The battery builders thread has gotten too long to read through for new battery builders, so this thread will guide you through building a brick-style battery pack out of li-ion cells. That being said, this guide will assume you have some prior knowledge about PEV batteries. In particular, this guide will NOT answer the following questions:
- How do I choose what cells to build with?
- What are parallel groups?
- How do I decide what S and what P my battery should be?
- How do I spot-weld?
- How do I solder?
- How thick wires do I need for X amps of current?
- How do I wire a BMS?
If you are unable to answer any of those, then please, use the forum’s search feature to understand them first! 
If you are an expert battery builder, please skim through this post and comment on any mistakes you notice / things you do differently. 
The characteristics of a brick pack
A brick pack is one of the more easy battery pack shapes to make, because there is no flex to worry about, meaning you can weld the cells groups directly to one another. However, this also presents some challenges, because brick packs are heavy, and can easily have the welds broken off if nothing else is holding the cells in place. They also require careful gluing to make sure they end up rectangular, and not skewed.
If the space and capacity requirements allow for it, I highly recommend using plastic cell holders to keep the battery in tact. You can get these in both grid and staggered configurations. These solve the hardest problems when building a brick pack, so please consider these / custom 3d printed cell holders as an option!
In this tutorial we will be building a pack without these, because I wanted to cram the biggest battery as possible into my mountainboard enclosure. I will be building an 12s8p 21700 pack with the last group and BMS stacked on top for a more narrow footprint. I am using reclaimed cells, so please ignore the markings on the cell ends.
Ready? Let’s go!
Step 1 - preparing the parallel groups
Measure the voltage of each cell in each p-group with a voltmeter to make sure that all cells in a p-group have the same voltage! If they differ, use a li-ion battery charger to ± even them out.
Align the cells using a dedicated glue guide (you can 3d print one) or improvise with some wood or books.
Use high temperature hot-glue on both sides of the group to keep the cells together and aligned. If done right, this will be stronger than silicone, and let you get each group done in a manner of minutes. You will need hot melt glue sticks and a special, heavy duty high temperature glue gun for this.
Then add fishpaper rings to the positive end of the cells, and wrap fishpaper strips around the positive end of the cells. (Only wrapping one end of the cell will still create a gap between the adjacent parallel groups, and let us save on resources. It will also give us a better glue bond in step 2.) If you are doing a staggered battery layout, make sure to indent the fishpaper in so that it conforms to each cell. This is an important step, because the plastic wrap around cells can be easily damaged, so the fishpaper acts as an insulator preventing a short-circuit between adjacent groups.
Step 2 - combining the parallel groups
To keep the groups together (so that it’s not only the nickel strips holding the pack together) we will be using neutral cure silicone. (Other silicone types can be acidulous, which will not only smell, but might cause a chemical reaction with the battery cell housings.) We are using silicone instead of hot-glue for this step for 2 reasons:
- Silicone has a longer drying time, which will allow us to adjust the alignment of the whole pack in 1 go and make it easier to work with
- Silicone is easier to cut through in the future in case a parallel group requires replacing
Because the fishpaper does not have a very strong bond with the cells, you want to apply the silicone in the middle of the parallel groups, so that it touches the cell housings of both groups. Every other cell group needs to be upside down, so that the negative ends are always next to positive ends, and vice-versa. See pic:
Once all groups are siliconed adjacent to one another, you will need to straighten and align the pack to make sure it holds its rectangular form and will fit in the enclosure you have in mind for it.
I measured the length of a parallel group, and sawed 2 pieces of wood to that length. Then I used 2 longer pieces of wood and a bunch of screwable F-clamps to make a box that I could apply pressure to.
Once you start applying pressure, it’s easy to create a parallelogram rather than a straight-angled rectangle. The simplest way to confirm your shape is straight is to take a tape-measurer and measure the length of the diagonals of the box. Both diagonals should be the same length. If not, then adjust the clamps to squeeze the longer diagonal smaller.
Once the box is straight, leave the silicone to dry. (This is usually 24-48h)
Step 3 - welding
Since all nickel strips will be so close to one another, it’s very easy to accidentally cause a catastrophic short-circuit during the welding process!
