[time-nuts] Battery backup of frequency standards

Dr. David Kirkby drkirkby at medphys.ucl.ac.uk
Thu Jun 9 09:15:58 EDT 2005


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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