How much current in wire to reach observed temperature?

[ethompson]

TL;DR Summary

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]