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Project №3: Underwater housings
Part 1: Battery canister
“If I had it [life] to do all over again, I’d have been a plumber” (C) Albert Einstein
Contents
0. Disclaimer
We are not responsible if you get hurt or injured in any way while following this tutorial. By following this tutorial, you agree that you are doing this at your own risk. Please follow all safety precautions and rules when working with any tools or materials.
1. What will we get as a result?
Let’s start with the most basic thing - with power supply. Today we will enclose our battery assembly based on lithium-ion batteries. This is the underwater battery unit with a voltage of 12 volts and a capacity of 3000 mAh, capable of diving 100 meters underwater, we will get as a result:
| Fig. 1 - The underwater battery pack - the final goal of this tutorial. |
Winding and unwinding long wires is below average pleasure, and autonomous power supply is fashionable, youthful and modern. In this case, we are extremely lucky: lithium-ion and lithium-iron phosphate batteries of size 18650 (recall, 18 is the diameter, 650 is the length) fit perfectly, one might even say with a whistle, into a plumbing polypropylene pipe Ф32 mm. Polypropylene is a noble plastic that does not glue together with anything, does not decompose and is quite resistant to ultraviolet and aggressive environments, and there are so many different fittings for it that we will definitely not have any difficulty choosing something for our underwater needs. And most importantly, it will be the most simple, cheap and universal solution.
2. What do we need?
2.1. Materials
Table 1 - List of materials
| № | Meterial | Amount | Notes |
|---|---|---|---|
| 1 | 18650 Lithium-ion battery | 3 pcs | |
| 2 | BMS for 3S Li-ion | 1 pcs | To be able to fit into a pipe, for example this one |
| 3 | Mounting wire 0.35 mm2 | ~1 m | It’s better to have different colors |
| 4 | Polypropilene pipe Ф32 mm | ~1 m | |
| 5 | Polypropilene stub Ф32 mm | 2 pcs | |
| 6 | Brass nipple for cable up to 8 mm | 1 pcs | For example this one |
| 7 | Cable in rubber insulation 2x1.5 | ~1 ь | It is very important that the cable insulation is rubber or polyurethane, otherwise the cable will harden in the cold |
| 8 | Heat shrink Ф25 mm | ~1 m | |
| 9 | Soldering kit - solder and flux | ||
| 10 | FUM tape or similar | ||
| 11 | Nickel strip | ~1 m | If the spot welding machine is not used, it can be replaced with a wire |
I rounded the lengths of all linear materials to the nearest meter - it is always better to have a small reserve.
2.2. Tools and equipment
Table 2 - List of tools and equipment
| № | Tool/Device | Purpose |
|---|---|---|
| 1 | Soldering iron or soldering station with the ability to set the temperature to 250 °C | |
| 2 | Side cutters/nippers | |
| 3 | Knife or scalpel | |
| 4 | Tweezers | |
| 5 | Tool for welding polypropylene pipes | |
| 6 | Scissors for plastic pipes | Roller scissors are preffered |
| 7 | Multimeter | desirable for checking |
| 8 | Spot welding machine for batteries | desirable, but batteries can also be soldered |
| 9 | Heat gun | For heating heat shrink tubing |
| 10 | Drill | For installing the cable gland |
| 11 | Ø3 mm drill bit | To mark the hole for the cable gland |
| 12 | Ø15 mm drill bit (1 mm smaller than the thread) | For drilling a hole for a cable gland |
3. Assembly
Let’s start, of course, with trying it on:
| Fig. 2 - Let’s try it on |
In the picture I already have the plug with the nipple (penetrator) soldered, but it would be a good idea to try everything on first. Anyway, according to the tape measure, the total length of the pipe we need is 26 cm. Yours may be slightly different in length: the BMS board may be slightly different, the distance between the elements may also vary slightly, for example, due to the fact that you can use soldering instead of welding, etc. It is always better to have a small reserve, especially when assembling the first sample.
Now let’s assemble the battery, and based on the actual resulting length, we will adjust the required length of the pipe.
Let’s conventionally mark the place where the cable is connected with the top of the battery, and the opposite side with the bottom. So, we start assembling from the bottom, weld (or solder) a nickel strip to the “plus” of the lower element, leave a small gap and weld the “minus” of the next element to this strip.
| Fig. 3 - Welding or soldering the 18650s |
Next, we cut the tape so that a tail of about 5 mm in length remains. Now, if we put the elements together so that they stand on top of each other, there will be a tail of the nickel strip sticking out between them - we need to make such tails on each element so that we can solder the wires to them and pull them to the corresponding BMS terminals.
