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'[OT] non-offgassing rechargeables?'
1999\02\06@183951 by William M. Smithers

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Hey guys,

I'm working on a PIC application that is meant to be exposed
to fairly high ambient pressures - something on the lines
of up to 250 PSI (it's for undersea use - a deep diving decompression
computer for diving with Helium).  This means that I can either use
a 1 ATA pressure housing, or pot the whole works to make it pressure
proof, and thus use a much flimsier housing by eliminating compressible
spaces.

Since this is a battery-powered device, with a total draw approaching
30mA (due to the need for some heavily compute-intensive code, and
thus a high clock speed), I am either going to use disposable batteries,
or rechargeables.  I would like to use rechargeables, but most of
the batteries I'm familiar with (lead acid, NiCad, NiMh) off-gas
too much during charging to reliably pot, which means I'm stuck with a bulky
(and expensive) 1 ATA housing.

Anybody know of a rechargeable technology that does not off-gas
appreiably during charging, or something else that might work?

-Will

1999\02\06@190540 by dave vanhorn

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>Since this is a battery-powered device, with a total draw approaching
>30mA (due to the need for some heavily compute-intensive code, and
>thus a high clock speed), I am either going to use disposable batteries,
>or rechargeables.  I would like to use rechargeables, but most of
>the batteries I'm familiar with (lead acid, NiCad, NiMh) off-gas
>too much during charging to reliably pot, which means I'm stuck with a bulky
>(and expensive) 1 ATA housing.

None of the nicads are supposed to outgass, they only vent if there's an
overcharge, but you have a bigger problem. The pressure will force
helium-o2 into the cells, and then when you depress, it will vent he-O2,
and water vapour, which will kill the cells after some number of cycles.

Lead acid can be sealed, but the chamber will eventually have to be vented.
A catalyst sponge might serve to convert the H2-O2 into water in a
controlled manner.

I think you might be stuck with Li-Ion in this environment, and you'll
still have to look at the problem of gas getting into the cells and then
carrying something else out with it. Thionyl Chloride is NOT something you
want venting into the cabin!

1999\02\06@194934 by Morgan Olsson

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At 19:04 1999-02-06 -0500, you wrote:
>>Since this is a battery-powered device, with a total draw approaching
>>30mA (due to the need for some heavily compute-intensive code, and
>>thus a high clock speed), I am either going to use disposable batteries,
>>or rechargeables.  I would like to use rechargeables, but most of
>>the batteries I'm familiar with (lead acid, NiCad, NiMh) off-gas
>>too much during charging to reliably pot, which means I'm stuck with a bulky
>>(and expensive) 1 ATA housing.

Maybe try Li-Vn cells.  3V nom, 3,4V constant voltage charge; you always
know clearly when they are full.

Used them ten years ago for memory retention, and found them very nice:
50yr(!) self discharge time, high energy per volume, low toxicity at
destruction.  I think they had low power rating that time when they were
new type, but that ougt to have improved by now.  Manufacturer was
Panasonic, Type no VL something.  I think i recently saw some new types; VM
series or something.

Regards
/Morgan
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1999\02\06@200847 by John Griessen

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In my experience, ( downhole oil well tools), stewart and stevenson blowout
preventer controls, potted electronics used in wet locations with no
pressure...), potting sucks.   Why not just
create a pressure vessel fo it?   Potting with silicone rubbery stuff
doesn't stop pressures like 250 PSI, and seawater is death to circuit
boards.  Short of making an openable pressure vessel like a cigar tube with
big O rings that just slides open and shut, you might get away with potting
the whole in hard hard epoxy  with glass fiber added and an empty chamber
designed in around the batteries, then wrap the battery area with hard
rubber and tighten down on that zone with band clamps and let it build
pressure from some batteries that are "sealable".  Like the lead acid
suggestion.

Assuming that any old potting approach will solve this will probably get you
a circuit that lasts as long as your batteries.

