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StockNazi
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PostPosted: Thu Dec 07, 2017 9:15 am    Post subject: Battery Cable Resistance Measurment Reply with quote

I would like to measure the resistance of me positive battery cable. I have been having some intermittent starting issues.
It's been replaced with thicker wire, so I would like to keep it if possible.

Is there a ballpark number I should be looking for when measuring the resistance of this wire?
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PostPosted: Thu Dec 07, 2017 9:30 am    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

I've never measured that but I would say Zero.

Anything else and there is corrosion / a dirty connection.
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PostPosted: Thu Dec 07, 2017 9:58 am    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

visual inspection is important, cracked covering, corroded ends loose crimps etc and yes no resistance.
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StockNazi
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PostPosted: Thu Dec 07, 2017 11:01 am    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

Just found this information on measuring resistance:

VOLTAGE DROP TEST

A voltage drop test is the only effective way to find excessive resistance in high amperage circuits. It's a quick and easy test that doesn't require any disassembly and will quickly show you whether or not you've got a good connection or a bad one.

To do a voltage drop test, you create a load in the circuit that's being tested. Then you use a digital volt meter (DVM) to measure the voltage drop across the live connection while it is under the load. Voltage always follows the path of least resistance, so if the circuit or connection being tested has too much resistance some of the voltage will flow through the DVM and create a voltage reading.
testing voltage drop with a volt meter

If a connection is good, you should find little or no voltage drop and see less than 0.4 volts for most connections, and ideally less than 0.1 volts. But if you find more than a few tenths of a voltage drop across a connection, it indicates excessive resistance and a need for cleaning or repair.
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PostPosted: Thu Dec 07, 2017 11:55 am    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

Quote:
Just found this information on measuring resistance:

VOLTAGE DROP TEST

A voltage drop test is the only effective way to find excessive resistance in high amperage circuits.

>>Well, not the only way. An IR heat imager is a great non-contact way to find excessive resistance in a wire as it will show the power being dissipated as heat.

It's a quick and easy test that doesn't require any disassembly and will quickly show you whether or not you've got a good connection or a bad one.

To do a voltage drop test, you create a load in the circuit that's being tested. Then you use a digital volt meter (DVM) to measure the voltage drop across the live connection while it is under the load.

Voltage always follows the path of least resistance,

>> Hmmm. Voltage doesn't 'flow' it just is. Voltage is like your height above the ground - if you are higher, you will smack the ground harder when you fall.

I think they are trying to say current, but thats not right either. Current flows from a voltage gradient - and the current will divide itself into all paths according to some simple rules, If you have three wires connected between two points of different resistance , all three will have current if the two points are at different voltages.

so if the circuit or connection being tested has too much resistance some of the voltage will flow through the DVM and create a voltage reading.
testing voltage drop with a volt meter

If a connection is good, you should find little or no voltage drop and see less than 0.4 volts for most connections, and ideally less than 0.1 volts. But if you find more than a few tenths of a voltage drop across a connection, it indicates excessive resistance and a need for cleaning or repair.

Well done- thats the crux of the technique for finding excessive resistance.


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PostPosted: Thu Dec 07, 2017 12:51 pm    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

A picture is worth a thousand words, from my automotive electrical book:

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Note the voltmeter test lead at the battery is on the post, not the cable end, and at the battery lug on the starter it's on the stud, not the crimped end of the cable. For the negative side of the starter, holding it on the aluminum case is OK.

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PostPosted: Thu Dec 07, 2017 1:08 pm    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

Double check the chassis ground at the front of the transmission. Without good continuity there you will have problems.
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StockNazi
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PostPosted: Thu Dec 07, 2017 2:50 pm    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

Looks like this will be a 3 person job. One to hold a test lead to the starter, one to hold a test lead to the battery, one to operate the starter.

The diagram shows the test leads attached to the lug battery/starter lugs and not the cable ends. This will test the complete circuit i suppose.

If the test reveals excessive voltage drop, can I isolate the problem further by putting the test leads on the cable ends instead of the lugs to check the cable itself?
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PostPosted: Thu Dec 07, 2017 4:32 pm    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

Or make up some custom test leads. Get some 16 AWG wire, a pair of banana plugs and a pair of test lead clips from someplace like Fry's. Make up a pair of 6 foot test leads. Clip the wires where they need to go. Have someone operate the starter, and note the voltage reading.
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PostPosted: Thu Dec 07, 2017 7:01 pm    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

It's a one man job with long leads. The extended leads don't have to be heavy gauge, nearly anything will work as it's not handling any amperage. Even some old speaker wire will work with alligator clips.
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chrisflstf
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PostPosted: Thu Dec 07, 2017 7:08 pm    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

Testing can be difficult at times due to dirt, oil and corrosion. You may have to dig a bit with the probe to make a good connection. I usually check stuff 2 or 3 times to make sure Im getting a good reading

Battery cables are cheap also
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PostPosted: Thu Dec 07, 2017 7:32 pm    Post subject: Re: Battery Cable Resistance Measurment Reply with quote

A good battery cable resistance is in the microhms per foot. To make an accurate measurement you will need to review this 4 terminal resistance measurement tutorial.

Image may have been reduced in size. Click image to view fullscreen.



https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/

Three feet of 4 AWG cable, 1.6mV/3=533 micro ohms per foot
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