Analog battery load tester. Harbor Freight.

Thread Starter

FJ Land Cruiser

Joined Jan 5, 2019
1
Hi. Does anyone know what load coil wire in a analog battery load meter is made
from? My wire vaporized during a vehicle battery test. It is a small gap, approx 1/2”.
I wonder if I can jump that gap with copper or some other wire to restore the circuit.
It is an almost black color metal, with a semi-rough texture. The model is Harbor
Freight # P-6317. They show no part number for the load coil.
It happened at the point where the yellow wire from the switch connects to one end
of the load coil. The wires passed through a black insulator material. That’s where vaporization occurred.
Thanks.
 

ian field

Joined Oct 27, 2012
6,539
Hi. Does anyone know what load coil wire in a analog battery load meter is made
from? My wire vaporized during a vehicle battery test. It is a small gap, approx 1/2”.
I wonder if I can jump that gap with copper or some other wire to restore the circuit.
It is an almost black color metal, with a semi-rough texture. The model is Harbor
Freight # P-6317. They show no part number for the load coil.
It happened at the point where the yellow wire from the switch connects to one end
of the load coil. The wires passed through a black insulator material. That’s where vaporization occurred.
Thanks.
In simple testers, its just a resistance to load the battery. More sophisticated testers chop the load and sample voltage for both conditions - its possible to calculate the battery internal resistance that way.
 

wayneh

Joined Sep 9, 2010
16,400
I'd be tempted to just use a hunk of copper wire because it's cheap and available. You lose that small section of resistance wire, meaning the overall resistance will be lower, but that will have a minimal effect on any readings you'll make.
 

shortbus

Joined Sep 30, 2009
8,044

MisterBill2

Joined Jan 23, 2018
7,299
A copper wire is certainly the wrong choice. Either nichrome or even steel wire is what is needed. Determining the correct size and length, given that the original is missing, is going to take some effort if you want the repaired device to have any degree of accuracy at all. What we need to provide any possibility of a right answer is what voltage is supposed to be developed across that resistance and at what current. Otherwise the best you can get is guesses, unless somebody has an identical unit and can measure the voltage and current.
 

ian field

Joined Oct 27, 2012
6,539
A copper wire is certainly the wrong choice. Either nichrome or even steel wire is what is needed. Determining the correct size and length, given that the original is missing, is going to take some effort if you want the repaired device to have any degree of accuracy at all. What we need to provide any possibility of a right answer is what voltage is supposed to be developed across that resistance and at what current. Otherwise the best you can get is guesses, unless somebody has an identical unit and can measure the voltage and current.
Some designers do it with copper tracks on the PCB.

Just design for a temperature rise that doesn't damage copper.
 

wayneh

Joined Sep 9, 2010
16,400
A copper wire is certainly the wrong choice. Either nichrome or even steel wire is what is needed.
I probably wasn't clear. I'm talking about a repair hack, not a replacement. I'm envisioning a long resistance wire with a brief gap filled with a hunk of copper connecting the loose ends, I'd use a fat enough piece that there is essentially no heating within the copper itself. Granted, making the connections could be tricky.
 

shortbus

Joined Sep 30, 2009
8,044
I probably wasn't clear. I'm talking about a repair hack, not a replacement. I'm envisioning a long resistance wire with a brief gap filled with a hunk of copper connecting the loose ends, I'd use a fat enough piece that there is essentially no heating within the copper itself. Granted, making the connections could be tricky.
I doubt very much if this originally had a gap. They have a stintered carbon resistance element in them when new, no gap. If the one used is too small of a value for the battery they vaporize or break down. That is probably where the gap came from. The gap for 6 or 12V to conduct would be so small.... This is the reason you need to match them to the battery to be tested, but since higher amperage ones cost more people buy the smaller cheaper one and complain because it breaks. The one the OP referenced is for lawn mower or motorcycle batteries, test with it on a car is just asking it to break. The link in my first post tells how to match them to the battery.
 

