Recently, we have seen several kits for assembling a battery pack out of 18650 cells. Here are some examples, so you can see if that is something that might interest you. And remember, these aren’t just for ebikes, you can assemble any voltage or size you want, for any job you need a battery to power.
The Original Gangster, Headway
The Headway company makes cylindrical cells that use the Lithium-Iron-Phosphate chemistry (LiFePO4). The other thing they are famous for is that their cells come from the factory with threaded studs on both ends of the cell. Also, their website provides all the parts to assemble a pack…including pre-cut metal bus-plates, and square plastic end-caps. I don’t know when they started this, but I wrote an article about a build that used these back in 2013, so this example of assembling a pack has been around since then, at the very least.
Since this is clearly awesome (because you don’t have to solder or buy a spot-welder), then…why aren’t they more common?
Headway cells don’t come in a variety of sizes (there are only two), and the smallest one is the 38140. This means that they are 38mm in diameter and 120mm long…and that, my friend?…that is BIG. The 38120 cell is 10-Ah for one cell, so a 2P pack would be 20-Ah. Although 20-Ah is a good size of pack (that I very much approve of), the only shapes you can configure them in is usually a big fat rectangle.
Our friend teklektik built up a Yuba Mundo longtail cargobike back in 2012 with these cells, plus the “Juicer” board tracker ebikes have used the 38120, and also, here is a pic of a pack that was assembled to replace a 24V lead-acid pack on an electric wheelchair.
Micah Toll’s pack-building kickstarter
We were excited to write about Micah’s kickstarter campaign. He realized that there were a lot of battery pack builders who didn’t want to buy a spot welder (commonly $300 or more for a very basic unit), but many of the home-builders were comfortable making the series connections with a fat wire and a soldering iron.
His idea was to provide paralleled modules of 18650 cells that had been spot-welded together. That way, there was no danger of the home-builder damaging the cells from the heat of a large soldering iron. We wrote an article on the internal construction of an 18650 cell, and we found out that the positive cathode can take a lot of heat, but the negative anode of an 18650 cell is very sensitive to heat-damage. Micah’s method meant that the home-builder would be soldering wire onto the nickel bus-plate, instead of directly onto the tips of the cells.
Even though Micahs system was awesome, some customers may want to use a different cell, or…maybe they wanted a different custom configuration of paralleled module…
Damien Rene pack-building spot-weld article
Damien Rene is an ebike enthusiast in Spain. One of our most consistently popular articles is the one on how he builds spot-welded battery packs for ebikes, found here. That being said, the most basic spot-welding unit you can buy direct from China is the Sunkko 709A, and it can be found for approximately $300 in 2017. Even as “acceptable” as that model is, you can google search “Sunkko fail”, and find discussion threads on how to repair and upgrade the guts on that unit.
There are 18650-cell spot-welding models that are much more robust and reliable than the Sunkko 709A, but…they also cost more than $300.
Generic 18650 Plastic Cell Separators / Spacers
For a very long time, there has existed a plastic end-cap design that helps the pack-builder to assemble a pack. The “slot” on top of the cells is an 8mm wide alley, which is often filled with a 7mm wide ribbon of pure nickel (nickel bus-ribbon that is o.15mm or 0.20mm thick are the most common).
As I found out when I was researching the 18650-format cell construction article, that the sides and bottom of every 18650 cell is one continuous conductor (ie: the sides and bottom of the cell are negatively charged). This means that…these plastic end-caps are very important. Vibration over the years can break through the PVC insulation-sleeve that comes over the sides of the cells, and a break right there will allow a short-circuit near the positive end.
The area near the shoulder of the positive cathode of the cell is the “danger zone”, and a compromise there is something that I call a “shoulder short” (the end with the white ring, in the pic below). It is VERY beneficial to add a self-adhesive fiber insulation washer to the positive end, and to use some type of plastic end-cap.
Once you decide on an available model of plastic end-cap, how do you attach each cell in series and parallel? The pic below is a common 2S / 4P bus-plate. In that pic, the four vertical pathways on the nickel bus-plate carry the high series current. If these are 10A cells, then each one of the four vertical runs carry 10A each, for a total of 40A for the whole pack.
