How much current in this wire to reach observed temperature?

[ethompson]

TL;DR

I have an odd failure in a battery charger system. I am seeing my positive wire melt MJF nylon. From other clues I am guessing a temperature of 270C.

My wire is 16 AWG (264 cores of 40 AWG) with 0.010 FEP Insulation. I have the insulation and the MJF nylon that it is touching melting! How much current would I need to have to get to these temperatures with this wire? This wire is inclosed inside the MJF nylon case, so figure for no air flow.

 

[Baluncore]

Welcome to PF.

My wire is 16 AWG (264 cores of 40 AWG) with 0.010 FEP Insulation.

What current is flowing in the melting wire?

Is it copper wire, or steel wire with a surface flash of copper from China?
Test the wire with a magnet.

16 AWG copper wire is rated for 18 amps at 90°C.

The heat generated by the warm wire will be a function of the power loss in the wire.
The temperature of the wire will be dependent on the thermal resistance of the insulation, plus the environmental temperature.

 

[ethompson]

Welcome to PF.

What current is flowing in the melting wire?

Yes this is the question!
My charger is limited to 10A, and I have tested ~20 of them. My battery can do more current, but thinking of a mechanism for this to be causing this issue. That is why I am asking to try to knock loose some data points and know approximately what current would cause the damage that I am seeing.

Is it copper wire, or steel wire with a surface flash of copper from China?
Test the wire with a magnet.

Copper!

16 AWG copper wire is rated for 18 amps at 90°C.

The heat generated by the warm wire will be a function of the power loss in the wire.
The temperature of the wire will be dependent on the thermal resistance of the insulation, plus the environmental temperature.

 

[berkeman]

@ethompson -- Do you have an appropriate size fuse inline between your charger and the battery? I know from personal experience how important those are in preventing fires. (Don't ask me how I know that)

 

[ethompson]

@ethompson -- Do you have an appropriate size fuse inline between your charger and the battery? I know from personal experience how important those are in preventing fires. (Don't ask me how I know that)

there are 20 and 30A fuses, but fuses usually are spec'd at blowing at 250% rating after by 1 min.

 

[berkeman]

My wire is 16 AWG (264 cores of 40 AWG) with 0.010 FEP Insulation

0.010 what units? Can you link to the datasheet for the wire?

16 AWG copper wire is rated for 18 amps at 90°C.

My charger is limited to 10A, and I have tested ~20 of them.

Tested 20 of which? 20 chargers or 20 batteries?

Do you have a meter that can measure currents in the 10-50A range? If not, can you just buy a simple analog needle display with that current range?
https://www.amazon.com/Fielect-Current-Ammeter-Accuracy-Measurement/dp/B08N6DWFM4?tag=pfamazon01-20

 

[Baluncore]

That is an AC meter.
Here is an example of a cheap DC VA meter, good for battery chargers and vehicles.
https://www.ebay.com.au/itm/387004089557?

 

[ethompson]

custom wire:
BN222216FEPUPU
22(7x12/44) 155 Strands Tin plated copper wire
0.032 Wire diameter
0.005 Inch FEP insulation
Wire O.D 0.042 Inch
2 Colors White and Green

16(7x38/40) TC
0.010 FEP Insulation
0.080 OD
Color Red and Black

Twisted together with 1.0 inch Left Hand lay
Core O.D 0.165 Inch Pressure Extrude
Final Jacket Black Polyurethane

I can measure currents resistively and using hall effect devices. I can dynamically watch currents of over 100A. I can apply loads of over 50A. I have tested over 20 of the charger supplies and verified that they will not supply over 10A continuously. At times I have seen in our system the batteries spike to 60A momentarily. On both sides of the hot spot there are 20A fuses. Fuse's are guaranteed to blow in less than 1 minute at 250% rating. I did verify this by putting on a 40A load and it blew fairly fast.

Now back to my problem. I have customers sending back melted chargers. I can't reproduce the problem. The highest temp that I see is 33.7C, but I see parts that have damage that needs 80C in one case and 200C in another. This wire case is the 200C instance. So the question is how much current would be needed on that wire to get these temperatures.

