Nickel welding strips - current capability ??

[confusedinkc]

Hey guys, I'd like to learn how to build a battery pack. I have read several threads on here regarding the current rating for nickel strips. There seems to be a consensus among the experienced pack builders and the resident experts that a 7mm/.15mm strip is good for 5 amps (or 7 at the most).

I generally like to verify data presented from charts or formulas to make sure they match up with my reality. Seeing is believing, right?
I obtained a sample of some common welding strips for testing. The part that goes between parallel banks is 7mm in width, thickness is .15mm. I wanted to determine the current capability of this small section of strip (about 20mm).

I connected the strip in series with a 60V supply and a load that draws 10A. There was a question in my mind regarding the proper test voltage, either the differential voltage between cells (4V) or full pack voltage, so I opted for the worst case just to make sure. The supply has it's own volt and current meter, and I put an additional meter right at the strip for verification. Please refer to the pics.

The result was that the strip remained stone cold, no heat at all, none.

I measured 0.018 V across the strip. So the resistance is:
.018V/10A = 0.0018 ohms. According to my math (somebody please check it), the power dissapation is .018V * 10A = 0.18W.

Granted that is not nothing, but seems negligible. Strange, huh? What's going on? All the related posts in this forum says that strip is only good for 5-7 amps! I held the strip in my fingers for several minutes, no heat at all. I'm not getting any correlation, not even close.

Did I do something wrong? What am I missing? Is it just my alternate reality? Can some technician or engineer verify my results please?
Note: I posted this request several days ago at the end of an old thread, that is probably dead, so got no response (moderator, please delete the other post if that's an issue). Hopefully someone will see it now and respond. Can anyone explain the discrepancy?
Thanks a lot guys.

Signed,
Confused in K.C.



View attachment 5
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[eee291]

Now try it with 4V and report back.

 

[Alan B]

Independent Calculations

Bulk resistivity of pure nickel is 6.99 x 10(-8) ohm meters
Strip is 7mm by 0.15mm by 20mm long, current stated at 10 amps
Resistance is length / (width * thickness) * bulk resistivity
0.20m / (0.07m * 0.0015m) * 6.99 x 10e-8 ohm-m is 0.00133 ohms
Heat is I squared R or 10*10 * 0.00133 is 0.133 watts (supply voltage does not matter)
Voltage across strip is I * R or 10 * 0.0133 = 0.133V

Verified by https://chemandy.com/calculators/rectangular-conductor-resistance-calculator.htm, however they use a slightly different value for bulk resistivity of nickel, perhaps assuming a different ambient temperature.

 

[jumpjack]

The "unadequacy" of spot-welded nickel strips is an urban legend, just like venting hole of old A123 cells, which even led to building nickel strips with a hole to prevent hole from being covered!

Useless holes in strips:

Details: https://jumpjack.wordpress.com/2013/05/25/struttura-interna-delle-celle-a123-ai-nanofosfati-vavola-di-sfogo-o-buco-di-riempimento/

Your nice post with math eventually debunks the myth of the unadequate strips.

 

[flippy]

now make the strip about 4~8 cells long. so 8~16cm and run 20~30 amps tru it. love to see those heating figures.

a lot of people have more cells in P so they get longer strips. longer strips and with each cell you add the current of that cell so 20~30A is pretty realistic for high draw packs.

back of the hand give about 4W of heating on a 20cm long strip (5 cells deep). that is more considerable and if you got a 16S25P for example that means 250W of heating over the entire pack.
now the heating is lower because current flow is not equal over the entire strip but still... stuff adds up.

 

[Matador]

It's not so much about how much it heats up... It's more about the voltage drop... And that (the voltage drop) increases proportionnally to the length of your conducting strip. Most battery have way way more than 20 mm of strip in total...

You might want to avoid droping your voltage from 36 to 26 volts when under full load, if you know what I mean... But if you're at the point where you can feel the heat coming from your conducters with your hands, that means voltage drop is insanely high, and your conductors are (insanely) inadequate... Unless your trying to build an electric heater instead of an energy efficient (and powerfull) eBike.

