Battery backup of frequency standards
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.
--
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.
Don't do that!
Under normal operation, it is desirable for the battery to be sealed.
The seal prevent the evaporation of the electrolyte. 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.
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.
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.
-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.
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.
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
--
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.
Hi David:
You might consider getting a BA-5590/U military battery, see:
http://www.pacificsites.com/~brooke/BA5590.shtml
These are really two seperate batteries, each one made up of 5 LiSO2
cells, 15 volts per battery, 30 volts if connected in series.
These are primary (not rechargeable) batteries with a shelf life well in
excess of 10 years. (I have some 14 year old units that test at 100%)
Between 170 and 210 Watt hours depending on which manufacturer and date
you get. You need to be extremely careful NOT to charge them.
Have Fun,
Brooke Clarke, N6GCE
--
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com
Chuck Harris wrote:
Hi David,
I think I misunderstood your solution.
Yes, we had our wires cross.
The plastic sheet that covers the caps is there to protect
the caps, and to keep them from popping off of the cells. The
caps usually aren't retained in any other way.
Mine are not retained by the plastic sheet, as that sits about 1mm above the caps.
You are absolutely correct, any gas or liquid that gets past
the rubber caps is lost.
If your solution provides the same level of support for the rubber
caps as they had before, and doesn't block or impede the passing
liberated gas, the SLA should be chemically safe.
It seems I'm OK there.
The plastic sheet is also a secondary safety release. It is welded
only superficially so that rather than blowing out the sides of
the battery, it will pop free in cases of extreme (rapid) outgassing.
Take your point. My solution does NOT do that, as any rapid outgassing would be limited by the rate
the acid and or gas can escape. The top will be stuck down fully on all sides, so will not provide a
safety measure in this case. It might be impractical for me to to keep this contained in such a way it
could come off in a rapid expansion of case, whilst at the same time providiing an air tight seal.
One option might be to mill the plastic until is is thin and hence weak, but perhaps not very practical.
I am not especially comfortable with modifications to the SLA's
safety mechanisms. If I make such a modification, then I take
responsibility for any failures that may occur. (including those
created by the battery manufacturer) SLA's can fail in dramatic
ways if the charging circuit gets too aggressive, and the safety
features don't work as advertised.
I might do it for myself (probably not), but certainly not for
a device that could be used by someone else.
I take your point. I did not realise that cover was part of a safety mechanism.
-Chuck (who has spent too much time designing rapid chargers)
I have no desire to rapidly charge them.
Given I have no desire to start the standard from batteries (using it only as a backup), the current
drawn by the rubidium, GPS, 10811A, shera board and a few other bits should not exceed 1A.
That would allow some protection with fuses, but I fully admit its not an ideal solution.
I guess a separate box for batteries, (that several suggested) is the least hassle. I was just keen to
keep the unit a reasonable size, and self-contained, but it certainly does not look easy to do.
Thanks for all your input.
--
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.
Hi David,
I think I misunderstood your solution.
The plastic sheet that covers the caps is there to protect
the caps, and to keep them from popping off of the cells. The
caps usually aren't retained in any other way.
You are absolutely correct, any gas or liquid that gets past
the rubber caps is lost.
If your solution provides the same level of support for the rubber
caps as they had before, and doesn't block or impede the passing
liberated gas, the SLA should be chemically safe.
The plastic sheet is also a secondary safety release. It is welded
only superficially so that rather than blowing out the sides of
the battery, it will pop free in cases of extreme (rapid) outgassing.
I am not especially comfortable with modifications to the SLA's
safety mechanisms. If I make such a modification, then I take
responsibility for any failures that may occur. (including those
created by the battery manufacturer) SLA's can fail in dramatic
ways if the charging circuit gets too aggressive, and the safety
features don't work as advertised.
I might do it for myself (probably not), but certainly not for
a device that could be used by someone else.
