EMX Battery Boxes
The push toward electrification of transportation is here, and it's starting to ramp up in the consumer market. While there is great debate about the environmental benefits and drawbacks of electric power, the performance advantages can't be ignored. From a purely performance standpoint, electric power has shown to be vastly superior to internal combustion and has done so utilizing powertrains that are exponentially simpler than their gas counterparts mechanically. In the offroad motorcycle world, energy storage has proven to be the biggest hurdle to overcome as consumers demand equal or greater performance compared to their gas powered bikes, as well as an acceptable run time. Because of this, creating systems for full sized motorcycles has proven to be extremely difficult with currently available technology at the consumer level, however Electro & Co decided to take a different approach to the problem. Rather than start with full sized bikes, they have decided to focus on the mini bike market and take advantage of the pitbike craze that is alive and well within offroad communities.
The Electro & Co EMX 14 minibike
Earlier this year Electro & Co came to me with a problem. They had selected components to power a smaller motorcycle previously driven by a 65cc gas engine, however they had no way to house the components. Due to the very limited quantities being produced and the unique application of the system, 3D printing seemed like a very feasible manufacturing method. The design of the box needed to meet the following requirements:
1. The design must fit within the given chassis parameters, while still being able to hold the required battery cells and battery management system (BMS).
2. The design must consist of an inner and outer box. The outer box would be rigidly mounted to the frame, and the inner box would allow quick removal and replacement of the battery cells and BMS as a sealed unit, making the system safe for consumer use.
3. The design must be adaptable to different chassis and battery cell sizes in the future.
4. The design must be able to handle the harsh environment of offroad riding.
5. The advantages and limitations of additive manufacturing must be considered.
First order of business came with creating an outline of the shape of the box. Every square millimeter of available space was needed in order to fit the large battery cells and BMS, and multiple iterations were tested. To use time effectively, a small sample of the box shape, usually about 5mm tall, was printed and inserted into the frame to check for interferences. This was truly a "napkin sketch to reality" type of story, moving from hand drawings and rough dimensions to a completed profile after multiple revisions.
Exploded view of major battery box components
The next hurdle was creating an inner box to house the battery cells and BMS, which would be sealed from the customer for safety reasons and removable as a single unit. This required an electrical connection interface that is proprietary to Electro & Co, however it can be disclosed that all necessary connections are made automatically as the inner box is installed into the outer box. Connections to the motor and controller are made via large gauge wires exiting the box through cable glands. By designing the box with the advantages of 3D printing in mind, features could be created that were not possible with other forms of manufacturing. Fully enclosed cavities as well as guides for wire routing were integrated into the box itself, allowing for geometry to be optimized without the limitations of traditional subtractive manufacturing.
A completed battery box
The final challenge came in selecting material. Given the size of the print, a low warp material that is relatively cheap and simple to print was clearly necessary. For normal FDM printing this left two popular options, PLA or PETG. Given that the box would likely see impacts during a crash as well as general debris during normal riding conditions, impact resistance as well as UV stability were also critical considerations. These are two areas where PETG excels greatly over PLA, enough to warrant its use even though it can be a bit trickier to work with. However, because the inner box would not see impacts or be exposed to UV light for extended periods of time, PLA became the chosen material for this piece due to its favorable printing qualities. For the door gasket, a low durometer TPU was selected to ensure a watertight seal during normal operation and washing.
Race testing during the Havoc Co. Pitbike Championships
So far, unit testing has been very successful. The bike has been race tested at a high level and the box has withstood the expected tip-overs and general abuse as intended. Printing has proven to be a bit of a challenge, but this can be contributed to the size of the piece and the limitations of the printing equipment rather than the design itself. Refinements to the design and printing process continue as more testing is done, and the packs just now being released for sale to the public.