spinningmagnets wrote:When it comes to electrical conductivity, aluminum and zinc are both similar or better than nickel, and also much cheaper than nickel. I am persuaded that using 0.20mm thick nickel strips to form the paralleled sub-groups is reasonable, but for the thicker high-current series bus-bars, it seems to me that copper is easy and cheap to use there...
I've made a few ampacity tables for different metal strips in order to try to understand what is "reasonable" for weld metal strips.
Here's my original tables in an excel file :
The thing is, "Ampacity" is NOT a "Cast in Stone " value
that is only proportionnal to the gauge (aka cross-sectional area) of the metal used as the conductor. Ampacity not only depends on the gauge (ex : AWG) and the type of metal, but it also depends on the lenght of conductor, the increase in temperature and the magnitude of the voltage drop that you're willing to tolerate.
I mean, Ampacity can be really high-rated if you don't mind your conductor rising at 105°C (221°F) (
loosing precious watts) in an open-air environnement, and if you don't care about loosing 10 volts in voltage drop just in one meter of conductor. Also, ampacity depends on the length of the conductor used... The shorter, the better.
Standard ampacity tables seldom specify these parameters when they give ampacity values. See for example : http://www.powerstream.com/Wire_Size.htm
From these charts, and looking at what Ohms/km (mOhms/m) values correspond to what ampacity values for different gauges of copper, you can extrapolate ampacity values for different metals of known resistivities : Hence I made this table extrapolating data from PowerStream.com (http://www.powerstream.com/Wire_Size.htm
) and from wikipedia (resistivity of different metals : https://en.wikipedia.org/wiki/Electrica ... nductivity
Just as an indicator of what ampacity values PowerStream.com expects from copper wire, they rate 10AWG at 55 amps (I personally think that would get hot). So if you trust PowerStream.com's ratings, use this first table.
So for you answer
... Is 0.20mm thick Nickel (let's assume 10mm width) enough for each parallel cell ? Well, it depends on the cell you choose to use and what amperage you intend to pull out of it. 0.20 x 10 mm should be okay for 7.9 amps. So if you intend to pull max 7.9 amps out of each cell (as in 31.9 amps in a 4P pack), you should be okay, but the strips might get a little warm or even a little bit hot at these sustained currents...
On the other hand, if you plan to use 20A
high-current rated 18650 cells (ex : Sony IMR 18650 VTC4 2100 mAh - 30A
; Samsung 18650 30Q 3000mAh - 20A
, LG 18650 HG2 3000mAh - 20A
, Sony IMR 18650 VTC5 2600 mAh - 20A
) then those Nickel Strips will be a serious bottleneck to what these cell could be capable of delivering with proper wiring. At 20 amps per cell or 30 amps per cell, these 0.20 mm thick nickel strips will heat up and fry (remember this : https://endless-sphere.com/forums/viewt ... 0#p1234386
). Even if there'is a copper bussbar collecting current from individual parrallel nickel strips. Even 10A cells
(ex : Panasonic NCR18650PF 2900 mAh - 10A
or SANYO NCR18650GA 3300mAh - 10A
) in my opinion would suffer a bit from bottlenecking if welded with these nickel strips rated for 7.9A. It's a bit like uprgading a car with a new big turbocompressor, but without upgrading the exhaust and intake pipe system ... The restriction from the way too small pipes will make the car underperform, counteracting the potential added benefit of the new turbocompressor ...
Personnally, I don't think the PowerStream.com Ampacity chart is suited to my own battery build. I mean, I wouldn't want my conductors to go over 50°Celcius (to prevent lithium cells degradation from heat). Bare in mind that these conductors would also be enclosed (not in open-air) and I would then be using about 10 meters total of conductor strips... So with theses parameters in mind, the ampacity values has to be seriously derated (see this website for a automated calculator : http://www.electriciancalculators.com/c ... toradv.htm
and this website for Neher-McGrath theory on calculation of ampacity : http://www.electrician2.com/articles/ampacity.htm
So extrapolating again (ampacity for max 50°C), I made another ampacity chart that is more stringent and more suited to my needs. Only 2 volts of drop when max amps approach ampacity. Here it is :
Just as an indicator of what ampacity values I personally expect from copper wire (based on calculation for max V drop of 2 volts for 10 meter, and max temp 0f 50°C), I rate 10AWG at 30 amps. So if you trust that rating more than PowerStream's ratings, use this second table
Just to give you an idea, in your house, your mains are required to be at least 14AWG for 15 amps rated circuits.
Well according to the PowerStream charts, 18AWG would be enough for 16 amps. Hence, I think they overrate.
With my own rating standards (from calculations made for 50°C, 2 V drop per 10 meters) ,14AWG would be rated for 17 amps... I think that is more comparable to household wiring expectations.