As a result, it should look something like this:
| Fig. 4 - Ready-made battery assembly |
It’s time to try on and cut the wires. I highly recommend using wires of different colors and generally getting into the habit of always using a wire with black insulation for the “common” and a wire with red insulation for the “plus”.
| Fig. 5 - Trying on and cutting the wires to the required length |
We solder the wires to the remaining “tails” and be sure to wash off the flux with alcohol:
| Fig. 6 - Don’t forget to wash off the flux |
We solder the wires to the board, the “lowest” one to the board output “0 V”, the next one to “3.7 V” and so on. The output and also the input of the battery are the contact pads “P+” and “P-“ - we solder short (~5 cm) sections of the mounting wire to them, and then we solder the KG cable to them.
We handle the end of the cable very carefully: a short circuit is not in our plans.
We must check with a multimeter that we have the appropriate voltage at the BMS output:
| FIg. 7 - Checking that the battery assembly is working |
Before shrinking the heat shrink, bend all the protruding tails so that there are no problems when pushing the entire package into the pipe. We also try to straighten the wires so that nothing sticks out anywhere.
| Fig. 8 - Everything is ready for shrink |
Now it’s time to start welding (or soldering?) polypropylene. The order should be as follows:
- first weld in the stub with the cable gland
- push the battery through, make sure that the battery does not stick out of the pipe
- release the cable through the cable gland
- weld the entire structure with the second stub.
To insert the cable gland into the stub, this stub must be drilled and threaded (at least nominally).
Since you will have to solder the stub with the cable gland already inserted, it must be placed as evenly as possible in the center, otherwise it may touch the heating element of the welding machine. Therefore, it is most convenient to first drill the stub from the inside with a thin drill, since it is easier to determine the center there.
Then you can drill from the outside, for example, with a step drill, 1 mm smaller in diameter than the cable gland. In my case, the thread of the gland is M16x1.5, which means that it is necessary to drill a hole with a diameter of 15 mm.
If you have the appropriate tap, you are lucky, and we cut the thread with the gland itself, carefully clamping the latter in a vice. The main thing is not to rush.
After the thread is ready, screw the gland onto the fum tape or its equivalent. Remove the complete O-ring beforehand. Tighten until it stops: so that the gland nut touches the end of the plug.
| Fig. 9 - We screw the cable gland onto the fum tape. At the end there should be no gap between the cable gland nut and the end of the stub |
Weld in the stub with the cable gland:
| Fig. 10 - Welding the stub |
It’s time to think about where the weak points in our design could potentially be, with the exception of destruction due to exceeding the maximum pressure, poor welding of polypropylene and leakage through the cable.
So, water can go:
- between the cable gland and the stub material
- between the cable insulation and the gland
- between the sealing of the gland and the gland itself
In the first case, we try to avoid leakage using fum tape (or its analogue), in the second, it is worth adding a little sealant to the indicated places. The sealant should be neutral, because there is a high probability that it will not polymerize/dry completely and can cause corrosion.
After we have coated all the places with sealant and it remains to tighten the compression nut of the gland, it makes sense to wait a while for the sealant to dry a little: 30-60 minutes is quite enough. Next, without fanaticism tighten the compression nut: the cable should not be overtightened like a sausage, but at the same time it should be firmly fixed.
Actually, that’s all. You are great! And with a new homemade underwater battery, you are doubly great!
4. Conclusion
As you probably guessed, it is not necessary to repeat everything down to the last detail: it was important for us to show the principle itself.
You can assemble a battery with a different number of elements and connect them in a different way, thereby obtaining a different voltage and/or capacity needed for your tasks. You can take a pipe and fittings of a different diameter, elements of a different size and/or based on a different chemistry, for example, lithium titanate or lithium iron phosphate. Moreover, in such a case you can assemble not only batteries, but also some electronics.
Of course, in order to use such a battery, a sealed connector must be installed at the end of the cable. For small depths (~10 m), you can use IP68 connectors, for example, like these or like these.
You can, of course, buy one, but if you are attracted to the path of the Jedi, you can make it yourself. We will tell you about this in the next part of this tutorial.