Nickel welding strips - current capability ??

Batteries, Chargers, and Battery Management Systems.
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confusedinkc   1 µW

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Nickel welding strips - current capability ??

Post by confusedinkc » Jul 29 2018 2:50am

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.

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eee291   100 kW

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Re: Nickel welding strips - current capability ??

Post by eee291 » Jul 29 2018 11:13am

Now try it with 4V and report back.

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Alan B   100 GW

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Re: Nickel welding strips - current capability ??

Post by Alan B » Jul 29 2018 11:15am

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/rectan ... ulator.htm, however they use a slightly different value for bulk resistivity of nickel, perhaps assuming a different ambient temperature.

jumpjack   100 W

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Re: Nickel welding strips - current capability ??

Post by jumpjack » Jul 30 2018 3:46am

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:
Image

Details: https://jumpjack.wordpress.com/2013/05/ ... empimento/

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

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flippy   1 MW

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Re: Nickel welding strips - current capability ??

Post by flippy » Jul 30 2018 7:07am

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.
Lithium beats liquid dinosaurs.

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Matador   10 kW

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Re: Nickel welding strips - current capability ??

Post by Matador » Jul 30 2018 7:58am

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?

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Alan B   100 GW

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Reality Checks on Ampacity of Battery Wiring

Post by Alan B » Jul 30 2018 9:35am

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/rectan ... ulator.htm)

Looking at the ES table (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   1 µW

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Re: Nickel welding strips - current capability ??

Post by confusedinkc » Aug 01 2018 1:45am

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   1 µW

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Re: Nickel welding strips - current capability ??

Post by BliraVolt » Jan 24 2020 4:18pm

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.
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