Then I will need to take a 400th look at the system and theorize how the heck to get this given the charging system / battery circuitry!

The hot wire is the 16awg red wire.

 

[berkeman]

Is there any chance that these customers are connecting the batteries backwards? Do the chargers have reverse polarity protection?

 

[ethompson]

Is there any chance that these customers are connecting the batteries backwards? Do the chargers have reverse polarity protection?

https://oceantechnologysystems.com/store/connectors/ots-4p-hi-use-connector/
https://www.amronintl.com/seacon-mi...ead-connector-with-teflon-wire-pigtail-mcbh4m

These are the connectors that the customer has to deal with. I see no way to plug them in backward.
Additionally I have tested all 4 shorting conditions with the second connector and the chargers protection worked in all cases. LOL even in one case that I still do not fully understand why a microcontroller is surviving the momentary voltages it is getting subjected to!

 

[DaveE]

I don't understand how or why you would do this sort of work without some very basic instruments. You can buy a cheap DMM, current shunt, thermometer, etc. to facilitate practical experiments. They don't have to be expensive. For example, this current shunt for about $5:
https://www.ebay.com/itm/110752301236?_skw=current+shunt&itmmeta=01JF0HVYJ6Y2SSYTAHTWGDCYV9&hash=item19c95a00b4:g:iJsAAOSwvc1ZaQXd&itmprp=enc:AQAJAAAA8HoV3kP08IDx+KZ9MfhVJKnxzKx+dYzT54GqSAUtWgMwaXweKfZ2/mJ3Jm7CmhmhmDdnnvc/nf5tHQAKd+yZvvc1n6ec0AsZsxddRLP5a3HaZG+2i8fxZ5SflFz/4bL9cicbZdU0vpcdc1OeH3tEuQpmSQyfCVClSxEXYxC1st1ifNtriKiIaBtAKQYU8dJSwSAfCa7EuVuRim+ejM5oSaMm1adYKr4Mw3X465WzuVJogD8aBB/UwLHwrSP5AIEx0IA2+RRQD9Nu3173x8ohJW8XuQw1/zY4fqnY3Do8AqtJ3z7xJzMHYI7KG8mq9NmRig==|tkp:Bk9SR6Lp75H4ZA

We aren't going to give you good answers, there are too many unknown variables. Melting insulation is perhaps the worst way to measure current. As others have said it's as much about the thermal environment as the heat source.

The ampacity tables I'm seeing say AWG#16 with 90C FEP insulation (probably 600V) is rated for 18A or less. Granted your insulation is thinner, but 20A or 30A fuses are questionable, IMO. Have you received safety agency approvals for this charger yet (CE, UL, CSA, etc.)? They would have should have checked and approved this. The issue isn't just that it fails, but that it isn't failing in a safe way.

 

[DaveE]

Is there any chance that these customers are connecting the batteries backwards? Do the chargers have reverse polarity protection?

I see no way to plug them in backward.

You haven't really answered this yet. The fact that you don't think it can happen is different than the question "what's the result if it does happen?". Customers can do amazingly stupid things from the viewpoint of the equipment designers. Connecting a battery backwards, even if you need a soldering iron to do it, is so common IRL that I wouldn't even say it's stupid, unless they did it on purpose.

Also consider an overload that isn't a short circuit, if you haven't yet. Like a bad battery that draws extra current but isn't 0Ω.

It sounds like you have multiple failures with similar evidence, I assume from different customers. So, I think it's safe to bet that there is an underlying design issue. Maybe you should try to think of ANY way you could cause this sort of failure, without too much regard to your assumptions about how the product is supposed to be used. I would also seriously consider something like a FMEA (Failure Modes and Effects) study, especially the "effects" part. I think hiring the right product safety consultant (for failure analysis, not just approvals) might be money well spent. Someone like Exponent* perhaps.

* Not really a recommendation, just an example. I used them a few decades ago and they were OK. YMMV.

 

[Babadag]

Sorry, I don’t see any 16 awg copper conductor. First, there are total 4 cables. No one is 16 awg [22*(7*12/44)=3.696 mm^2 and 16*(7*38/40)=20.8544 mm^2) and 16 awg it is 1.30-1.43 mm^2.

