1
$\begingroup$

I am trying to build a battery pack from 18650 batteries, each interconnection is made from steel strip, most '18650 strip' has a steel core with nickel plating.

I have a question regarding the purpose of the nickel coating found on 'nickel coated steel strip' which is used for welding 18650 batteries together to build li-ion battery packs.

enter image description here

Is the nickel plating's purpose merely to stop the steel strip from rusting/corroding? Or does the nickel aid in the welding process as well?

I am using a spot welder similar to the one in the image below, high current is sent between the two electrodes creating high heat on the contact patches between the steel strip and battery top/bottom. This heat melts the metals together at the contact points. enter image description here

Nickel is a better conductor than steel therefore if i am correct the nickel coating does not help in the spot welding process since the electrical resistance of nickel is lower than that of steel so more current needs to pass through nickel to get the same amount of heat production. Is there another explanation why (if true) nickel aids the welding process or is it merely to prevent rust/oxidation of the steel?

Does the same apply when using copper strip to connect the batteries together?

EDIT: I was thinking maybe molten Ni has a better viscosity (more fluid) when molten compared to steel, which could be positive in the welding process as the gaps between the strip and battery get filled better. I have found the following table with some info regarding viscosity of molten metals: enter image description here

Source: https://www.pnas.org/content/pnas/73/4/988.full.pdf

It appears Fe (iron) has a higher value for ϕ (Fluidity in reciprocal centipoise) so my theory is probably wrong...however nickel has a higher melting point than nickel so perhaps in the welding process the average fluidity of the nickel is still higher than the average fluidity of the steel at the surface of the strip (at the contact plane between the strip and battery housing). Could someone please tell me if this is of any influence and if my theory is somewhat accurate (and how much difference this (if existent) difference in fluidity would make)? I have not yet figured out how the temperature influences the viscosity of the molten metal and how to calculate/obtain the factor which determines the ratio between fluidity/temperature... perhaps someone more knowledgeable will be able to give some insight into this

Thank you very much!

$\endgroup$

2 Answers 2

1
$\begingroup$

I can see 5 purposes for using a Ni plated steel strip:

1) Protection against corrosion.
2) Improved electrical conductivity. Ni has about half the resistivity of 1010 steel.
3) Mechanical resistance (due to steel).
4) Low price (also due to steel)
5) Weldability

If the steel could be Cu plated, item 2 would be maximized, while keeping item 1 property. But there are problems with liquid Cu diffusion into iron grain boundaries that poses a great challenge to make Cu plated product. See for example: http://files.aws.org/wj/supplement/WJ_1978_01_s9.pdf

Increased electrical conductivity impairs weldability as you said, and it would be much more difficult with Cu.

Another candidate is Zinc plated steel. It is very common as a way of protection against corrosion, and the electrical resistance of Zinc is slightly lower than Ni. However the low boiling point of zinc ($ 871^0$C, well below iron melting point) is a problem for the welding.

$\endgroup$
1
+50
$\begingroup$

The welding process is an important step for the production process of a battery pack, because if we obtain an insufficient contact to the battery, the electrical resistance increases and we haven't gained much. And the welding process itself is an "art", where everybody has his/her own opinion. Of course there are many parameters, like the contact material on the battery side, which has to be taken into account, because there exists many combinations, which result in a brittle, undesirable joint. Also the surface consistency (waviness and cleanliness) is important, and nickel is definitely preferable compared to steel.

Nevertheless, I reckon the main reasons for using nickel plated steel are not found in the fabrication process, but in the functionality of the final product. The key idea is to obtain a "high" electrical conductivity -- that's why people use pure nickel instead of nickel plated steel if they need "high" drain currents. The second reason, which you already mentioned, is probably the corrosion resistance. Again, I haven't gain much, if my product is not lasting.

So, while the simplicity of the fabrication process is an important factor -- copper has a higher el. conductivity, but the fabrication process is more involving -- the functionality is the main reason.

$\endgroup$
4
  • $\begingroup$ Could you please elaborate on why "nickel is definitely preferable over steel" (end of first paragraph of your answer)? Thank you! Nickel has a slightly lower melting point than steel (1455 degrees C vs 1510 degrees C) which might help it 'flow' a bit better at the temperature the welding occurs at in order to better 'fill the gaps' to create a better contact between the strip and the battery. But I don't suspect this has much of a positive effect since the difference in melting temperature is quite low. $\endgroup$ Jan 21, 2020 at 22:56
  • 1
    $\begingroup$ There exists hundreds of different Nickel alloys. So maybe I was exaggerating. Of course, not all of these alloys have great properties regarding their machinability and cleaning. However, all alloys I have encountered (which are only a few) where superior to steel with respect to their "consistency": The passivation of a surface due to a predictable oxidation layer is something, is especially great for cleaning. $\endgroup$
    – Semoi
    Jan 21, 2020 at 23:10
  • $\begingroup$ Thank you for clarifying! that's good to know. Do you think the lower melting point of Nickel could have a significant advantage when welding, the nickel layer will be very thin so the effect of this (lower melting point) might be negligible (probably 0.01mm thickness). Unfortunately it turns out the viscosity of nickel is higher than for steel (if i understood the data correctly) so that will not give nickel an advantage in welding? (please check the image/link/explanation under image I added to my question). Thank you! $\endgroup$ Jan 21, 2020 at 23:23
  • 1
    $\begingroup$ A lower melting point always help. Just think of the thermal stress due to cooling. However, whether or not it is crucial, I don't know. But, I guess that the melting point becomes more important, if you are limited in temperature. The same is true for the viscosity. As said before, welding can be taken as an art form. However, it still is an "experimental" process with lots of trial and error. $\endgroup$
    – Semoi
    Jan 22, 2020 at 5:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.