Chuck Harris wrote:
Don't do that!
Seems I'm in trouble!
Under normal operation, it is desirable for the battery to be sealed.
Agreed, although experience of others using thousands of batteries suggest about 10% do leak even if
charged very carefully.
The seal prevent the evaporation of the electrolyte.
What seal?
SLA batteries,
because of the way the electrolyte is gelled, don't have very much
extra electrolyte. Any amount of evaporation is harmful to the battery.
But the battery never was sealed, except by the rubber caps on each of the 6
cells.
The top cover put over those caps seems (and I may be wrong) to serve little purpose other than to
prevent the rubber seals getting damaged. It is certainly not a seal. Given it was spot welded at only
about 6 points along the side, it would suggest there is an airgap around most of the battery. The
fact these channels I mentioned go to the side, suggest to me that the sides never were sealed.
So I suspect (and may of course be wrong) that by sealing the sides, but providing a single vent, I
have not significantly increasesd or decreased the ease of evaporation. I would think that once any
liquid has got past those special rubber seals, it is effectively lost anyway.
Underneath the plastic cover you found rubber caps that sealed the
individual cells. That rubber is special. It allows the cells to
pressurize up to a point, and gives way to provide a vent during
catastrophic cell boiling.
I have not touched those rubber caps - from what I understand, they vent when the internal pressure is
about 150mbar higher than that externally. They sit about 1mm below the surface of the plastic cover,
so certainly not directly affected by its removal.
Sulfuric acid wicks around and through silicone rubber. Battery
manufacturers have spent a lot of time and money developing seals that
work fairly well with Sulfuric acid, and yet you still get corrosion
arround
the terminals of your automotive battery. If something as simple as a
bead of silicone rubber would solve the problem they would already be
doing it.
I appreciate what you are saying. I will test the seals periodically.
-Chuck
Dr. David Kirkby wrote:
For obvious rasons a frequency standard should be battery backed. I
was intending puting the batteries in the same box as my GPS/rubidium
etc, but was advised this is not a good idea. I think I have found a
solution to putting sealed lead acid batteries into an enclosure with
electronics, such as needed for a frequency standard.
The larger (100Ahr) lead acid bateries have the factility to vent
fumes externally, but this is not so with the smaller ones most likely
to be used for frequency standards.
Despite the fact the batteries I have (Powerfit S300 12V 7Ahr, RS
Components Ltd 422-5944) have no facility to vent the fumes
externally, adding such a facility is not too hard.
I simply removed the top plastic cover (easily unclips, as weleded in
only about half a dozen places, drill a hole into this top cover,
attach some form of connector for a pipe, replace the cover but this
time using a gas tight seal on four *edges* of the cover. Now the only
way for fumes to enter/leave the batteries is via the pipe, which is
vented externallly.
Silicon rubber, whilst not totally inert with sulphuric acid, would
appear to not react too strongly. In any case, it is easy to check the
seal, as blowing (or sucking) air from the pipe should soon show any
leak.
When I removed the top of the battery, it is clear there are small
ducts (about 3mm wide, 0.3mm deep) connecting the 6 cells and the
outside. This is the intended path for fumes. Hence you ideally want
to locate your pipe connector above one of these - any one, as they
are all linked. I did not realise this at first, but its easy to
extend the ducts somewhat.
Obviously this is not as secure as a separate battery box, and would
not work in the event of catastropic failure of a bettery, if the
volume of gas and/or liquid was too great for the tube to handle, but
for me anyway, it is a reaonsable compromise. I'm using tube with an
ID of about 4mm, for each battery.
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--
Dr. David Kirkby PhD CEng MIEE,
Senior Research Fellow,
Department of Medical Physics,
Mallet Place Engineering Building,
Gower St,
University College London,
London WC1E 6BT.
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