Re: Sealing NiMH Battery Pack?

Robert wrote:
> Hi Mike,
> This is a diving related project and dive shops just don't have the 
> enclosures needed. It would have to be built from scratch. If I am going 
> to put it in an enclosure I will just seal it completely with orings and 
> open it after the dive to release gases. My original idea was to seal 
> the pack in resin. Would it work?
>
> mike wrote:
>
> > Robert wrote:
> >
> > > This is actually going underwater (deep, 100ft+) so tape or 
> > > rubberbands are not going to work. Any ideas on resealable 
> > > enclosures? The pack is small 6in x 2in x .75in.
> > >
> > > mike wrote:
> > >
> > > > Robert wrote:
> > > >
> > > > > Hello,
> > > > > Can I permanently seal (make waterproof) a pack of NiMH batteries. 
> > > > > The reason I am thinking I can't is because of off gassing of the 
> > > > > cells. Should I be concerned about this or not. If it is a concern 
> > > > > does anyone have any ideas about making a pack waterproof?
> > > > > Thanks.
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > All you need is a clear path from every cell to a vent that opens
> > > > before the sealed part explodes.  If you don't need it to reseal,
> > > > just make a hole and tape over it.  The tape will blow off if the
> > > > pressure increases.  Wet lead acid batteries do it by drilling a hole
> > > > in the side of a closed pipe and put a rubber band over the hole.
> > > > Think "check valve".
> > > > mike
> >
> > Ah yes, the usual belated disclosure of the real and much more 
> > challenging requirements.
> >
> > A check valve is a hole with a plug and a spring to set the release 
> > pressure.
> > Hole, rubber band, works just fine if you use quality rubber band and 
> > correct size hole.  1000 psi on a .001 in^2 hole exerts only a pound 
> > of force in the backward direction.  Can be set to any (differential) 
> > level you want in the release direction with the rubber band tension.
> > I think I'd be more worried about crushing the sealed enclosure with 
> > 12 in^2 on each side.
> >
> > Sounds like you need to visit a dive shop.
> > mike

Unless you're seriously overdischarging them, it's difficult to vent a 
cell on discharge.  So, if you're not charging them under water, the
o-rings may solve your problem.
If you have ANY extra volume in the battery case, it's gonna be 
difficult to build up anywhere near as much pressure as you have on the
outside at 100 feet.
If you pot the cells in something non-porous, you have ZERO extra volume
and you can explode something.

Once vented, the cell vent is compromised by debris.  If you want 
reliability, you want to control charge and discharge so that the
cell NEVER vents.

There's a variation on the rubber band.
A stepped rubber plug

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stuffed into a hole will blow out on internal pressure, but seal 
external pressure.  Stated another way, stick a cork in it ;-)
mike

mike

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