Yes, because those are connected to a hard/heavy brick of cells on only one side, and there’s no source of external flexing to cause problems.
Yes, but there’s rigid, and then there’s rigid. In my view, anything esk8 is considered semi-flexible, because in esk8, everything flexes. The deck, the enclosure, the battery, the motors, the trucks, everything. It’s a fact of life, and you must build around it. For an ebike pack that’s basically a brick of cells that isn’t attached to a long bendy piece of wood supported only at the ends with a dude bouncing on top, it’d be totally fine.
But you wouldn’t catch me dead with an esk8 pack built like that.
This. Everyone thinks things are rigid until you look at a slow motion video of said thing in action, then you realize solid metal and carbon fiber is flexing around like a noodle as it tackles hits and terrain.
So, you got me curious about the fatigue limit of Nickel. I did some reading. Based on my understanding, Nickel should not show any fatigue in this application. It in fact seems to have far better properties than aluminum or copper, and behaves more like steel that has a finite fatigue limit.
If anyone has more relevant data, please correct me, but it seems like folding nickel in a semi-rigid pack is safer than riding Precision CNCd Aluminium trucks.
You’re assuming that the nickel is vibrating in elastic deformation not plastic. This is why flexibility is important. to let the nickel move with the board. If too much strength, the nickel will pull on the much smaller weld points and possibly go into plastic deformation, from then its just a matter of time.
Post all the chart, but grab a strip of nickel and bend it back and forth a few times past its elastic point. It will break. thats why they take the time to cut double rows of nickel with bridges inbetween. It’s easier to just put a big thin sheet over, but the bridges and gaps serves a purpose.
I see.
Do we have any hands on data on this for our use?
Has someone built a pack like this? I assume many have.
Has anyone experienced a fatigue fracture from flexing? I would love to hear about that.
Why am I interested in this: I am trying to form my own opinion, rationally.
I have seen welds ripped apart. But your question unfortunately has too many variables. How flexible is the deck, how often is it ridden and over what terrain? Even what wheels would make a significant difference as to how much stress is put into the battery.
You could however, build 2 similar P groups, one with a sheet across and one with just strips, bend each back and forth a number of times and count how many times before a failure occurs. That will give a baseline on the difference.
That should be the same if the cells in a p group were epoxied into a block.
Let’s look at the case where the p groups are rigid. And cells are yet connected with strips. So, only bending forces and vibrations should attack the strips. Has anyone’s battery broken from that?
(And not from bad welds)
I’m really curious. Looking at the material properties, I think it should perform better than copper - which is today’s go to. Obviously it’s not directly comparable because we use copper in fine strands, but still, I think this could be a legit method.
See you named the fundamental principle there. The reason why copper wire is in fine strands, is multiple thinner pieces is more flexible and allows more bending before fatigue sets in, than one solid piece of copper. Same as nickel strip. Multiple thinner strips better than one thick one for flexibility endurance.
On point. Now it would be interesting to know how much the fine strands offset the significantly lower fatigue limit. I do not have numbers on that, though.
“Work hardening , in metallurgy, increase in hardness of a metal induced, deliberately or accidentally, by hammering, drawing, or other physical processes.”
Pretty sure this should not happen with only one slight bend, and even if it does we do not care how hard the nickel is…
Almost all batteries are built like this, some of mine have been going for 3 years with no issues
People need to stop overthinking and being chair warriors on line talking about how stuff should be done…
I will sure do some practical tests on my own to get a better feeling for it.
Edit: However, some fundamental properties matter - like steel having a finite fatigue limit and Aluminium not, which matters long term, even though they might initially test similarly.
to others having debate if nickel can crack - it probably could on battery with all series connections out of nickel on flexible deck. But in case when series in length of battery are silicone wires, no way. Many people do 180 degree series nickel (LHB for example) and many will do - many times there is no way to do it other way.
And you didn’t count one important thing here - cells also have their cycle life. They will fail before nickel.
