Luna Cycle has usually been tight-lipped about its battery technology, and recently has decided to come out of the closet about their assembly methods. Thanks to Luna recently publishing pictures of its “Naked Pack” it has been confirmed that Luna Cycle is using a wire bonding process in its “Fusion” packs (usually hidden behind a potted plastic box) and has become the first ebike company ever to use the high tech wire-bonding assembly method. With the revelation that Luna is using wire bonding in its packs, it becomes clear that Luna is really striving to be a high tech leader in the high-power ebike business. You can see the new Luna Cycle Wolf pack here.
In May 25th of last year, Luna Cycle took delivery of the latest technology in wire-bonding and shot a photo of the giant wooden box it came in, which was posted on the fast electric bike facebook group where the Luna founder Eric Hicks often posts. Here is what he said…”What’s in this one box, we gambled everything for…and this has the potential to change this ebike game forever.”
By “gamble”, he meant that this was an expensive wire-bonding machine, and it would be very difficult to implement all the steps necessary to use this new technology. There was a lot of speculation about “what is in the box?” from the forum members. One forum member retouched the photo a little bit:
Announcement of the “Naked Pack” and the wire-bonded revelation
This new wire-bonding machine enabled Luna to bring one of the hottest technologies available in the car industry into the Ebike world.
Last week Luna gave a sneak peek to their soon-to-be-released “Naked Wolf” pack which clearly showed the wire bonding technology being used in all the Luna Fusion batteries. Fusion batteries are the in-house built batteries that go on the Luna ebikes (see the Fusion line of ebikes here).
What is wire bonding?
This is a system and method that links batteries in parallel to conductors using wire bonds that act as fuses in the event of an overcurrent condition in a battery. To protect the wire bonds in the case of a larger overcurrent condition, a fuse may be added in series to the parallel batteries.
Ultrasonic wire-bonding is actually nothing new, and has been used for years in the semiconductor industry, as far back as the 1970s. Tesla decided to use wire-bonding to build its production battery packs for its electric cars from the very start.
In 2006, Tesla filed a US patent that described a method for using wire-bonding techniques to connect multiple cells into a larger battery pack. Groups of individual cells are more commonly connected together with a soldering or spot-welding process (which can apply a large amount of heat to the cells). Tesla’s patent application claimed to find problems with these approaches, saying that “the welding process is time-consuming and prone to failure. It is also difficult to test the connection between each battery and the conductors”
Tesla recently (and generously) released its patents to help proliferate good EV technology. This has laid the groundwork for small companies (such as Luna) to use very advanced technologies that are normally reserved for the big guys. Ironically Luna Cycle is currently leasing a building in El Segundo that was once occupied by Tesla’s sister company, “SpaceX”.
Attaching cells together with no heat
Heat is dangerous for an 18650 cell. Most existing processes such as spot-welding and laser-welding use high heat to connect cells together.
Wire-bonding is a combination of 3 parameters that form the bond…ultrasonic vibrational power, downwards force, and a very precise amount of applied power. The power and vibrational elements comes from an ultrasonic transducer that generates mechanical vibrations that weld the 18650 cell without heat. When done properly, and with the right equipment…wire bonding is virtually a cold process.
There are some DIYers on the power-wall building forums that try to do wire bonding using a mickey mouse method of soldering huge paralleled lines of cells together…this is not recommended since it misses one of the big advantages of wire bonding which is that…when it is done right? it should not create any heat inside the cell at all.
Also, soldering is a very unreliable method of assembling 18650s and your chances of failure are high. Be aware that…the negative end of an 18650 cell is the most sensitive part of the cell to risk of damage from heat, but…once you are set up to assemble a pack with a reliable method? you might as well bond both ends to a common bus-plate with the same method. This Garage-built method of wire-bonding shown below is bad, and we do not recommend that anyone reading this should attempt wire-bonding at home.
Advantages of ultrasonic wire bonding over spot welding
- No heat is used in wire bonding, unlike the other methods.
- Wire-bonds are easy to test for a strong bond…unlike spot welding, which are hard to test and are notorious for coming loose. Luna Cycle uses state-of-the-art equipment to perform a wire-bond strength test to ensure that each bond is strong. As a contrast, there is no known way to effectively test the strength of a spot-weld.
- Spot welded packs are not flexible, which makes the connection easier to break, where wire-bonding has some flexibility, which in the case of a pack flex, the bonds will not break.
- Wire-bonding allows for real and effective “single cell” fusing.
- Wire-bonding allows us to use highly effective and efficient CNC equipment, to greatly decrease the amount of labor that goes into a pack, and to make it affordable to have a high-tech pack built in the USA.
