[time-nuts] Battery backup of frequency standards

[Dr. David Kirkby]

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.