The two horizontal pathways carry the paralleling current, which only keeps the cells at an equal voltage as they charge and discharge. It will typically be less than 1A, even for very high-powered battery packs. The only reason the series and parallel pathways are the same width and thickness on this common style of bus-plate, is that…doing this makes it fast and easy to laser-cut the shape, and the nickel is stiff enough to add some structural rigidity to the resulting pack.
To be clear, if you are charging your pack with a 5A charger, that 5-Amps is charging “in series” too, so…the parallel connections will only carry a tiny amount of balancing current, under all charging and discharging conditions.
In fact, nickel is not a great conductor. It is only used for two main reasons. I ‘would’ state that it has very strong anti-corrosion characteristics, but…nickel-plating over copper bus-plates [or aluminum] has the same corrosion protection, and that composition would conduct better and costs less (nickel is expensive, and getting pricier each year). The first reason is that…pure nickel is stiff, and provides some structural strength, while copper or aluminum is malleable (soft and easily bendable).
The second reason is that pure nickel [as a bus material] will spot-weld quite easily and very fast, which helps the assembly-line move along rapidly. Although many packs are still spot-welded in China one cell at a time by a person, the trend is to have robots spot-weld a pack together very rapidly.
Notice the slot that is located over each cell in the pic above. Electrical current-flow will take the “path of least resistance”, and that means that…if you include a slot over the cell, then the majority of the spot-welding electrical pulse is forced to pass through the end of the cell, instead of just passing through the bus-plate (from one welding probe to the other), which is what would happen if the nickel ribbon did not have that slot. This provides a more consistent result, when spot-welding cells to bus-plates.
Personally, I am a fan of the “honeycomb” configuration, over the now common rectangular “rank and file” shape (shown above). For roughly the same volume of battery pack, the honeycomb layout provides more air-flow between each cell. Whether you are trusting in passive heat-shedding, or a pro-active fan-cooling system…if you have a more consistent air-flow envelope around each cell? It will help each cell to shed any of the heat that develops.
NESE Modules, from Agniusm
The endless-sphere forum member “Agniusm” is a very smart enthusiast from Lithuania. I am posting his “NESE” modules first because…the 6P modules have been tested at 80A (13A per cell), and the bus-strips only warmed to 30C / 86F.
This business model recognizes that any new clever design can be copied and produced very cheaply in China, so…rather than create the modules himself and ship them around the world…he provides the design code for free, so each builder can have them produced locally from a 3D printer service. The difficult part that he does provide for the customers is…the metal 6P bus-plates, which can be trimmed-down to fit in any of the other sizes of case, along with the Poron foam backing (6P, 5P, 4P, 3P, 2P, etc).
The metal bus-tabs are 0.60mm thick (0.023 inches), and they are made from copper, and then zinc-plated for anti-corrosion protection (the early version shown below is a shiny nickel-plate, but the nickel cracked when the bus-tab end-tab is bent in order to mate with the connector-screw, so now…the plating is zinc). The most common nickel ribbon that is spot-welded to commercial packs are 0.20mm thick, so…these are three times thicker and also made from copper.
Since there will be a minor amount of heat-expansion / cold-contraction…some type of “spring” needs to be used to take up any slack, so the electrical connection is maintained with a consistent compression pressure. These modules use a thin black “Poron” elastomer backing to maintain pressure against the bus-strip. I was skeptical at first, but my research verifies that Poron is an excellent long-term material to use in this application.
The pic below shows two modules being paralleled to increase the amp-hours, while remaining at 4.1V when fully-charged (4.1V per cell is MUCH better for long life, compared to the common 4.2V). The 4P version shown below has three radial slots that make a convenient place to bond a pair of modules with zip-ties.
Here is a pic (below), showing a “crosswise” assembly, this pic shows a “10 modules in Series” (10S) pack, which is generally considered to be 36V. If you were using these 6P modules with 3500-mAh cells, the pack shown would be (6 X 3.5 =) 21-Ah.
