[time-nuts] Battery backup of frequency standards

Chuck Harris cfharris at erols.com
Thu Jun 9 09:14:29 EDT 2005

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.


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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