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Creating an AC-free 5V, 12 V, 24 V DC UPS

I've had a traditional UPS powering my network rack for a few years now, but it isn't all that efficient because it sends DC battery power to an internal AC Inverter so that it can power gear, and that gear goes around and converts it back to DC.  It seems reasonable to skip this lossy process.  Here's how I did it.

Uninteruptable DC Power Supply Diagram

 

Backup Voltage and Power

My network's gear uses various voltages: 12 Volts, 24 Volts, and 5 Volts.  So my overall goal is to have three voltages available for my various devices.  Sadly, the computer UPS market doesn't seem to want to deliver any devices to satisfy these needs... at least nothing affordable.

But there is another huge industry that had this need, and that's the world of small business security.  And in particular, small apartment buildings that use electronic locks!  They don't want their buildings to be inaccessibly locked during a power outage, and so they use battery backups all the time.

This is when I decided to buy an Uninteruptable Power Supply box designed for door access control systems.   This little box delivers 12 Volts DC either through its built in AC power supply, or if that fails, through an internal rechargeable battery.  This is a great first step!  With this box and a proper battery I can get 12 volts DC with or without wall power.    Note that this box doesn't include a battery.  It requires a very common 12 volt, 7A SLA battery.  I had an old one laying around, so I took advantage of it.  A larger battery could be used outside of the housing, but I haven't had a need yet.

The Good:

  • Supplies 12 Volts at 5 Amps.  That's a lot of power.
  • Fast switching: if wall power is lost, battery power is delivered near immediately. If wall power is restored, the battery is switched off and wall power resumes.
  • Automatic recharging: the device automatically charges the internal battery. 

The bad, which I have addressed:

  • Only 12 volts!  I need 12, 24, and 5 volts.
  • Not regulated when on battery!  When powered from the wall it delivers an even 12 volts, but when the battery is switched in, the power supply delivers the battery voltage.  And the battery voltage migrates from 14 volts to as little as 9 volts as the battery goes from fully charged towards nearly empty. These variable voltages could be a problem.

Addressing The Limitations

In order to address these voltage limitations, I'm using a set of XL6009 buck/boost converters.  These very affordable converters can take the variable +14 to +9 VDC power and convert it into regulated 12, 24, and 5 volts DC power.  I can add new voltages if required. 

UPS Duration

I get about 2 hours of power through my weak, 10+ year old SLA battery, which is longer than what I was getting out of my old dual-battery UPS.  I suspect I can get about 8 hours out of a new battery, but I'm good for now.

Future Possibilities

The power box has numerous control features that could be used to detect the status of the power.  These features could be leveraged by an Arduino or Pi Pico.  I haven't implemented anything in this area yet, but it is on my radar.


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