[time-nuts] Re: Backup DC power with current battery technology?

[Robert LaJeunesse]

John,
 
My suggestion is to think of charger/battery and operating supply as independent items. Connect them each to the load with beefy (Schottky?) diodes. Set the operating supply a bit above the float voltage such that the AC supply through its diode runs the load normally.  When AC drops out the diode from the battery conducts, keeping the load powered. If you are concerned about efficiency/dropout use a SPDT relay to bridge one diode or the other based on the AC presence. That also makes the design less sensitive to the relative supply vs charger voltages.
 
Using this approach you can optimize the float charger for battery life, and have as much operating current as needed without concern for battery loading.
 
Bob L.
> Sent: Sunday, May 26, 2024 at 10:39 AM
> From: "John Ackermann N8UR via time-nuts" <time-nuts at lists.febo.com>
> To: "Discussion of precise time and frequency measurement" <time-nuts at lists.febo.com>
> Cc: "John Ackermann N8UR" <jra at febo.com>
> Subject: [time-nuts] Backup DC power with current battery technology?
>
> I need to replace the batteries in my 28 VDC power distribution system
> in the clock room. It's currently two series 12V AGM batteries with an
> IOTA Engineering float charger.
>
> This system has two purposes: (a) primary power for some OCXOs and other
> gear; and (b) failover power for some AC/DC gear like HP standards.
> Under normal conditions, power draw is 2 or 3 amps. During mains
> outage, it could reach 10 amps. Now that I have a house generator, long
> run-time isn't a big issue so 25 Ah or so of capacity is plenty.
>
> I haven't explored LiFePo4 batteries until now, and I'm having trouble
> finding a charger that can provide significant continuous load current
> while maintaining a battery floated across the line. Does anyone have
> recommendations for something like this?
>
> Thanks,
> John
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