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.
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.
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:
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!