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Dear Pierre.
Saw your question to Dan. I noted that you said you
were going to use a 1 atm battery pod. I am assuming you intend to
only
use it to one atmosphere or around 33 feet if I am
interpreting you correctly. In this case you may
follow Dan's design as he has
described it and I also offer an alternative.
However, If I am NOT intrepreting you correctly and you intend
to take this to hundreds of feet, the pvc pipe in
my pod design although thick and strong may not suffice at extreme
depths
beyond the normal limits of contempory
scuba.
My pod design is for a wetsub that is intended to be used at normal scuba
diving depths with a max of around 100 feet depth. I am building a battery
pod for my own wetsub and am using 12 inch pvc pipe that is approx 5 ft
longthat holds 4 deep cycle marine batteries connected
so as to provide 24 volts for full speed and 12 volts for half speed
for my 74 lbs thrust minnkota motor.
I am using one closed end cap that will be glued in place onto the pvc pipe and have
epoxy all around the seam as well. The other end
of the cap will open to allow access to the batteries. On this end I am glueing a threaded
female fitting that will also have epoxy all around
the seam. Inside
this female fitting is a flat surface that the 12 inch O ring goes against. The
male end plug screws into the female fitting
and seals its own
flat surface tightly against the O ring squeezing the o ring between the male
end plug and the female fitting providing a watertight seal.
The male end plug has a large square in the middle
of it on it's outside side that you can get a large wrench around to tighten or loosen the
plug. I will be using a scuba
regulator 2nd stage (mouthpiece) with attached air
tank to equalize both the battery pod and the motor housing.
What this entails is attaching the scuba regulator
either directly to the battery pod or by attaching an air line from the
regulator to the battery
pod. As you descend the water pressure will open
the diaphram in the regulator and expell air into the battery pod until the air
pressure in
the battery pod is equal to the outside water
pressure whereupon the regulator diaphram will close and the battery pod will be
equalized to the outside
water pressure. Since my battery pod will have a
flexible tube into which my wiring routes to my motor housing, this will
equalize my
motor housing and battery pod at the same time.
Simple yet effective. As you accend the air will expand inside the motor housing
and battery pod and be expelled thru the scuba
regulator's exhaust valve. Since scuba regulators sometimes leak a few drops
past the air intake valve I will
be placing absorbant diapers into the battery pod
to absorb any small amount of water droplets that might engress that are easily
removed and replaced when
you unscrew the end plug to recharging the
batteries which I could rig to recharge without unscrewing the end plug but
prefer to unscrew the end plug to vent
hydrogen gas when recharging. Also by using the
regulator you can actually help vent out any very small amounts of hydrogen that
form when using
the batteries when you accend and the air with any
small amounts of mixed hydrogen with the air vents out as the air expands and is
expelled thru
the regulator exhaust vavle. Most hydrogen forms
when charging but some small amounts do form from discharging the batteries
also. I could actually make
the air push out any small amounts of water inside
the battery pod similiar to the way a purge valve works in a scuba mask but it
really isn't necessary I believe
since the amount would be very small and taken care
of by the absorbant diapers. I do intend to install a hydrolator (to
neutralize any hydrogen) to the upper portion
of the pod or install hydrocaps on each battery cell as an alternative just to be safe.
This might work even at depths much deeper than I
am designing it for
since the regulator will keep the pod and motor
housing equalized against the water pressure, but I cannot recommend that. Not
sure if my design will be of help but
I offer it in case it might
be.
Kindest regards and good luck with your
project.
Bill Akins.
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