Beneath the orange 3D-printed protective insulating caps, these modules shown below are connected positive-to-negative in order to “series” them into the desired volatge. Do your home work, and exercise caution!
There are many DIY options, and the pack example shown in the pic below is a “lengthwise” assembly.
The Vruzend kit adds plastic cell-end caps to each end of a cell, and each cap has a metal contact inside, along with a tiny bolt that extends out to create a threaded stud for attaching bus strips.
Like the previous styles that may have inspired it, each cap has slots and protrusions on their sides that can connect to each other.
The pic below shows one Vruzend cap cut open, and the metal contact has been removed to show its exact shape. It is a “corrosion-resistant” stainless steel, and it acts as a “leaf spring” that has a “C” shape. It also performs the duty of being the conductor, so these are limited to 3.5A each. A future version may have a copper foil overlay to provide a higher-amp version, but those are not available yet.
As you can see, this makes assembling a pack out of 18650-format cells much easier, compared to soldering (which can damage the interior of the cell from too much heat), or…using an expensive and unreliable spot-welder. Reliable spot-welders exist, but the ones that ebike battery factories use are very expensive…
The original kit was based out of India, and due to its popularity…there is now a US-based dealer in Florida…which is managed by our friend, Micah Toll.
If you decide that this kit is the one for you, I recommend you make certain to also order the 5.5mm nut-driver that they can provide. It is an odd size. If they had asked my advice, I would have specified a bolt that uses a 6mm wrench on the nut (which is much more common), and the bolt should have been brass. I understand that many global customers are price-sensitive, and every penny counts, but…they can produce two versions, an affordable “low amp” version, and a more expensive “higher amp” version.
If you wish to experiment with a copper foil overlay to increase the safe amp-carrying capabilities of these contactors, I recommend your initial experiments use 0.25mm thick copper foil (also called, 10-mil, or 30-ga)
If this kit appeals to you, you can view their website by clicking on the link, here.
The advertising for “Battery Blocs” seems to indicate that they feel their best customer base is the expanding field of DIY “Tesla Walls”. By that I mean…if you have a source of affordable 18650 cells, you can assemble a huge battery pack to mount in your garage wall, and it can be charged from either A) solar panels while you are at work, or…B) You use the “Power Wall” as a back-up because you live where the grid is unreliable, and…you keep it charged from the grid… while you charge it in the middle of the night, when the cost is lower.
The “Battery Bloc” system has a wide variety of sizes for each 18650-cell paralleled module. One interesting feature is that they use a long plastic bolt to connect both ends. It is interesting because the entire sides of an 18650 cell are negatively charged. Although swapping-in a common steel bolt would not touch the sides of the can, the amount of compression needed is low enough that a plastic bolt is acceptable, and the added safety is always a good design goal (if your battery is involved in a crash).
The most noteworthy feature of this kit is that…they use a small-but-strong neodymium “button” magnet as the connector to each cell-end. These magnets are mostly composed of steel, so…they are not good for high amps. This is not any issue for a very large “low-amps-per-cell” Power Walls. However, if you want to use these for the battery of a a longtail cargobike, upgrading the amp-capacity of the connections may involve adding a copper foil ribbon in-between the magnet and cell-tips (for initial experiments, you might consider using 0.25mm thick, 10-mil / 30-ga copper sheet).
Battery Bloc kits are available in a variety of configurations (4P, 6P, 8P, etc), so take your time shopping to see if these would work for you. You can find their website by clicking on the link, here.
The Energus company makes modules very similar to the NESE modules at the top of this article. However, they prefer to produce complete modules, which already have the cells inserted. As such, you must specify which cell you want inside the module (max amps, max range…or something in-between). and they also have a “plug and play” wiring harness available for the BMS, if you want.
If you like these, their website can be found, here.
If you know of another battery-pack building kit, contact me and I will check it out. If it looks like something that an ebiker might use, I will add it to this list.
written by Ron/spinningmagnets, June 2017