[time-nuts] Oscilloquartz BVA has been sold. Thank you all who expressed an interest.

[Mark Spencer]

Thanks Paul.

The bit I am struggling with re using a 24 volt battery system in this application is what happens when AC power is removed and the terminal voltage of the battery  starts to fall.

Between voltage drop in the regulator, the voltage drop in any extra diodes etc, a nominal 24 volt lead acid battery doesn't seem enough to me if the goal is to provide the BVA with a stable 24 volt power source for a lengthy period of time.   I acknowledge that one could perhaps decide to run the BVA from a some what lower voltage and still be within the voltage spec, but I don't want to do that and in my view that probably won't really solve the issue if one wants to fully use the capacity of typical lead acid batteries.   (I expect a typical 24 volt lead acid battery system could provide useable power down to 22 or perhaps even 21 volts ?).   The voltage regulator and diodes will add voltage drop.  

Maybe I am missing something ? (Maybe fancy regulator schemes that automatically bypass themselves when the input voltage goes below a preset level ? But from my hobbyist perspective that seems more complicated than simply using a higher voltage battery system.)



Thanks 



Mark Spencer
mark at alignedsolutions.com
604 762 4099

> On Sep 25, 2020, at 10:01 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> Mark Spencer writes:
> 
>> I like the idea of using a diode arrangement to facilitate changing the power
>> source for the BVA.   I expect I will also add some form of over voltage
>> protection as well.
> 
> If you are after the low noise, be weary of "integrated" lithium batteries of 12V and higher, many of them have a built in buck-converter to take the real battery voltage down to 12V and provide over-current protection.
> 
> If you go lead-acid, don't forget rule #1:
> 
>  Your fuses can never be too numerous or too close to the battery.
> 
> Lead-Acid batteries float-charge at 13.8V[1] so two 12V in series you gets you 27.6V.
> 
> Subtract the drop over a series diode and add a linear voltage regulator and you have nice and quiet 24V.
> 
> (A lot of professional fire and burgler-alarms run on such configs, and the hardware is actually pretty nice, being a nieche and high-margin business, look out for it in scrap-heaps.)
> 
> When the charger looses power, you will drop below 24V after some time (1-60 minutes depending on battery capacity), but if the rest of your lab is dark anyway, that can still keep the BVA warm until power comes again.
> 
> If on the other hand you want to disconnect the charger to make low-noise measurements running from battery, you will need more voltage headroom.
> 
> I would not go with five 6V batteries.  6V batteries are almost universally lower quality than 12V, probably because nobody "serious" uses them, so manufacturers do not get yelled about on quality.
> 
> Three 12V blocks will put you at 41.4V when float-charging, which is a lot to swallow for a linear regulator, both in terms of voltage and power.
> 
> Poul-Henning
> 
> [1] Tons of footnotes on this but find and trust the manufacturer is a very good starting point, unless you want yet another hobby.
> 
> -- 
> 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.
>