Reloadron

Joined Jan 15, 2015
5,667
What they use is a flat strip of flat nichrome wire having a resistance of 0.12 Ohms total which they measure across. One major downside is when testing a fresh 12 volt battery you are dissipating about 1,200 watts. The load resistance gets real hot real fast and when you figure the dissipated power they are not designed to sustain it. So what you have is a resistive load of right about 0.12 Ohm made of nichrome wire (flat ribbon nichrome cable) which they measure a voltage drop across. The same unit will measure 50 amps on a 6 volt battery.

Considering they cost about $20 new I would replace it and keep the toasted one for parts. Keep in mind if you try to bridge the open break your bridge needs to comfortably handle about 100 amps so you are talking a beefy chunk of copper which is of a much lower resistance than what was in there and you need a reliable way to affix the new section to the existing ribbon cable.

Ron
 

MisterBill2

Joined Jan 23, 2018
7,299
Right, I assumed a gap had opened due to overheating.

@Reloadron's post has convinced me that my hack is probably impractical. Too hard to make the connections and still have a device worth using.

I retract.
Probably you could replace the entire damaged wire with a section of steel wire of an adequate thickness. That might be from a thin coat hanger or even a section of baling wire. . And you may find a suitable stainless steel strip inside of a discarded wiper blade. A lot of those blades do contain 2 skinny strips of stainless steel, which is similar to nichrome wire, but not the same. The advantage of the strip is more heat radiating area. The result would not be perfectly calibrated but consider what that battery checker is supposed to do. Pass or fail testing. And you could try it on a bad battery and observe the readings to see how close it was.
 

shortbus

Joined Sep 30, 2009
8,044
What they use is a flat strip of flat nichrome wire having a resistance of 0.12 Ohms total which they measure across. One major downside is when testing a fresh 12 volt battery you are dissipating about 1,200 watts. The load resistance gets real hot real fast and when you figure the dissipated power they are not designed to sustain it. So what you have is a resistive load of right about 0.12 Ohm made of nichrome wire (flat ribbon nichrome cable) which they measure a voltage drop across. The same unit will measure 50 amps on a 6 volt battery.
I know the old ones used the wavy strip in them, but the one I looked at a few years ago at Harbor, had the carbon/graphite in it. Haven't looked at one for a while though. But if he buys the same one he originally had to test the same battery it will blow too. Think he needs the $49 one rated for 500A, not the 100A he had.
 

MisterBill2

Joined Jan 23, 2018
7,299
Monitoring the battery voltage with a decent voltmeter during engine cranking is a good way to check the battery condition, along with watching the battery voltage as the various lights are switched on with the engine off. A bit harder to interpret on an analog meter, but very informative when using a good digital voltmeter, one that reads volts and millivolts. (2 digits to the right of the decimal point)
 

ian field

Joined Oct 27, 2012
6,539
Monitoring the battery voltage with a decent voltmeter during engine cranking is a good way to check the battery condition, along with watching the battery voltage as the various lights are switched on with the engine off. A bit harder to interpret on an analog meter, but very informative when using a good digital voltmeter, one that reads volts and millivolts. (2 digits to the right of the decimal point)
Sampling and/or averaging in a DMM hides a lot of info on loading tests.

An analogue (moving coil) movement is better. You can put a Zener in series with the multiplier resistance so the movement sweeps full scale for only the relevant voltage range.
 

MisterBill2

Joined Jan 23, 2018
7,299
Monitoring the battery voltage with a decent voltmeter during engine cranking is a good way to check the battery condition, along with watching the battery voltage as the various lights are switched on with the engine off. A bit harder to interpret on an analog meter, but very informative when using a good digital voltmeter, one that reads volts and millivolts. (2 digits to the right of the decimal point)
My Beckman DMM is plenty fast enough. Yes, an analog meter will also give a fairly good answer, a fast digital meter an even better answer,
 
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