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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. http://www.sea-technology.com/
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
(http://www.sonnenschein.org/) 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!
Al...
<Rubicon> wrote in message news:43435ee1.1327604@xxxxxxxxxxxxxxxxxxxxxxx
> Hello,
>
> 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.
>
> Cheers,
>
> Andrew.
>
>
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