 

[Babadag]

According to #8 D1=0.042; D2=0.08; D3=0.165
However, if we take at BN 22(7x12/44) 155 strands[ actually there are only 154]
one strand is 12*(44 awg) 4.3* 0.00200"=0.0086" diameter and 12*4=48 cmils
7 strands=3*0.0086=0.0258" diameter and 7*48=336 cmils
22 of 7 strands=5.8*0.0258"=0.14964" diameter and 22*336 =7392 cmils [7.392*0.5067=3.7455 mm^2)
In my opinion, it is about 11 awg.
The same calculation for TC cable gives 0.34875"diameter and 41.157 MCM
16 awg (solid) diameter 0.0508" and cross section area 2581 cmils

 

[Babadag]

 

[ethompson]

View attachment 354513

Yes that is the cable.

 

[ethompson]

You haven't really answered this yet. The fact that you don't think it can happen is different than the question "what's the result if it does happen?". Customers can do amazingly stupid things from the viewpoint of the equipment designers. Connecting a battery backwards, even if you need a soldering iron to do it, is so common IRL that I wouldn't even say it's stupid, unless they did it on purpose.

Also consider an overload that isn't a short circuit, if you haven't yet. Like a bad battery that draws extra current but isn't 0Ω.

It sounds like you have multiple failures with similar evidence, I assume from different customers. So, I think it's safe to bet that there is an underlying design issue. Maybe you should try to think of ANY way you could cause this sort of failure, without too much regard to your assumptions about how the product is supposed to be used. I would also seriously consider something like a FMEA (Failure Modes and Effects) study, especially the "effects" part. I think hiring the right product safety consultant (for failure analysis, not just approvals) might be money well spent. Someone like Exponent* perhaps.

* Not really a recommendation, just an example. I used them a few decades ago and they were OK. YMMV.

Thanks. To be clear, there is no way the customer could plug this in backwards without physically modifying the cable. This modification would be very noticeable and it would not be able to be hidden. This has not happened with these.

Late friday I did get some good high temperatures. Unfortunately I do not see any mechanisms to cause that fault. There is a sweet spot of pulling just enough current to not blow the fuses that will get the wire fairly warm. I just do not see how to get the charger to charge the battery fully, then apply a 0.5 ohm short to the battery for 1 hour before the battery runs out of power. Then have the 0.5 ohm short disappear! Heck the best tap water would do is 2 ohms. As for bad batteries are concerned, I am still waiting for the associated batteries to be returned.

 

[Baluncore]

I once had a problem like that, where the interconnect wires were getting far hotter than could be explained by the rational circuit diagram.

The problem turned out to be an intermittent short circuit between the +5V rail inside a PC power supply, to the external chassis of that supply. Ground and earth were separated somewhere they should not have been, so the fuse did not blow. DC current flowed from the PC chassis, out through the mains cable earth, to the wall socket, then back in through a mains cable earth to the computer monitor. Then through the shield of the video cable to the ground on the motherboard. It all worked, but three different cables were getting hot, since +5 volts was being dropped, in the ground loop.

 

[Averagesupernova]

What is this charger exactly? Make and model numbers please.

 

[ethompson]

What is this charger exactly? Make and model numbers please.

The COTS part of the charger is WADUKS WA360 we have an adapter that verifies communication to the battery before letting the voltage connect.

 

[Averagesupernova]

The COTS part of the charger is WADUKS WA360 we have an adapter that verifies communication to the battery before letting the voltage connect.

I am not familiar with that. I did a quick Google search and came up with a unit but Google suggests Waouks which comes up with a number of chargers. So between the battery and the charger you have wires melting if I understand correctly. I've had this happen.
-
What caused the meltdown in my case was charging a lead acid battery with an Astron linear power supply. This is something that the company does not recommend and I soon learned why. I dialed the voltage until I got a safe charging current. I knew the power supply was not capable of overcharging because it was not a large enough supply. But what these supplies have on the output is an SCR that will fire if it senses an over voltage on the output thus short circuiting the output. I was running on generator power and a transient or something caused this SCR to fire. It shorted the output alright. Also shorted the battery. I wouldn't think a decent charger would be set up like this but your case made me think of that. Don't be so sure that for some reason the battery isn't supplying the power that is melting your wires. Not the other way around.