Remember... If there is heat from conducting strips, the voltage drop is way past significantly high, it's way past unacceptably high .... If the conductors heat up, it`s a fire hazard waiting to happen. Once the nickel strip melt through the plastic covers of the cells and bridge the positive end to the negative casing (which comes very close to the positive), your battery will short and become a bomb.

Personally, I see every Watt of power lost in heat as less power injected in the motor... Why lose prescious power in heat when you can use it to propell you even faster?

 

[Alan B]

Measuring temperature rise should be done with some kind of instrumentation, fingers are not accurate.

To more easily compare 7x0.15mm nickel strip to other conductors, it has a resistance of about 20 milli-ohms per foot, about 4 times worse than the same volume of copper. (https://chemandy.com/calculators/rectangular-conductor-resistance-calculator.htm)

Looking at the ES table (https://endless-sphere.com/forums/viewtopic.php?f=14&t=68005), the 20 gauge copper value says less than 11 amps is optimal. The PowerStream wire table (http://www.powerstream.com/Wire_Size.htm) for 20 gauge wire says 11 amps is max for chassis wiring single conductor wiring in open air (and 1.5 amps max for power transmission based on the conservative 700 circular mils per amp rule). 7x0.15mm nickel has two times as much resistance as 20 gauge copper.

The National Electrical Code calls for a 15A circuit breaker on 14 gauge copper(http://www.usawire-cable.com/pdfs/nec%20ampacities.pdf), which has 8 times lower resistance than 7x0.15mm nickel.

As has been mentioned, there are many factors involved in choosing the ampacity of a conductor for a given application including heat production and thermal rise, insulation temperature capability, thermal environment, voltage drop, mechanical strength and the cooling of the interconnects to name only a few.

 

[confusedinkc]

Alan, thanks for taking the time to reply with that data. It basically confirms the raw data from my test and validates it was done correctly. The small differences can be attributed to my power supply not being able to put out the full ten amps at 60V. I am satisfied my raw data is correct. So the question is whether the strip gets hot or not at those values (about .1 watt).

I have been busy the last couple days but tomorrow I will repeat the test at a lower voltage and at the full 10 amps, then increase the current until it gets warm. I will try to get a temp sensor on it for a specific read out, eliminating any subjective observations.

Everyone will have their own opinions, I will be satisfied if the strip does not feel warm to the touch, indicating (to me) that any power loss is negligible.

 

[BliraVolt]

First of all thanks to all of you that have posted and lead me to calculate the specifics I need for my battery projects. Just as an add on regarding the table we can find pretty easily. I've made some calculations regarding the information in the table and what Heat loss they are referring to and this is what I got. And it's pretty consistent throughout the table. Heat loss in watts should be Optimal is .03, acceptable is .045 and the upper limit is .061. This is considering the table uses 1M as the length in the DC resistance length.

 

[fubgumfaw]

First of all thanks to all of you that have posted and lead me to calculate the specifics I need for my battery projects. Just as an add on regarding the table we can find pretty easily. I've made some calculations regarding the information in the table and what Heat loss they are referring to and this is what I got. And it's pretty consistent throughout the table. Heat loss in watts should be Optimal is .03, acceptable is .045 and the upper limit is .061. This is considering the table uses 1M as the length in the DC resistance length.

So...that graph you posted...
I had some pure nickel in .2mm x 7mm. I have an 82v battery pack partly run down and a 10 amp charger. I also have a laser temp meter and a watt meter and a clamp-on DMM that can measure DC amps. I used two 1" long sections of that nickel strip. In parallel and at 10 amps, there was zero heat. My laser temp meter could not read anything above room temperature. I then removed one section of nickel and effectively doubled the total resistance. Same 10 amps, no readable heat increase. So I soldered both sections in series which would again double the total resistance. Same 10 amps, no readable temp increase. I'm highly skeptical that the chart you posted is accurate! When 12 amps is "hot" according to the chart, then 10 amps ought to make some amount of heat and it did not increase the temp of the nickel even 1 degree! I'll have to pull out my 30 amp charger and try this again, but I bet that section of nickel doesn't get hot with that either.

I'll have to try 1m lengths and see how that goes...