Personally, I believe that the safest way to approach this is:
- Weld groups 1 and 2 together
- Cover the nickel with kapton tape as a low-effort insulation against accidents
- Flip the pack upside-down
- Weld groups 2 and 3 together
- Cover the nickel with kapton tape
- Flip the pack over
- … repeat until done
A tip I recommend when doing spotwelds is to use 2 strong neodymium magnets to keep the nickel straight and in place, helping avoid accidents.
I was also too lazy to cut my own nickel with rounded corners (which is highly recommended to avoid the nickel poking through insulation layers), so I used pre-cut nickel strips for an 8P battery from https://www.duckbatterysystems.com/
It’s safer and easier to use a single piece of nickel though, so do as I say, not as I do.
If you choose to go a similar route with a middle nickel piece bridging the other 2, you want to make sure your welds on the connecting piece are lower current, and only on the areas where there is not a cell end exposed underneath. (Welding directly above the cell might result in a poor weld between the 2 nickel pieces) Likewise, make sure not to weld above plain air, as that will blow a hole in the nickel 
I used a Kweld spot welder to weld this pack, using 0.2mm nickel. I needed 50J for the nickel to cell welds, and 32J for the nickel to nickel welds. Just for reference.
I recommend cleaning the probe tips with some fine grit sandpaper every now and then for consistency. Large packs need a lot of welds after all!
This picture is a good guide for how good welds should look:
That being said, even a perfect looking weld from the outside can turn out to have bad contact with the cell. To avoid this, I highly recommend:
- cleaning the cells ends and nickel strips with isopropyl alcohol before welding
- test welding to some spare cells beforehand, and try ripping the nickel off with pliers. The nickel should tear when pulled off, meaning the weld is stronger than the nickel itself. If the nickel comes off clean, then the weld was not good. This usually means you were applying too much pressure with the weld probes. (You want very slight pressure, so that a miniscule air-gap between the nickel and the cell adds resistance and creates a hotter, stronger weld.)
Step 4 - Insulate the battery pack
Apply fishpaper to both sides of the battery pack, potentially the sides as well if you have space. Use fiberglass tape to keep the fishpaper from peeling off.
Bonus step - Adding another p-group
For my pack I only had space for 11 parallel groups, so to make it 12s I added an extra group on top. For this reason I folded some nickel, added kapton tape beneath the folded nickel for protection, and soldered the group on with some 12AWG wires. I secured the additional group with kapton tape in a diagonal pattern to keep it from moving around.
Step 5 - Adding the BMS
- Make sure there is plenty of fishpaper insulation between the BMS and the battery
- Use hot glue / tape the BMS to the battery pack.
- Route the balance wires along the battery, using hot glue to keep the wires in place and in order. You can route the wires on the side of the battery between p-groups to protect them. If you need to cross any balance wires over one another, put fishpaper between them so that they cannot rub against one another can cause a short.
- Double check that your balance wires have not changed order before you shorten them and solder them. This is an annoying mistake to fix if you make it.
- Use an exacto knife to cut through the fishpaper and kapton tape to make a small hole available to solder the balance wires to the nickel between the p-groups.
- Cover the solder spots with fishpaper or hot-glue to insulate it again.
Step 6 - Solder the main discharge leads
You want to solder the wire along the length of the nickel strip to make sure the nickel isn’t carrying the current to a wire at only one end.
Step 7 - Finishing touches
- Glue down your BMS temp sensors
- Fuse your charge port / charge circuit
- Add kapton tape / fishpaper to insulate any areas that are still missing insulation
- Add main discharge fuses to protect your battery in case your ESC shorts itself
- Surround your battery in shrink-wrap, if you have space for it
As you can see, my battery barely fit my enclosure, so I was unable to shrink-wrap it. I surrounded it with some straps so that I can lift if out if need be.
If you are designing a battery for a certain box size, definitely leave some extra space for insulation, inaccuracies, extra wiring, foam. I got extremely lucky with my battery fitting, in hindsight I should have just made a smaller one with less cells.
Step 8 - Charging the battery
Test charging the battery, and connect to your smart BMS to set up its settings if applicable. After the battery is fully charged, you should disconnect the charger and measure the voltage of the p-groups to make sure they are all fully charged and holding their charge.
