[time-nuts] Battery backup of frequency standards

[Dr. David Kirkby]

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.