 

[flippy]

I'm highly skeptical that the chart you posted is accurate!

you should be secptical. the chart is wrong.

and dont be afraid to use your "real" account. too many people here are too scared to say something that might offend something or brings their plans and ideas back to reality with dangerous concequences just so nobosdy get their feelings hurt. wich is an exxelent way to get someones house burned down....

ps: i generally buy 8mm wide nickel for straight packs and 10mm for honeycomb packs. :wink:

 

[spinningmagnets]

Current flows in series in a battery pack. A strip that connects one cell to the other might only be one inch long, but...there are 15 of them in a 14S 52V pack.

If the controller and motor are drawing 30A, then pass 30A at 52V through 15 inches of nickel strip to see if there is any voltage drop. Now try 50A and see how much voltage drop there is. If you are happy with the results, then you will be fine.

Here's a discussion that may be helpful.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=108006

 

[fubgumfaw]

I'm highly skeptical that the chart you posted is accurate!

you should be skeptical. the chart is wrong.

ps: i generally buy 8mm wide nickel for straight packs and 10mm for honeycomb packs. :wink:

OK I'll bite! What is the real current rating for .2mm x 7mm in pure nickel? A buddy of mine referred me to this thread when he saw me build a pack with .2mm x 7mm and claimed my pack would be a giant fire as soon as I tried to pull more than 10 amps per section of .2mm. I disagreed with him just from experience with building packs, but I honestly don't know what a single strip can handle.

I use a kweld which does great with .3mm so I'll be using that pretty much for everything I build once I run out of .2mm nickel. Even with 18650's there's room for wider than 7mm nickel. This is a good idea to add a bit more width.

I'm in the USA. Do you have a good source for nickel here?

Current flows in series in a battery pack. A strip that connects one cell to the other might only be one inch long, but...there are 15 of them in a 14S 52V pack.

If the controller and motor are drawing 30A, then pass 30A at 52V through 15 inches of nickel strip to see if there is any voltage drop. Now try 50A and see how much voltage drop there is. If you are happy with the results, then you will be fine.

Here's a discussion that may be helpful.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=108006

Yeah...this is known. My latest pack is 21700 cells at 2 sets of 20S5P. There are 2 layers of 20S5P for a total of 20S10P of 4800mah cells. My total length is 30" of nickel per layer. There is never less than 2 nickel sections per group of 5 cells. I did the math based on 100 amps draw and across both layers of 20S5P. I will see 19.8 watts of heat in the nickel. That's hardly worth mentioning really.

 

[fubgumfaw]

Delete/hold

 

[flippy]

OK I'll bite! What is the real current rating for .2mm x 7mm in pure nickel?

simple: it depends on the application.

ok, fair enough. not so simple...

the "heat" issue is really a time issue. you can pump 75A into such a strip if the time was short enough.

you have to look at the time that the current is being drawn. same goes for the cells. short pulses dont do much for cell temperature but sustained loads will heat up the cells and poor pack construction can cause (the middle) cells to heat up a LOT more then the outer ones that are not bunched up. same goes with the strips. one has to look at time and location.

fun fact: most of the heat of a strip gets absorbed by the cells...

if you can: take a piece of strip (length is not super important but 1ft gives you enough room to work), wrap a half dozen turns of kapton tape (as a heat insulator) and a temperature sensor to it and then simulate your current profile and see how the temperature holds up. anything below 50C is fine for high current applications and when using high end materials. 40C if you use cheap crap from ali/ebay. going above 50C means you will damage the cells. but at those currents i suspect those cells witll be cooking as well...

comments like this:

Now try it with 4V and report back.

simply mean a simple minsunderstanding of the physics involved here. thinking that voltage matters in the current capacity is exactly why this forum desperatly needs a wiki or a couple stickied topics that explain the basics. there are simply too many people that have "have heard something" and taken it as gospel (like the chart) without understanding what they are actually recommending is built on false premises or a simple misunderstanding of the material.

 

[Chalo]

My total length is 30" of nickel per layer. There is never less than 2 nickel sections per group of 5 cells. I did the math based on 100 amps draw and across both layers of 20S5P. I will see 19.8 watts of heat in the nickel. That's hardly worth mentioning really.

