Build Blog for OpenROV #358 (aka "Little Boy Blue") - Post 3


Hey Everyone!

Today's blog will be covering Step 6: Assembling the Battery Packs, and Step 7: Assembling and Waterproofing the Thrusters. This was a day of blunders, blisters and mistakes. Nothing too big, thankfully. Read on.

Step 6: Assembling the Battery Packs

It's been a couple years since I've had to solder, so I needed a quick refresher before working on the battery terminals. If you've never soldered before, I highly recommend these two YouTube videos for a primer on the proper technique: & The second video also discusses using flux when soldering which prevents oxidation of the wire being worked on, among other things.

Once reacquainted with this technique, I soldered the 18 gauge wire to the button and spring terminals. Having a helping hands tool with alligator clips to hold parts being soldered makes things so much easier – do buy one.

**Note: I know this sounds obvious for anyone with common sense – and after several years of being in a hands-on field I should know better – but don't solder with shorts on...especially on a drafting table that slants downwards. That handy-dandy liquid-hot metal has the tendency to roll off the table and onto one's lap. That is all.

The War Zone.

I secured the terminals to the acrylic circles using hot glue, then attached the spring terminal to one end of the battery tube. I accidentally made wire loops on both the button and spring terminals, but reheating the glue with the tip of my glue gun to remove and reposition the wire on the spring terminals was a simple fix.


Next came cutting a small hole in each black rubber end-cap to feed the battery terminal wires through. I completely massacred these end-caps with my Xacto knife – ended up applying too much force when cutting through the rubber, making the opening slits too long. Is there a simpler way to do this? I might have to go back and make sure those slits are sealed up with epoxy, so as not to spring a leak when underwater.

Like the tutorial states, use the batteries as a guide to getting the correct wire lengths to push through the battery tubes. I also found having the batteries in the tube helped to keep the spring terminal cap level when hot gluing.

Use the batteries as a guide to help you determine the appropriate amount of wire to string through the tube.

For adhering the rubber end-caps, I purchased Loctite Instant Mix Epoxy, which comes with two self-mixing applicators. One thing to note is the 5-minute set time for the epoxy – once it hardens the applicator nozzle becomes unusable. I suggest preparing both battery tubes so you can epoxy them at the same time.

Feeding the wire through the end-cap now filled with epoxy. Let sit vertically when done.

Step 7: Assembling & Waterproofing the Thrusters

Use a very small Flathead screwdriver or razor blade to pry off the snap ring from the motor. Be careful not to stab into the wires when cutting and removing the heat shrink tubing. I found cutting with the blade pointed upwards helped. Or, you can make multiple passes on the tube, cutting slightly deeper each time. Don't be afraid to give the heat shrink a small tug to peel the rest off. The remaining motor dis-assembly and attachment of new wire was smooth sailing.

(Left) Using an Xacto with the blade up to cut through the pre-existing heat shrink. (Right) Peeling off the heat shrink.

Working with the solder and liquid electrical tape really revealed my lack of finesse for tasks requiring fine-motor skills. Basically working with the liquid electrical tape reminded me of when I paint my nails: failure of epic proportions requiring massive clean-up. The supplied brush is pretty huge – application with a small paintbrush would be more precise.


Good soldering technique is crucial – don't leave on too much solder when attaching the wires to the motor. Doing that required me to use a larger heat shrink tube, causing a tighter fit when shrink-wrapping all 3 wires together. I had one large tube tear on me when heated. I don't have a heat gun, so I quickly and lightly rubbed the soldering iron along the length of the tube in order for it to contract. The last procedure in Step 7, Propeller Assembly, was straight-forward and easy.

(Left) Heat Shrink tore due to too much tension from the wires squeezed inside. (Right) Applied a new one.

I'll end this post with a quick question: what exactly needs to be sprayed with silicone? The tutorial stated to spray the inner and outer part of the motor. Should all surfaces be coated?

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