DIY Powerwall #3 (battery pack assembly)

Hi guys!,

This third entry about the build a DIY Powerwall, we are going to make the first battery pack assembly. As we discussed above, we will use 18650 Chinese (aliexpress) cells, Liitokala’s NCR18650B cells.

If you have not seen the previous entries, I recommend it, as we explain how to check the cells before moving on to their assembly and how to prepare the supports for the cells. You can access through the link DIY Powerwall #1 (testing cells) and DIY Powerwall #2 (prepare holders).

Assembling the first pack of cells

The first step will be to insert the cells into the holders. Remember, these are cells we’ve checked before. It is very important that you do, as a faulty cell could ruin the pack. I leave you photos:

Here are the empty holders
We insert all cells into the holder. As in our case they are new cells, they all have more or less the same capacity, so we will not distribute them by capabilities. If they were recycled cells, so there may be larger differences and it would be advisable to distribute the cells of both bus bar so that they have a similar final capacity
Finally we put the top bracket and squeeze the whole set so that it is well inserted. It goes pretty tight into the cells, so it’s a little hard to leave it right.
Here’s the end result

Welding cells 18650 to bus bar

The next step will be to weld the cells at both poles to the bus bar. After several tests, I have decided to use 5A glass fuses for the positive pole, and 0.3mm enamelless copper wire for the negative pole. I’ll leave you some pictures:

0.3mm copper wire without enamel
5A glass fuse bag
Here the fuses in more detail

I’ve tested both components and the copper wire can withstand more intensity than 5A fuses, so in principle it should never act as a fuse. However, if a fuse is defective and does not blow, it could act as a fuse, adding more security to the system, which is never left over.

I have also checked the temperature of the fuses and the wire depending on the intensity that passes through them. Up to 3A nothing heats up, so we won’t be losing energy in the form of heat. From this intensity if they start to heat up so it would not be very efficient. In our case, as we calculated in the previous entry, a maximum of 1.7A will be extracted from each cell, so the system will not be heated at any time. All this, as I have already commented on in the other entries, it is necessary to calculate it for each particular case, taking into account both the capacity of the powerwall, as well as the number of maximum watts that we consider to extract from it.

Why have I decided to use glass fuses and non-copper wire for the positive pole?

Because it always blow at the same intensity, it does not depend on many factors such as copper wire (the intensity it holds varies with the length of the wire, if it has any wrinkles, etc.). Another feature that I like the most is that with glass fuses if we make a short to a cell for example, melts instantly and does not get to heat anything, while the wire takes some time until it gets red hot and burns, it can melt some plastic pack (plus leaves residue, not like fuses).

Let’s go on to explain the process of how I welded the cells. I started with the negative pole, where we’re going to weld the cells to the bar bus with the 0.3mm copper wire. First of all, what I do is apply some tin to the bus bar at the height of half the cells to later facilitate the soldering of the copper wire to the bus bar. I’ll leave you some pictures:

I apply tin to the bus bar in the middle of each cell. This then makes it easier to weld the copper wire to the bar bus

Then we weld the copper wire between each cell pair:

We weld the wire in the inner cell:

Then we passed it over the bus bar and weld on the outer cell

Once we have welded the wire into all the cells, we proceed to weld the wire to the bus bar:

We weld the copper wire to the bus bar. Here it helps us a lot to have pre-tinned the bus bar before
Here’s the end result

Now I move on to welding the positive pole of the cells. As we discussed above, in this case we will use the glass fuses. First of all I apply tin to the bus bar, as we did in the other holder, but this time I apply it at the height of the left edge of the cells, since the fuse is longer and must go sideways. Once the bus bar has been ready, we went on to prepare the fuses. Let’s bend one of the fuse legs 90 degrees with a pliers. We’ll bend it pretty attached to the fuse glass. Once we have everything ready we will go to weld them in the cells. You can see in the photos as I do:

I apply tin to the bar bus to facilitate welding with fuses. I apply it to the height of the left edge of the cells, not as on the other support that did it in half
We prepare the necessary fuses, bending 90 degrees one of the fuse legs. As you can see in the photo, I do it pretty close to the glass
It doesn’t look very good, but I pay the fuse leg that has the angle to the positive pole of the cell, adjusting the other leg of the fuse so that it is about 45 degrees from the bar bus, so that it matches the tin that we applied before
We repeat the operation with the cells on the opposite side, but reversing the orientation of the fuses, causing the fuse legs to match that go to the bus bar
Here we see the result after welding the fuses
We solder the fuses to the bus bar, applying tin to the junction between the fuse legs
We cut the leftover sections of wire from the fuses
The final result
We see it in detail

Finally, we check with a polymeter that there is continuity between the poles of the cells and its bus bar. We do it at both poles and with all cells:Check the continuity between the cells and the bus bar

Finish the powerwall pack

Finally, we’re going to add some protection to the pack, to prevent damage to any fuse or copper wire when we handle it.

First of all we will add an insulating that has a sticker on one side. I used the 80mm wide model.

I’ll leave you some pictures:

This is the insulating roll for batteries
We cut it to the size of the cell pack, and paste it. We do it on both sides. It should be noted, that with the bus bar does not have good contact surface and does not stick very well, but we do not care since the next step we solve it

Once the insulating is glued, we will apply a special adhesive tape for electronics, which is also heat resistant. In my case I bought the 50mm wide model. This will keep us from moving the board we put in earlier.

I’ll leave you some pictures:

This is the heat-resistant tape roll
I give the pack a full turn
The end result. The insulating or joking joint no longer moves

Finally we will add a shrink plastic to the pack. The size I bought is 180mm wide. This gives us a bonus of external protection that never badly. To apply heat, I used a hairdryer.

I’ll leave you some pictures:

Here we have the 180mm wide heat shrink roll
We cut it to the pack, leaving some margin on the sides, but without covering the copper terminals
We put inside the pack of cells
We focus it before applying heat with the dryer
We’re warming up with the dryer little by little on all the sides. This makes the case fit the pack
Once the faces are adjusted, we hit it by the edges, so that they fit as much as possible
This would be the final result of the pack. There is very compact, robust and well protected.
Some more photos in detail
Another one

We already have our first pack of cells. Now you only need to do 13 more to complete the powerwall (almost nothing…).

In future entries I will show the rest of the packs, and how I will roll them.

I hope it will help you if you are considering doing something similar.

If you have any questions, you can leave it in comments.

Many thanks and greetings!

2 thoughts on “DIY Powerwall #3 (battery pack assembly)”

    • Thank you! Glass fuses work very well, and they weld relatively quickly. It is worth the work. At the moment I have 5 packs mounted. I am waiting for more cells to arrive (covid does not help…)


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