Nickel Strip becoming work hardened by vibration, and breaking

ESK8 Mechanics
, , , ,
1

So I finished my first build a few weeks ago, and up until now it was running like a dream! Unfortunately I begin to have connection issues, where the voltage would drop to basically nothing in certain orientations. I was expecting broken solder but when I opened it up I found broken nickel strips.

I had some u shaped nickel strips to bridge the junctions of some of the batteries like this: 20190126_155650

While the solder joints connecting them to the strips on the batteries are fine, they are broke at the U bend, most likely from all the micro vibrations ever so slightly bending them, work hardening them, and making them brittle like a paperclip would if you bent it back and forth enough. I see signs of this same wear on a few others as well.

I’m not sure what the best way to fix this is, because if I just put a different U shaped strip in there I think there’s a good chance it’ll just happen again in a month or so. Any advice or ideas?

Cheers!

3 Likes
2

Lots of conductive materials will work harden (copper/nickel/aluminum…) after stress so the best thing is to use lots of thinner pieces like braided flat wire…if you’re hurting for space then you can connect the batteries to plates and mount the plates at one point to the deck to prevent the batteries bending

What does the entire pack look like?

3

Hmm… That’s an interesting idea with the flat braided wire!! You think if I jam some strips of that in there are fill it with solder, the capillary action will suck it all up and create a solid connection with the terminals/strips?

Here’s a shot of the entire pack: 20181029_225155 Since taking this photo, I’ve swapped out the BMS on top with a thinner one that can handle more amperage (30A). Only using a 4A charger with this.

4

hmm, only one layer of nickle?

5

I’ve been told soldering directly to the battery usually doesn’t work, but you might be able to get copper braid to stick to nickel strip, if you tin the nickel and copper first it should work a bit better

6

There are two U bridges of nickel between each of the junctions in the middle of the pack. Are you suggesting that I add another layer of strips across all junctions? My max amperage output currently is 60A, and unless I drastically change my setup, I don’t see that ever increasing.

1 Like
7

those are 0.15x 8mm strips right?, people usually do 2 on parlell (just to be safe) and 3 on series. each strip can handle around 8 amps i believe

8

Soldering directly to the cell most definitely does work, but it’s not recommended because it’s nigh impossible to do that without getting the cell too hot and degrading it.

Soldering to nickel that’s right next to the cell is much better but you still need to beware of putting heat into the cells, and be quick about it.

2 Likes
9

How do you connect to an 18650 w/o a spot welder?

10

Well you can solder directly to the cell, but it’s not recommended at all and even if you did, you’d have to have solder skills

This cell is soldered

20171227_031414%20(another%20copy)

2 Likes
11

Disclaimer: NOT RECOMMENDED to solder to the cells. This was done with a super-hot high-quality iron, with 30 years of soldering experience, and I had a piece of aluminum handy so the cell had an iron on it for about 1 second, then a piece of cold aluminum pressed against it, sucking the heat off the endcap before it traveled into the cell. Without shorting the cell out, of course. And it probably still damaged the cell a little bit.

1 Like
12

you know that spot welding the same spot 6 times in a row will cause the cell to heat up a lot too right? (not by a shit tone though)

52c on the same spot for spot welding 212c for soldering (unsafe)

1 Like
13

Spot welding doesn’t generate as much heat, it works on a much smaller area than a soldering iron which is a big piece of metal and has tons of heat capacity. I can touch the terminal immediately after a spot weld and its not scorching hot, it is after soldering.

OP: Rather than bending metal tabs to meet the other segment of your battery, just solder some lengths of an appropriate gauge wire to some nickel tab, then spot weld those nickel tabs onto the battery. That will allow the wire to do the bending instead of your nickel tabs. This is how a lot of large factory-made packs are done.

Certain cell layouts are easier to wire without needing as many connections like this - a big long serpentine arrangement is really great if you only have 2-3 parallel, avoids having to turn corners or connect cells head-to-toe. (i realize that’s not practical for a lot of eSkate builders as we need typically thinner packs that have more parallel cells.)

Be careful to insulate everything really well! if you have enough rattles to break those connections then think about how the cells might also rattle around in there, rubbing against each other or the case. The factory shrink wrap on most cells is not very durable.

2 Likes
14

I still think it is fine to solder on the batteries some times if you:

  1. Have a good soldering iron and appropriate tip.

  2. Are fast; in and out.

  3. Know what you are doing.

  4. Aren’t a moron.

2 Likes
15

What happens if I am:

  1. using old kit
  2. have a weak solder pull out game
  3. am a dumbass
  4. <see #3>
2 Likes
16

Don’t.  

10 Likes
17

Unlucky guy

1 Like
18

image image image

Friend owned inboard M1 nickel broke. I am a little skeptical about the battery build that folds nickel. Is there a good way to secure the series single stack securely? I think the Chinese 10S2P double stack is robust.

19

Personally the pack design isn’t great, the series connections should ideally run down the length of the battery and then at one end have a bridging WIRE to connect either side of the lots of cells. This way no strips are bent and the two half’s can flex to the concave of the deck. Like this diagram from @mmaner

image

If you can redo the pack that is the best, but adding braided wire for the serial connections is another good way

4 Likes
20

If you decide to go with nickel strip, one way to reduce the amount of bending that the nickel sees is to increase the radius of the bend. Space the cells farther apart, so there’s more room for a less-sharp bend, and then put something stiff and slightly compliant (like a 1/4" thick strip of firm rubber) between the cells to keep them from smooshing back together. This can also be paired with using several thinner strips instead of one thick one.

If you go with copper braid or wire, use one that has lots of fine strands, and keep the radius large just like above. It does zero good if the whole thing is saturated with solder, that just makes it behave like a solid bar. You want solder on the ends, and none in the middle. When I was doing my packs, I always used a pair of hemostats clamped on the braid to suck the heat out and keep the solder from flowing up into the middle, which worked great.

Side note on soldering to cells: If you’re at all uncomfortable with it, don’t do it.

[spoiler]If you do do it, have a good temperature controlled iron with a clean fat tip.

Use an aggressive flux (not just the stuff in the solder wire), I use “ruby fluid” liquid flux, sold at my local hardware store for soldering copper pipes. It becomes active at a lower temperature than rosin flux, and that means it takes less time to tin the cell. Less time means less heat is transferred.

I add a small drop of flux to the cell, and with a blob of solder to the iron, I quickly tin the cell, then cool it back down to room temp. Then I tin the copper. Then I put the copper on the cell, and use the heat of the iron to melt and smoosh them together, using the absolute minimum amount of heat and time. Immediately cool the cell again.

I find that having the iron very hot (~400C) allows it to bring the joint up to temperature much faster, and thus the actual amount of heat transferred to the cell is less than if you tried to go with a low temperature.[/spoiler]

3 Likes