To begin with I’ve looked at other batteries to make sure that I was making a good choice with 18650 batteries. Luckily it turns out it’s the most efficient way to pack as much power into the canister as possible. I’ve chosen batteries, after reading some battery tests, and the winner for me was the Sanyo 3.7V 2600mAh. This is the battery that I’ve previously looked at and features good price/performance. I’ve talked to my good friend Mikael, a electrical guru, about the charger and he recommended going with a balanced charger. Unfortunately all these chargers are real “gadgety”, ergo ugly, but they offer significantly better battery life as they make sure that both of the parallel batteries are charged equally. I choose the version that had a external power supply as this makes it possible to charge the battery from 12V in a car or on a boat. In total I spent $88 on batteries and charger.
Now onto the design of the canister. I’ve changed the wall thickness to 5 mm on the top part (above the battery compartment) and scrapped the idea of having screws on the lid to the battery compartment. I instead plan of having an O-ring in a groove on top of the lid to hold the lid in place (instead of a circlip). Basically I think that a circlip would not hinder the lid from rotating, which is important when inserting the top lid and you fit the banana contacts. I figure the O-ring will push enough to keep the lid in place and via friction hinder it from rotating. The lid itself will have a radial O-ring to keep the compartment dry.
This is the initial design of the lid to the battery compartment. I’ve read up on O-rings and it’s pretty straight forward, some good reading for the Swedes, (which is a condensed version of Parker O-Ring Handbook). Together with ERIKS O-Ring Calculator and Inventors built in O-ring creator I didn’t have to do much hand calculations.
To this I’ve found suitable banana connectors and added chamfers, etc. This is what it looks like now. Only ring connectors are missing. I looked at a sealing washer, but they seem to need pre-loading (deforming the soft metal washer, often in copper) and that won’t work here as the bulk material is softer than the washer, ergo just deform the lid instead. Furthermore the head of the banana contact is in plastic so that might break first.
- Some type of industrial glue in the unthreaded part of the hole, but I’m hesitant as it would be permanent.
- Threading the lid and using that part with thread sealant, but I haven’t found one that bonds well with metal-plastic. Maybe just need to find a primer, will need to look further on this one.
- Engulfing the whole thing in industrial glue
Ideas are most welcome!
One other thing I’ve thought of is that the drop in temperature when diving in cold water will cause the moisture in the battery compartment will condense. I’m thinking that a few silica bags will fix this.