-Chuck (who has spent too much time designing rapid chargers)
I can recommend buying a so-called "battery-backed" powersupply for
atomics. They are built to be usedin fire-alarms, burglar-alarms
etc, and consist of a power-supply that understands how to charge
SLA's and a box. Mostly one has to buy the batteries separately.
They come in 12V and 24V versions.
Look for instance at RS-components 310-2058
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Brooke Clarke wrote:
Hi David:
You might consider getting a BA-5590/U military battery, see:
http://www.pacificsites.com/~brooke/BA5590.shtml
These are really two seperate batteries, each one made up of 5 LiSO2
cells, 15 volts per battery, 30 volts if connected in series.
These are primary (not rechargeable) batteries with a shelf life well in
excess of 10 years. (I have some 14 year old units that test at 100%)
Between 170 and 210 Watt hours depending on which manufacturer and date
you get. You need to be extremely careful NOT to charge them.
You must also be extremely careful NOT to fully discharge, or short
circuit them. If you do, explosion, or explosive outgassing of Hydrogen Sulfide
gas is a very real failure mode.
There are very good reasons why LiSO2 batteries are never used in commercial,
or consumer equipment.
-Chuck
Hi David,
Mine are not retained by the plastic sheet, as that sits about 1mm above
the caps.
One way you can tell if a battery has been abused, is you will
find the rubber caps stuck to the plastic lid. Under normal
conditions, there is no gas generation, and the rubber caps stay
down where they were installed. But the way they (the usual style I
have seen) vent is the sides of the cap move outwards, and the
cap raises up to protective plastic cover.
One option might be to mill the plastic until is is thin and hence weak,
but perhaps not very practical.
Then you would have to do testing to see if it failed in the way you
had planned, and suddenly this becomes a big research project.
-Chuck (who has spent too much time designing rapid chargers)
I have no desire to rapidly charge them.
I didn't think that you did, but in the process of, uhmmm.. Agressively
charging batteries, I have had the opportunity of witnessing the protective
safety measures in operation. (A fancy way of saying I have blown a few
batteries up.)
I guess a separate box for batteries, (that several suggested) is the
least hassle. I was just keen to
keep the unit a reasonable size, and self-contained, but it certainly
does not look easy to do.
It would make your instrument more managable, weight wise, and would
keep all the yucky stuff in one place. Plus, it would allow you to use
a variety of means to keep your system operational during power loss.
If you keep the charging and load lines separate, you can even explore
using a large primary battery, as Brooke suggested.
-Chuck
Hi Chuck:
I don't think it's possible with a BA-5590. There's circuitry inside
that limits the current draw with a dead short. Fully draining the
battery is no problem, in fact there's a one way action switch on the
BA-5590/U that puts a resistor across each internal battery in order to
fully drain it.
You may be thinking about Li-Ion or Li-poly batteries where fire comes
out of the battery when it's mistreated (too much current either charge
or discharge, over or under voltage, or mechanical puncture). Like the
video at: http://www.valence.com/SafetyVideo.asp
Not true about LiSO2 batteries and commercial products. The "D" cell
that's inside the BA-5590 is used in a number of products, medical
emergency equipment and head mounted flashlights come to mind.
Have Fun,
Brooke Clarke, N6GCE
Chuck Harris wrote:
Brooke Clarke wrote:
Hi David:
You might consider getting a BA-5590/U military battery, see:
http://www.pacificsites.com/~brooke/BA5590.shtml
These are really two seperate batteries, each one made up of 5 LiSO2
cells, 15 volts per battery, 30 volts if connected in series.
These are primary (not rechargeable) batteries with a shelf life well
in excess of 10 years. (I have some 14 year old units that test at
100%) Between 170 and 210 Watt hours depending on which manufacturer
and date you get. You need to be extremely careful NOT to charge them.
You must also be extremely careful NOT to fully discharge, or short
circuit them. If you do, explosion, or explosive outgassing of Hydrogen
Sulfide
gas is a very real failure mode.
There are very good reasons why LiSO2 batteries are never used in
commercial,
or consumer equipment.
-Chuck
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts