Construction of an electronics box

Bag or box?
Is it worth going to easy?

If not the most difficult, then certainly the most labor-intensive stage when assembling an electric bicycle is the construction of a battery and electronics box. Of course, you can go easy, like I did in my first project, and use a textile bag mounted in the triangle of the bicycle frame. At first glance, this seems like a cool and aesthetic solution. Unfortunately, realities quickly verify the idea as good only for a moment. While it looks really neat and does not arouse much interest in bystanders, it turns out that such a bag has, unfortunately, several key disadvantages. The choice of solid bags is limited, and when we hit a nice model, we won’t fit too much into such a bag. In my case, the bag fits 78 cells of size 18650 (13S6P), 48V 18A controller, 5v power supply, cables and charging socket. I will not stuff more if I wanted to. This set weighs about 4 kg and this bag was certainly not designed for such a load. Once, I even managed to break the attachment strap when I drove into a larger hole on a dirt road. The most glaring fact for me is that the textile clamps fastening the bag to the frame and the bag itself at the point of contact with varnish can scratch all the paint into bare metal. At the end I leave the questionable watertightness of the bag itself in case of stronger rainfall.

I prefer solid box

In my subsequent projects it was certain that I would not use a textile bag anymore. I decided to make a box of polyester resin laminate and fiberglass mats. On thematic forums you can read a lot about how people make containers from this material. There are as many methods and formulas as there are makers. Most often, however, visually such boxes are very “heavy and rough” and I did not like them. I will not post pictures of boxes that I do not like, because their authors might feel offended, and it is not my goal to insult someone. I hope that everyone who reads my scribbles knows what I mean by writing “heavy and rough”.

I set a goal that my boxes must optically match the bicycle based on a regular frame. It is known that such a frame is made of pipes or more modern hydro-molded profiles. The cross-sections of such sections are max 50mm, and our box has the widest point around 130mm (you can’t do more because the cranks would hook the edges of the box). If I put in a nearly rectangular box, there would be a significant change in width around the frame tubes, which would not look good to me. Therefore, I decided to add bevels, running parallel to each of the tubes of the triangle of the frame. This solution, unfortunately, reduces the space inside the box, which we have to use for battery and electronics, but it fits much neatly with the entire bike. Practice shows that despite the reduced capacity of the box, you can easily stuff the 45A controller and about 1200Wh battery, which in an e-bike design based on a classic frame is sufficient to cover a distance of about 100km.

Let's go design!

Therefore, I have assumptions, it’s time to begin the project that will be supported by 3D CAD software.

1. I start by measuring the arms of the triangle formed by the frame. At this stage, you can help by cutting out a cardboard template, fitted to the frame and measuring its sides. I measure the dimensions inside the frame.

2. From the middle of the frame, I take a photo of the whole bike, which I then place in the software. Using measurements and photo as a background I created the rest of the project.

3. At the moment, I am mainly guided by visual aspects and I try to do everything to make it as pleasant as possible. I try to remember to leave about 10mm of space around the container, so as not to bother with its subsequent installation. At the first design of the laminated box – for “Pienius bike”, unfortunately I left too little space and I had a problem with the seal connecting both halves. Under the upper tube, the derailleur cables and the brake hose run and are almost in contact with the box. In the second design of the resin container for “Minion bike” I have already corrected it.

4. When theoretically everything looks OK, I proceed to check what I can fit into the modeled box. At this stage, you can still make small adjustments to the project, which can result in adding the additional battery section in or take one out.

Time to roll up your sleeves 🙂

Method one

1. Okay, the battery and controller are in the box, it’s time to finish the project and start construction. Theoretically, I made both boxes using the same method – by laminating. However, the devil is in the details. The „Pienius bike” box was made first and then I came up with the idea that I would make the mold based on inside dimensions for it, and after thar i will stick everything with resin and fiberglass mat. I did a template made of cardboard and adhesive tape. Such 3D PUZZLE.


2. I think the devil in its own person was sitting on both my shoulders and told me to follow this path. In so far as assembling the mold and laminating did not cause any major problems, but finishing this SHAPELESS THING (that looked like ham/gammon) that i laminated, required countless hours (at least 10 hours) and many sand papers for an orbital sander. After the this part, my garage was suitable for thorough cleaning. The foil I hung up to separate the dirty part from the rest did not help. Dust from grinding laminate settled everywhere, in every smallest, furthest, smallest corner of the room was a minimum half a millimeter layer of fine dust! Dust that you couldn’t just sweep away. Everything with Ania cleaned the next X hours and even the walls had to be wet cleaned … Anyway, see for yourself.

3. After initial sanding, I applied a layer of colored gelcoat to fill larger holes and sand again. Theoretically, this was to result in a navy blue substrate, however, I sanded a lot of it again, and I lost my nerve for the next layers of gelcoat.

4. After grinding, I halved the whole with a mini-grinder with a cutting disc. I tried to cut on a drawn line, but it is impossible to get two identical halves with this method. Fortunately, the whole is connected by a rubber gasket, which will mask the larger shortcomings of the cutting line. Somehow it all managed to bring order to order. After the first adjustments, it turned out that even it begins to fit.

5. In the workshop where I used to work, I managed to add some mounting plates to the box. Made of 1.5mm thick sheet metal with M5 welded nut and several spacers. Then I fixed it in the place for a water bottle. It takes a little bit of effort to adjust it all, but in the end it all worked out. While fitting the box to the bicycle frame, I roughly marked a point for holes, and then I first drill with a small drill d3, and then drilled to d6, taking care that the laminate did not tear. Then i adjusted the box on the mounting plates. At the top of the box I used a bracket printed on a 3D printer fitted to the bicycle frame.

6. It remains to fill the shortcomings that remained and paint in the target color. Unfortunately, I was not able to finish everything to high aesthetic level, my impatient nature won. It would be good to fill everything up twice, because after painting you can still see the irregularities of the laminate. However, I already wanted to put it all together so left it as it turned out. It’s time for ready assembly and adjustment of the rest of the equipment and for affixing the finishing labels of the whole design – laser cut from 1.5 mm thick aluminum sheet. I ordered colorful acrylic sprays and colorless matt lacquer at a popular auction site, choosing similar colors to the bicycle frame.

Time for the second method!

As they say “The first pancake is always spoiled”. Having learned all the hardships associated with laminating resin on the interior mold of the box for “Pieniuś bike”, I came to my senses.

1. I made the second box, this time for “Minion bike” by laminating the two halves separately (unfortunately I don’t have photos from the whole process …). Left and right forms, this time mapping the outer surfaces of the box, I also made of cardboard, of course, all previously modeling in CAD 3D. Corners with a radius I glued with pulp available in every shop with artistic products. I smoothed the exact radius in the cellulose mass with a disc, printed on a 3D printer, with a diameter corresponding to the corners.

2. This time sanding after lamination was just a cosmetic. In an hour I sanded both halves. In several places, you also had to use putty to level the surface on which there were cavities from air bubbles. The contact surface of the two halves was ground by sticking sheets of sandpaper on the table top and the whole half of the box was scrubbed on this surface with sandpaper. I consider such a laminating solution with a projection in the form of external planes to be recommended.

3. The box colors have also been chosen according to bicycle frame and ordered acrylic sprays. In addition, I also bought a colorless spray with a structure effect that is painted first. It gives the entire surface a sandblasted structure. This eliminates the visual effects of laminate unevenness and the whole looks very aesthetically pleasing. Finally, all the electronics had to be adjusted to the inside of the container again, and finally secured everything with mounting foam.

It's good 😀

I consider the goal almost achieved. It looks good, houses the equipment that I need. The mounting foam holds all the components inside as it should, and thousands of kilometers after verses are not able to loosen anything. Such a box can be opened if necessary by unscrewing one half (it is best to rethink the arrangement of components in this regard). The only drawback is the problematic fitting of the two halves relative to each other – even on seals. It is difficult to press perfectly so that there is a tight connection. Unfortunately, there is a risk that in unfavorable weather conditions water will get into the box through not tight connections at the top of the box. To minimize this risk at the joint, on the gasket, I additionally stick black insulating tape.

Bicycles with such boxes look really great. I highly recommend this solution to everyone, as the final e-bike based on a regular frame, with powers up to 3kW and a range of about 100km. If someone needs more (range or power), then better go straight to the custom frame.

Pieniuś i Minion
PIENIUŚ left / MINION right

We have also third way!

30.08.2018 – we have and third way. But what I will describe, you just have to watch it. I say we have, because it was created with the participation of Radek, from the TABA ON BIKE channel. The largest electric ebike channel in Poland.

Stay tuned 😊 :

FB group e-BIKEL-owcy technically about electric vehicles

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