[time-nuts] float chargers for oscillator backup power

[Bob Camp]

HI

We seem to have drifted a bit.

This started out as a supply for the Z3xxx Lucent GPSDO’s. In those units, you have a 24 volt switcher that is driven off of the battery or linear supply. Since it’s designed as an isolated supply, I doubt the switcher in the GPSDO is looking for a very clean input. The same is true of it’s need for a tightly regulated input. Yes it’s efficiency will vary a bit with input and so will heat in the box. Still, not a real big deal if the voltage drops a volt or so when you go onto backup. The GPS is still running to keep everything lined up. 

How fast to you need to re-charge the battery bank? For me, not very fast. To keep the batteries healthy, maybe a bit faster than I would like. Lower charge current is another reason to go with a smaller battery if you can get away with it. 

Bob


> On Oct 26, 2014, at 6:03 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> In message <CAGVVbuFAM=u5Uhdf+wgTUNuoeMZ+=PzWQ4JpugjNJdd60FGp0A at mail.gmail.com>
> , Brian Lloyd writes:
> 
>>> There are a variety of inexpensive wall-wart packaged float chargers for
>>> lead acid batteries around. Might be easier to just get something off the
>>> shelf.
>> 
>> Some of these things are *extremely* noisy.
> 
> Well, yes and no.
> 
> It's amazing how much noise doesn't make it past the lead-acid battery
> due to its low internal resistance.
> 
> The real issue is not the chargers noise in float mode, where it is
> barely loaded, but in bulk-charge mode, where it works full bore.
> 
> Unless you want to burn a LOT of heat charging your batteries with
> a linear regulator, you are better of shaving the noise after the
> battery.
> 
> For OCXO's, the 14.5-11.6V supply range is going to be much more
> important than the noise from the charger.
> 
> In other words, you will need some kind of regulation between
> the battery and the OCXO, and that is where you should cope with
> the noise.
> 
> A couple of other concerns should worry you too:  Isolation and
> short-circuit current.
> 
> The design I will suggest, is to find a good charger which takes
> care of your battery, and have it do only that.
> 
> Driving the load with the charger almost invariably means the
> battery doesn't get optimal conditions which is why it will
> croak in 5 years instead of the 20 years in the datasheet.
> 
> Find another power supply for your load, at a voltage slightly
> above the chargers bulk-mode (14.5 V) voltage, and use two
> solid diodes to "or" the battery and the "production supply"
> onto your "DC-bus".
> 
> Don't skimp on the ATO fuses, put one right next to the battery.
> 
> Your OCXO should be driven using a small isolated DC/DC converter
> from this DC bus.
> 
> It is important that the converter is isolated:  It prevents
> ground-loops, but it also allows you to common-mode filter the
> supply to the OCXO to get rid of any noise from the DC/DC
> converter.
> 
> Yes, it is alot more complex, but the result is also much better.
> 
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at 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.
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