The nickel strip is one thing; the spot welds are quite another. They're harder to characterize, less consistent, and harder to measure than the strip material itself. My guess is that they always have more resistance than the nickel strip, but like I said-- that's a difficult value to measure.

Trying for a high performance battery pack and using homemade resistance welds are at cross purposes with each other. Both threaded terminals and cell tabs are easier to join with low resistance and without damaging the cells.

 

[fubgumfaw]

the "heat" issue is really a time issue. you can pump 75A into such a strip if the time was short enough.

if you can: take a piece of strip (length is not super important but 1ft gives you enough room to work), wrap a half dozen turns of kapton tape (as a heat insulator) and a temperature sensor to it and then simulate your current profile and see how the temperature holds up.

Yes...current over time = heat.
I let my test sit there for 10 minutes and checked it again. Admittedly my section of .2x7 was really short, but there was still no heat in it at 10 amps. At that point I posted what I did about being skeptical about that chart.

My 20S5P battery layers have about 30" of length in nickel in them. I think that's a real world test scenario...since it is real world. I have kapton. It would not be hard to cover the nickel in kapton, coil it up so it builds heat all in a tight area and see if my calculated 19.8 watts at 100 amps is real or not. Actually, since there is now where in the 2 layers with less than 2 parallel sections of .2x7, I can cut that 30" to 15" and get the same results as two 30" lengths in parallel. I'll try that! I can't create a 100 amp load outside the EV so any results would be calculated and not directly measured. Best I can do is about 28-30 amps with my largest charger. At least I can measure the voltage drop across the length of nickel, calculate for that amp load and guesstimate for the rest.

The EV has 3 battery packs in it of which I built 1 from semi used multistar LIPOs and the other 2 from LION cells. The 3 packs are all in parallel of course. The highest current draw I have seen so far is 500 amps across all 3 packs so that's more like 166 amps per pack and that never lasts very long. I've yet to see any heating issues in the packs and they all have temp sensors and smart BMS's on them. More "normal/aggressive riding" current draw is more like 300 amps across the 3 packs...hence the 100 amps I keep referring to.

If voltage created heat, then 3000 volts on a 32 awg wire ought to melt it instantly. This is the sort of voltages found in a taser. Lots of people don't think this through that it is current that makes heat, not voltage.

 

[fubgumfaw]

My total length is 30" of nickel per layer. There is never less than 2 nickel sections per group of 5 cells. I did the math based on 100 amps draw and across both layers of 20S5P. I will see 19.8 watts of heat in the nickel. That's hardly worth mentioning really.

The nickel strip is one thing; the spot welds are quite another. They're harder to characterize, less consistent, and harder to measure than the strip material itself. My guess is that they always have more resistance than the nickel strip, but like I said-- that's a difficult value to measure.

Trying for a high performance battery pack and using homemade resistance welds are at cross purposes with each other. Both threaded terminals and cell tabs are easier to join with low resistance and without damaging the cells.

I messed with solder paste and spot welds to .15 nickel a while back. The idea was to reduce the weld resistance as low as possible. I have a 4 wire milliohm meter. The solder paste did melt around the spot welds, but the reduction in resistance was negligibly better. I used 2, 2" lengths of nickel on the side of a dead 18650. I had 4 spot welds on each length. I then used the same cell and did it again with solder paste. I tried to keep nickel lengths the same and the distance on the cell can the same. Weld strength was not changed in the kweld. I should have gotten pretty consistently similar results leaving just the added solder making any difference. The reduced resistance was small enough that my meter couldn't consistently read a difference between spot welds and spot welds with solder.

My solution is to make my spot welds as powerful as possible without blowing through the cell. This did take some finessing! I did ruin a few cells before I got it right! I have looked at other peoples kweld settings and I tend to run mine (joules) hotter than they do. This seems to get me the least resistance between the cell case and the nickel.

I want to mess with the solder paste some more, but use it with copper instead. Who cares if you spot weld or solder, just as long as you don't heat up the cell too much in the process. A spot weld on copper won't weld without crazy current, but it will melt solder instantly.