 

[ethompson]

I am not familiar with that. I did a quick Google search and came up with a unit but Google suggests Waouks which comes up with a number of chargers. So between the battery and the charger you have wires melting if I understand correctly. I've had this happen.
-
What caused the meltdown in my case was charging a lead acid battery with an Astron linear power supply. This is something that the company does not recommend and I soon learned why. I dialed the voltage until I got a safe charging current. I knew the power supply was not capable of overcharging because it was not a large enough supply. But what these supplies have on the output is an SCR that will fire if it senses an over voltage on the output thus short circuiting the output. I was running on generator power and a transient or something caused this SCR to fire. It shorted the output alright. Also shorted the battery. I wouldn't think a decent charger would be set up like this but your case made me think of that. Don't be so sure that for some reason the battery isn't supplying the power that is melting your wires. Not the other way around.

The only way would be the battery doing it, as it is the only thing capable of the required current. I just do not see how the load can happen in our dongle and then dissipater.

 

[Tom.G]

The only way would be the battery doing it, as it is the only thing capable of the required current. I just do not see how the load can happen in our dongle and then dissipater.

Then take apart the charger, and anything else connected to the battery, to see if there is a "crowbar" circuit across the output as @Averagesupernova suggested above in post #21. As those "safety devices" across the output exist to protect the device itself, they are more common than expected.

In any case, please keep us updated about any solutions/non-solutions you find - we like to learn too!

Cheers,
Tom

 

[Babadag]

Referring to #15, if we take only the strands in the parentheses BN (7*12/44) and TC (7*38/40 ) then the diameters are match with D1 and D2. However, TC is indeed 16", but BN it could be 25. BN presents 30.87 ohm/1000 ft and TC 4.057 ohm/1000 ft at 20oC [DC]

 

[Averagesupernova]

The only way would be the battery doing it, as it is the only thing capable of the required current. I just do not see how the load can happen in our dongle and then dissipater.

Is the battery discharged after this event? You said it yourself, the battery is the only thing capable of the required current. Go from there. I don't know why you insist on making this so difficult.

 

[Babadag]

If the all 4 wires are in parallel and the maximum total current it is 57 A then in BN will be 3.3 A and in TC 25 A and the temperature of conductors will be 200 dgr.C

However, it is for single cable only.I don’t understand what is the number total

of the cables and what is the running system-cable tray or else.

 

[Averagesupernova]

We need a diagram here. This is ridiculous. You are now talking about parallel wires?

 

[Babadag]

By the way, MJF PA12 Nylon parts have good temperature resistance. Thick parts will remain dimensionally stable in temperatures close to 175 °C, while thin parts will start to soften and lose their shape when temperatures exceed 95°C.
So if FEP layer is at 200oC then MJF thin layer will melt indeed.

 

[Ivan Nikiforov]

Insulation melts from a lot of heat. A large amount of heat is the result of too much current. Too much current is a consequence of: 1) Voltage is too high, 2) Resistance is too low. The voltage may be too high due to a malfunction of the charger. I suggest measuring the output voltage of the device without connecting to the battery. Resistance that is too low may generally result from: 1) Battery plates are short-circuited. 2) Incorrect connection polarity 3) Improper use (battery of different voltage, other devices). Given that the failure rates are massive, it seems to me that the problem lies with the charger itself. Either the charger outputs too much voltage, or the output circuit of the charger cannot withstand the reverse voltage of the battery, that is, the battery current flows through the charger and heats the wires.

 

[Ivan Nikiforov]

I'm sorry, I didn't understand the problem exactly. If only the positive wire melts, then most likely the problem is there. I suggest you compare the positive wire with the negative wire in terms of cross-section and material. If the wires are exactly the same, then the problem may be a bad contact of the positive wire when connected to the battery or when connected to the charger. Poor contact can lead to local heating and heat is transferred through the wire due to the high thermal conductivity of copper.