I read in a previous post about the induction method of charging an electric toothbrush.
In my wanderings around the net I've seen a series of commercial marine fishing devices that report the depth and spread of their nets to the ships. These devices have two stainless contacts on their exterior for re-charging their batteries in a cradle. It'm wondering how these re-charging contacts work, especially when they're in salt water and does anyone have a circuit. My reason is that I'm thinking of putting together a "Kontiki", a floating battery powered torpedo shaped fishing device that takes a line out past the breakers. The prop is either at the rear or under its front for better self-correction and it's on a timer. As they're battery powered (sealed lead acid or ni-cad pack) it'd be good to be able to re-charge it without having to be constantly opening and closing it. As I write this it occurs to me that an overcharge would possibly cause the batteries to vent and build up pressure and fumes inside the watertight casing. I understand that sealed lead acid batteries and saltwater don't mix (chlorine gas) but it they seem to be in regular use as a power source.
Any help here is greatly appreciated as I'd really like to know just how they accomplish this.
I've done tons of UW batteries in the past. Aluminum (or other) pressure case that is basically some Sched. 40 pipe with caps on each end... (involves o-ring groves and surfaces to be designed into the system)
I've always put pressure relief fittings in them to make it easier to get the tops off if the temperature is cooler when trying to open them, than when they were sealed. (I'm talking about 8"-14" diameter cases here...) Also, if you got seawater in on your batteries, you get lots of gas... you don't want to be unbolting a case that is under extreme pressure, design the case to fail by blowing past the o-ring in such cases)
There are several manufacturers of UW rated connectors, many that can be mated and de-mated under water. Pick up a copy of Sea Technology Magazine for listings.
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In my experience, as long as you use a charger rated for Gell Cells (different technology from Sealed lead acid batteries slightly, do a search) you will get no gassing. But you have the purge plug there to be safe before undoing the cap bolts.
Things to think about:
If you don't go a pre-made route by using a trolling motor, you will have to deal with issues of shaft drag due to hydrostatic pressure.
Make sure you get a connector that can take the maximum current that your charger (or motor) will draw.
If you don't want to get into the expense of machining o-ring seals in heavy containers, many Oceanographers have made battery cases of lighter materials, and filled the extra space around the batteries (again you must use gell cells) with mineral oil, this eliminates the pressure differential around your prop shaft, but doesn't get rid of the necessity for a water proof connector. It also means that if you don't get it right, you have a hell of a mess when you want to make changes.
I expect that in your exposed contact object, they had a diode in there yes...
For all the instruments we built (dozens) in my time working in Oceanography, we either used gell cell batteries
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if rechargeability was the major concern, or welded packs of alkaline D-cells if time on station was the major concern (long term measurements). We never used nicad packs. Things may be different now with higher power densities in NiMH and Lithium based technology maturing.
Not knowing anything about what you have in mind, regarding some commercial product, how much thrust you need, etc..... I'd get a trolling motor, and using some polyurethane compound (two part liquid rubber that is mixed and cures like epoxy, but is flexible when cured) to seal up the hole where the steering shaft goes in, I'd have a waterproof connector on a neoprene jacked cable sticking out of this. (only works if your depth isn't deep enough to cause the cord to "extrude" into the motor due to pressure) I'd build a pressure case out of big bits of ABS or PVC pipe (if the depth wasn't going to be too great, (aluminium if I was worried about shock loading and depth) and fit a bunch of smaller 12V gell cell batteries into it wired up in parallel. Design a bullet nose out of polyurethane foam (stuff you find in ski cores) to put on the front of the battery case, and mount the trolling motor to the front of the case with SS rod to clamp it and provide protection for the prop. Stick a matching UW connector on the case (you need to machine an 0-ring surface here) and plug in the motor to drive it... use the same connector to charge if saving money is really important... (design of the electronics, located in the case would have to accommodate this kind of thing)
You can try to make your own potted polyurethane connector in the battery case too, They even make plastic "cinch" connectors design to keep rain water or gas out of electrical boxes, but in my experience if you go cheap on equipment designed to work in the ocean (especially at the beach interface), you will get a few runs (at best) out of it before you have failures.
This should give you a few things to think about... fun sounding project!
This couldn't be as simple as a diode between the external positive contact and the battery packs positive terminal and perhaps a thermal fuse against the battery pack in case of overcharging?
Yup, it sure could! :-) If you want to get fancy-schmancy, you could put a thermistor against the battery pack and bring out a third contact for your smart charger to use.
