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Underwater battery charging contacts question

Discussion in 'Electronic Basics' started by Rubicon, Oct 5, 2005.

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  1. Rubicon

    Rubicon Guest


    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.


  2. Alan Adrian

    Alan Adrian Guest

    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.

    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

    For all the instruments we built (dozens) in my time working in
    Oceanography, we either used gell cell batteries
    ( 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

    This should give you a few things to think about... fun sounding project!

  3. Jasen Betts

    Jasen Betts Guest

    I expect there's a diode after the contacts to stop current leaking back

    sealed lead acid is probably a bnetter choice unless you intent to put the
    charge controller inside the torpedo body.

    . . . . . . . . .
  4. Rubicon

    Rubicon Guest

    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?

  5. Alan Adrian

    Alan Adrian Guest

    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.


    Better than nothing! No model submarine shaft
  6. Rich Grise

    Rich Grise Guest

    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.

    Good Luck!
  7. Rubicon

    Rubicon Guest


    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

    Alan. It's not a commercial project but a private one. There are
    already plenty of manufacturers of expensive commercial units -

    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?

  8. Jasen Betts

    Jasen Betts Guest

    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

  9. Mammary? Or endocrine? ;-)
    "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.""
  10. Rubicon

    Rubicon Guest


    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


  11. Jasen Betts

    Jasen Betts Guest

    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...

  12. Rubicon

    Rubicon Guest


    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.


  13. Rubicon

    Rubicon Guest


    Not such a wacky idea as small rockets are used to get light cord
    lines out to ships to drag out heavier cables.


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