DIY Powerwall #2 (prepare holders)

Hi guys,

This second entry about the build a DIY Powerwall, we will prepare the holders on which we will mount the cells. As we discussed above, we will use 18650 Chinese (aliexpress) cells, Liitokala’s NCR18650B cells.

If you have not seen the previous entry, I recommend it, as we explain how to check the cells before moving on to their assembly. You can access it through the link DIY Powerwall #1 (testing cells).

Cell holder 18650

Let’s use some plastic holders that are 5×4 cells. These holders come prepared to be joined together, both lengthwise and wide. In our case, each pack of the powerwall will be 60 cells, so we will hook 3 holders. I’ll leave you some pictures:

You can buy these holders in the following link: buy holders 5×4 cells 18650

Here we can see the outer hooks that serve to join various supports
We joined three supports to form one of 60 cells
This is the end result

We need to make two 60 cell holders, one for each end of the cells.

Prepare cell holders 18650

The next step is to make a few small modifications to these holders to be able to fix the “bus bar” later, that is, the central copper cables to which we will connect all the cells. You can see them later.

The first modification we will make is to make two small holes as seen in the photo. We’ll do them on the 3 holders we have attached. To make these holes I used a wire that I’ve heated with a tin welder. I put some photos where you can see it:

Here I am making one of the holes. As you can see, I apply heat to the wire with a tin welder. This way the holes are made quite quickly.

The next modification will be to make a small rail on the outside of the holders. This rail will help us to make the bus bar settle properly. It’ll be firmer and keep him from moving.

To make the rail I also use the tin welder. I slide it through the outside of the stand, so that it melts it a little and forms the rail. I don’t use the tip of the welder, but I do it with the thickest area of this. Be careful because this gives off a lot of smoke. I recommend doing it in a ventilated area. I’ll leave you some pictures:

Here I’m doing the rail with the tin welder
This is the result. It seems to be quite ugly, but it’s something that brings a lot of solidity when we ride the bus bar.
Another photo of the final result

Prepare the “bus bar”

To make the bus bar, we will use “naked” copper cable, that is, it has no outside protection. In my case, I bought in a hardware store 35mm2 copper cable, which consisted of 6 wires about 2mm in diameter each.

It depends on the power you want to load from the powerwall, it is necessary to calculate that amperage can reach to circulate through the bus bar. In my case, for example, I will be able to load up to 5000W of power. By making a small calculation, let’s look at amp stories this translates: 5000W / 48V = 104A. So we’ll need to support 104A in every powerwall pack.

Each copper wire is 2mm in diameter, and can hold 25A without problem (I have performed tests to check it and nothing is heated with that intensity). Therefore, if we use 2 wires for each bus bar, and put two bus bar for each pole of the pack, they could circulate the 104A we need. We will need to perform these calculations to adapt to the particular needs of each installation.

What we will therefore do will be to destrench the copper cable to extract 2 wires. These two wires, we will hook them with a drill at one end, and on the other we will hold it with a bench screw for example. It is important that it is an adjustable speed drill to go slowly. If you don’t have a bench screw (as is my case), you can do it between two people, one grabbing the drill and one at the other end grabbing with some pliers (that’s how I did it). You’ve got to roll it up enough to make it well consistent. I leave you some photos so that you see how it looks:

Here you can see the original 35mm2 cable. He’d already removed some wires. It originally had 6 wires of about 2mm diameter each.
Here I untwist it to remove 2 wires, which will be the ones that we screw with the drill to make the bus bar
Here she’s giving the drill my partner, while I grab it at the other end with pliers.
Here you see the final result of the bus bar.

I used a length of 2 meters, since then we will cut it in sections of 40cm. So it would give us to make 5 bus bar. I leave you some photos of the bus bars already cut off:

The cell holder measures approx. 30cm. With a 40cm bus bar, we have 5cm left over on each side. The next point is to make a mark with those 5cm, then fold the bus bar with a pliers about 30 degrees. I’ll leave you some pictures:

Marking 5cm at each end, then make the angle of 30 degrees approx.
Folding the bus bar with a pliers
Here’s the end result. We’ve got the bus bars ready to assemble.
Here we present the bus bar above the holder to see how they look

The next step is to attach the bus bar to the stand. To do this we will use some wires that we will introduce through the holes that we made previously. First let’s prepare the wires. We will cut them into pieces of 6cm approx, and fold them with a pliers making a U-shape to facilitate insertion into the supports:

Cutting pieces of wire approx. 6cm.
Then we fold them into a U shape

Once the wires are prepared, we will insert them into the supports, in the holes we made earlier, from the inside to the outside of the supports:

Inserting the wires
The final result

Now we’ll fix the bus bar with these wires. First we screw the wires with our hand taking care to leave the bus bar well centered. Once we have it right, we will give it the final tightening with a pliers:

We put the bus bar
We tighten by hand by adjusting the bus bar so that they are well centered
We give you the final tightening with the pliers
Once tightened, we cut the leftover wire with a pair of tongs, and fasten the remaining wire so that it is not punctured upwards
This is the end result

The truth is very very robust. Bus bars don’t move at all. It’s a very good base for welding the cells afterwards.

The next step is to add the terminals to the bus bar. In my case I used copper terminals, for 25mm2 cables (it is the model SC25-6). You can buy them at the following link: buy 25mm2 copper terminals

Before putting the terminals in, we must bend the tips a little so that they are more parallel, and enter well into the terminal. We’ll finally tin with the terminals. It is necessary to use a blowtorch, as it is a lot of surface and does not look good if we do it with a normal tin welder. I’ll leave you some pictures:

We bend the tips of the bus bar so that they are parallel and we can insert the copper terminal
We enter the terminal. You must leave the flat part of the terminal outwards, to make it more comfortable to join the packs after
This is the end result. Now all that’s left is to add tin to fix it
We heat the terminal with the blowtorch and add tin until it is completely filled
Here’s the end result. It looks pretty good.

Once this is complete, we have the brackets ready to assemble the cells. It gives some work, but it is worth it, since, as I mentioned above, there are some very robust supports. The bus bar doesn’t move at all.

I’ll leave you a picture with the final result. We have ready the two brackets (top and bottom) to put the cells in:

All ready to assemble the first powerwall pack!!

In the next entry we see how to mount the cells on the brackets, weld them with the fuses and apply the final finish to the pack. You can access the following link: DIY Powerwall (battery pack assembly)

Any questions, leave it in comments.

See you at the next entrance! Best regards!

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