if the car has one. Virtually every car that is less than 25 years old will have a solid state voltage regulator. Meaning the alternator will still put out useful charge current at idle. If you don't get a useful charge at idle, either you have a very old design car that uses a mechanical voltage regulator, or you have a charging system fault.

A modern car will have an alternator rated to a max current of somewhere in the 100-150A region. Now imagine if you try to jumpstart the OPs Pajero at 450A. Sure you might get away with it if you use poor quality jumper leads that limit the current. And some car manufacturers on purpose undersize the wire that takes the alternator current to the battery - partly to allow for this exact situation. But some cars like my Merc Vito, use the starter motor wiring for carrying the alternator charge current. My Vito has a 120A alternator, and if I jumpstart something like a Pajero while running the engine fast. I can very easily destroy the alternator. As there is no current limiting anywhere in the charging system. But by leaving the engine at idle, the alternator won't be able to output enough power to destroy itself.

Now imagine if the dead car has an engine fault that means that it won't turn over, a starter motor will easily draw over 1000A if it can't turn. Either way im not going to risk damaging something in my car and getting myself stranded. Just from helping someone else.

If the battery in the dead car is in good condition, but just low on charge. Even 5 min of it being charged by the donor car is often enough, for both batteries working together to get the dead car started. It is those situations where the dead cars battery is badly sulphated, or is open circuit. Where the donor car will be supplying the full current. Also keep in mind that alligator clips will never be able to carry 450A without really high voltage drop anyway. When jumpstarting, turn off the headlights, aircon, heater fan and any other unnecessary loads. So the electrics in the donor car will only be drawing 20A or so to keep the engine running. Meaning even at idle, plenty of charge capacity left for jump starting.

Also carry a multimeter with you in the car. Jaycar sell a simple digital multimeter for $12, so no reason not to. Perfect for checking what is going on with jump starting, including checking that the donor car's charging system is working.

richms

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#1831825 26-Jul-2017 20:00

Can get enough charge on a working battery very quickly to get enough to turn over 1 or 2 times on the flat car if the battery is ok. Its when its stuffed that you will be sitting for ages to put enough into the car to get it to crank. Remember its not supplying the full current, there is still the good battery in the donor car in parallel with it providing a lot of current over the jumperleads.

Richard rich.ms