While the bodge motor/fan sounds like it would be fun to play with, I suspect that you will drown a couple before you find the sweet-spot where you don't have too much packing drag, but don't let any water in. Using a fan sounds to me like you will be sacrificing lots of effiiency...
If it were me, I'd buy a whole trolling motor (must be able to mail order a cheap one?), rip off everything above the bottom bit.. Inteface my PIC to the controler that will be sitting in the top bit, park the electronics in with the batteries. Seal.... (as in previous message)....
What I used for purge plugs weren't pressure relief fittings... just capped stainless hydraulic fittings. I opened one of them (I used two for when I wanted to purge my gear with nitrogen) before undoing the battery cap bolts. Any hydraulic supplier should be able to get you the fittings and the cap.
First of all I must apologise for the stupid question about the diode. Yes it's the same as a reverse polariy diode or a solar panel diode that prevents the batteries charge during the day from dissipating during the night. I had it stuck in my head that when in seawater it must somehow be more complex than a simple diode and I couldn't shake that false assumption.
Continuing on thanks for the replies and the information on construction.
Alan. It's not a commercial project but a private one. There are already plenty of manufacturers of expensive commercial units -
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A pre-made trolling motor unit would be nice but so far I've been unable to locate an international supplier or any supplier of only a trolling motor and prop. I was thinking of using an 18 volt rechargable drill motor/gearbox and its fast charge ni-cad power pack with a fan blade (only about 1 inch pitch) to get it a couple of hundred meters offshore. Better than nothing! No model submarine shaft sealing rings available here (not popular) so a short home-made brass tubing, plastic tube and grease drive shaft would have to be made. The boys nextdoor at work at Brittens Motorycyles could help me out with any case machining but being "downunder" there's also limited if any local access to the major parts - trolling motor and prop. I have had a litte experience with the marine environment having put together a B/W underwater camera tested at 20m and an electric trim-tab position indicator so I hope I understand the need for a strong casing, "over-the-top" sealing and thick lacquering of any PCBs.
I was thinking of programming a PIC 16F84/A to control the motor via a power transistor and do the timing cutout in five minute increments. All set by a couple of magnetic reed switches. I was also thinking that I'd make it so the 16F84/A could be replaced with a 16F628A when I get around to learning PIC pulse width modulation of the motor. It has the same pinout.
Can you suggest a supplier of the pressure relief valves and the pressure range it needs to be within for safety on a surface vessel?
-- Cheers! Rich ------ "There was a young plumber named Lee Who was plumbing his girl by the sea. She said, "Stop your plumbing, There's somebody coming" Said the plumber, still plumbing, "It's me.""
so which parts don't you need - it seems to me that only the mounting hardware and control handle would go to waste if you purchased a complete trolling motor.
I was thinking of using an 18 volt
maybe you could seal the shaft using O-rings or some sort of gland.
or maybe a switch that stops the motor when you tug on the line ?
depends on the pressure your torpedo body can handle,
I think he's just talking of a small bung you can undo to equalise the pressure so the the end cap can be removed... if you're cheap a stainless steel pan-head screw with a small O-ring under it would do.
I've no idea what he means by schedule 40 pipe... but 110 or 80mm PVC drain pipe can witstand a metre or so of outside pressure (don't use stormwater pipe its much weaker) pressure rated PVC pipe is much stronger but harder to find in sizes over 60MM.
There's also pre-fabricated O-ring sealed screw-caps available in the
110mm size.
How do you intend to keep thins thing going straight when it gets into the surf?
Yes a complete trolling motor is a good option. There's also some divers propulsion units going cheap on a local net-auction site. It's a battle between the challenge of making something odd-ball that works and doing what's easier. Though mostly very frustrating I also use a project to teach me more about electronics, programming and... patience. If I went the home brew way I'd probably import the shaft sealing rings that model subs use. A fan prop isn't very efficient that's true. Thanks for setting me straight on the stainless hydraulic fittings.
Thanks for the info and advice. They're kept as straight as possible via an adjustable rudder particularly on the classic rear powered "pushing" torpedo type. You take a guess about the offshore current and set it before launching. The front mounted motor "pulling" type are I'm told more self-correcting and have a much smaller rudder.
yeah, with the prop up front it'd tend to head directly away from the line, and pulling on the line would straighten it out....
here's s wacky idea, how about attaching your line to a cricket ball and using a land-based machine (air cannon?) to cast... a couple of hundered metres should be doable...
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