Even with the use of high grade equipment, spot-welding is tricky and difficult to get right. And it is a process that does not lend itself well to CNC, so it still requires many hours of hand assembly, which is not conducive to American manufacturing (this is why most ebike batteries are still made in China). There is a recent trend for home-builders to DIY a spot-welded pack together with inexpensive equipment and no Quality Assurance (QA) testing after it’s built. We would advise anyone who is assembling a home-built battery to be very careful, and to do your research.
Aluminum bus bars instead of nickel
Bus bars are what connect the cells together in series. Most ebike battery packs use nickel ribbon instead of custom laser-cut bus bars which Luna Cycle is able to make, since they now have a state of the art laser cutter. Luna Cycle makes their bus bars in-house from sheets of pure aluminum, which is much more conductive, effective, and reliable…compared to nickel strips.
Most existing ebike packs use nickel strip bus-bars to connect the cells together, using a very low-tech process we would compare to slamming your balls against the walls. Nickel is not the ideal electrical conductor, and is only used because it is easy for robots to spot-weld. Nickel does conduct electricity (obviously), but…nickels’ conductivity is only 25% of the conductivity of copper, according to the international IACS chart that is readily available. Aluminum is more than twice as conductive, which means fewer watts are thrown away as waste heat, and that means aluminum buses can carry higher current.
The larger the 18650 pack, the longer some of the connections must be, and a nickel bus would need to be thicker to even work. The thicker the nickel is, the harder it becomes to spot weld to, and more heat and pressure is required to make an acceptable spot weld..and if you do not use thicker nickel, inefficient and dangerous resistance bottlenecks to current-flow will be created.
A lot of builders advocate using double-layers of nickel throughout the pack, which is not necessary. The problem is that…certain areas of the pack must have 2x-to-4x the thickness of nickel ribbon to allow for high current, if you want high-performance. Nickel is fine as a material for battery cell-buses when using low current, but…when you begin to demand high performance?…no matter how thick you make the layered nickel ribbon, it simply becomes a resistor instead of a conductor. When it comes to conducting high current, aluminum and copper are the only options (of course, you “could” also use silver and gold as high-performance conductor options!)
Luna Cycle “Fusion” batteries, made in the USA
Wire-bonding allows Luna to use highly effective and efficient CNC equipment to greatly decrease the amount of labor that goes into building a pack, and that makes it affordable to have ebike battery packs made (with love) in the USA. Most importantly, using American engineering and American ingenuity, Luna is able to make a product that sets a new bar in the industry for the level of quality and safety that is possible for an ebike battery.
There are many advantages to buying from a USA company, both in the quality and support of the product you get, and also the intangible “buy American” stuff, like supporting fellow American workers, and keeping as much of your money as possible within our borders. It is also helping to bring to life the American dream of a new and innovative product that has a global demand, and it is assembled in an American factory, located in southern California.
Wire-bonding…a complex process
Building a wire bonded battery is high tech process and not an easy one to do. It requires meticulous planning during the design process, sturdy spacer design and construction, multiple jigs and above all?…a strong process control.
In order to build a wire-bonded battery pack, over 50 pieces must be custom-fabricated using a combination of CNC equipment and 3D printers. All these parts are made in-house in the USA, and also assembled in the USA, using space-age methods, such as wire-bonding. The process to make wire-bonding work is painstaking and intricate, and most of it should be done in a “clean room” environment.
But once it works and the pack comes together? it is well worth the effort…It is the most advanced ebike battery we have ever seen.
Enter The Luna Wolf Pack
The new Luna Wolf battery pack will replace the shark pack on all the Luna Cycle ebikes. It will make our Fusion line of batteries more affordable and will eventually be available to our kit buyers as well.
The Wolf Pack is currently made up of Samsung 30Q cells is 52V and is 12-Ah. It is a high-power pack capable of putting out 50-amps continuous, to feed our most powerful motor systems (2500 watts, like the Ludicrous BBSHD, among others). The Wolf Pack is not only wire bonded it is also “potted” in an epoxy which completely weather-proofs the battery and makes it fall-proof as well.
The Wolf Pack uses a state-of-the-art Battery Management System (BMS) developed just for Luna, and made here in the USA. The Luna BMS protects the battery, and shuts it down if it senses any unsafe condition. Like the rest of the battery, this BMS is the most advanced BMS we have ever seen used on any ebike battery, and is capable of putting out up to 70 amps of current when needed. At the heart of any great ebike…is a great ebike battery pack.
Written by Eric, March 2018