With the unknowns you face, a reusable pressure vessel is what I recommend.
Just think cheap materials, simple cylinder and sphere shapes, band clamps,
O rings, pipe, stamped and drawn metal shapes like the Zero Halliburton Co.
makes, and design one.  Glass tubing will take compression.  You could
design feedthroughs out of the system, and have the display read through the
glass tube wall, and the ends seal with multiple O rings....

John G Austin TX

> -----Original Message-----
> Hey guys,
>
> I'm working on a PIC application that is meant to be exposed
> to fairly high ambient pressures - something on the lines
> of up to 250 PSI (it's for undersea use - a deep diving decompression
> computer for diving with Helium).

1999\02\06@224306 by William M. Smithers

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On Sun, 7 Feb 1999, Morgan Olsson wrote:
> Maybe try Li-Vn cells.  3V nom, 3,4V constant voltage charge; you always
> know clearly when they are full.
>
> Used them ten years ago for memory retention, and found them very nice:
> 50yr(!) self discharge time, high energy per volume, low toxicity at
> destruction.  I think they had low power rating that time when they were
> new type, but that ougt to have improved by now.  Manufacturer was
> Panasonic, Type no VL something.  I think i recently saw some new types; VM
> series or something.

OK, I was aware of the Vanadium Lithium Pentoxide rechargeables,
but all I've been able to find is 3v button cells, which are
rated for a max drain rate of about 20uA.  That's *alot* of
cells to come up to a 30mA anticipated drain....does somebody
make these things in sizes more appropriate for real CPU
drive currents, not just memory backup levels that could
be nearly as well served with a super cap?

-Will

1999\02\07@003828 by Alexandre Guimaraes

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>I'm working on a PIC application that is meant to be exposed
>to fairly high ambient pressures - something on the lines
>of up to 250 PSI (it's for undersea use - a deep diving decompression
>computer for diving with Helium).  This means that I can either use
>a 1 ATA pressure housing, or pot the whole works to make it pressure
>proof, and thus use a much flimsier housing by eliminating compressible
>spaces.
>
>Since this is a battery-powered device, with a total draw approaching
>30mA (due to the need for some heavily compute-intensive code, and
>thus a high clock speed), I am either going to use disposable batteries,
>or rechargeables.  I would like to use rechargeables, but most of
>the batteries I'm familiar with (lead acid, NiCad, NiMh) off-gas
>too much during charging to reliably pot, which means I'm stuck with a
bulky
>(and expensive) 1 ATA housing.
>
>Anybody know of a rechargeable technology that does not off-gas
>appreiably during charging, or something else that might work?
>
>-Will



   Most batteries will implode at those pressures.. I guess you are really
stuck with the 1 ATA housing.

1999\02\08@054218 by g.daniel.invent.design

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some options:
1) cut the frequency of calculations ! Micros and human biology run at
very different speeds.

2) The Atmel range has pretty good speed/power consumption ratio.

3) Power up sensors only long enough to stabilise readings.

4) Use vaseline etc to fill entire PCB container (choose something with
thermal expansion close to water.)

5) Super caps may be an option also.

6) Lifejackets use a battery that requires water entry to activate
(electrolyte.)

7) High computational intensivity ? try working the answers out first on
PC etc and then interpolate with the PIC/Atmel etc. Old enough to
remember log tables etc ? turn a complex operation into a table lookup
and addition etc !

8) Look at the mixing volume, bigger volume would again increaase time
required between calculations.

regards,
Graham Daniel.

William M. Smithers wrote:
{Quote hidden}

--
Steam engines may be out of fashion, but when you consider that an
internal combustion engine would require recovery of waste heat by
transfer just before top dead centre then fashion becomes rather
redundant, USE STRATIFIED HEAT EXCHANGERS ! and external combustion.

You heard it first from: Graham Daniel, managing director of Electronic
Product Enhancements.
Phone NZ 04 387 4347, Fax NZ 04 3874348, Cellular NZ 021 954 196.

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