Running from DC (48V up to 60V), pico-psu (DC-ATX) considerations, alternatives.

Notice: Page may contain affiliate links for which we may earn a small commission through services like Amazon Affiliates or Skimlinks.

Mithril

Active Member
Sep 13, 2019
354
106
43
Feels like the right forum, but I could be wrong.

One of my "slow burn" projects is moving all of the critical gear in my rack to DC input, using the "normal" UPS output for unmodifiable(or not yet modified) equipment mostly. This allows for much better "on battery" time during power outages, in some cases much better efficiency than a terrible power brick or cheap PSU (yes, I could buy high quality replacement power bricks or PSUs, but if I'm going to spend the money I might as well achieve multiple goals).

I'm currently using a few DC-ATX PSUs, one is a "name brand" (of sorts) pico PSU, and another 2 generic/ebay jobs. I've been looking at getting into Lithium Iron Phosphate batteries which are "48V" (nominal charged voltage is about ~52v and charging voltage can get close to 60V). Most/all of the "wide range" DC-ATX PSUs cap out around 28v/32v. Does anyone know of quality DC-ATX/pico PSUs that can tolerate 60v? Or am I better off with a quality DC-DC to step down to ~12.1-12.2 (to account for some droop) and use 12v DC-ATX PSUs which expect a steady 12v supply.

I would expect feeding even a high quality buck converter into a 12v DC-ATX would require some additional filtering and bulk-caps close to the inputs. Am I better off using a single high quality high capacity buck converter for 12V output, or one converter per PSU? (assuming at this stage we are not dealing with adding redundancy.

Unfortunately most of the more commercial DC-ATX psus seem to be telco standard, which means positive ground (AKA, -48V, not 48V).
 
  • Like
Reactions: zunder1990

edge

Active Member
Apr 22, 2013
203
71
28
I would suggest a higher capacity UPS over going to DC. You are likely to lose any efficiency gained by DC to the isolation circuitry required in your devices' DC power supplies.
 

Mithril

Active Member
Sep 13, 2019
354
106
43
I would suggest a higher capacity UPS over going to DC. You are likely to lose any efficiency gained by DC to the isolation circuitry required in your devices' DC power supplies.
I'm trying to tackle both angles and figuring out where to spend money now. With simi quality DC-AC-DC you're facing a double conversion in the range of 85-95% at both stages, with a final efficiency of 0.72 - 90% (and possibly lower). A decent buck converter, even with a wide difference between Vin and Vout can be 80-90%. The issue with ALL of these is finding quality units. Be that quality DC-ATX (like picopsu) that can tolerate up to 60v for safety, or quality DC-DC buck converters. Or quality inverters, which very much has an efficiency of scale in $ to output ratio. Considering that this system will at some point also have solar and be powering more things, efficiency of power use while on battery is important. The Inverter in my UPS I think is around 65% at lower load and effectively has a cutoff around 10% where efficiency drops rapidly.

the 2 most critical systems are the HA firewall pair, and even with a very expensive platinum PSU are going to be operating in the less efficient range due to low power draw. Keeping the NAS on for 1+ day would be nice but not required.

I don't believe even the "extended run" rackmount UPS that take 48v are intended for multi-day runtimes, and 48v is much easier to deal with once it's outside of the unit.

There is a LOT of quality 48v gear out these days due to popularity of solar systems. I had hoped that included more consumer focused DC-ATX.
 
Last edited:

colinb

Member
Jul 19, 2022
43
3
8
@Mithril, did you manage to find anything?

At home we are just in the process of installing PV and a very large "48V" (52V in practice) battery bank (24kWh initially for 3days' power, 52.8kWh in a future upgrade for 6days all assuming no changes in behaviour in event of a power cut). Keeping as much on DC as possible and avoiding the losses associated with repeated conversion feels like a no-brainer, but as you've noted, the devil is very much in the quality details!
 

Mithril

Active Member
Sep 13, 2019
354
106
43
@Mithril, did you manage to find anything?

At home we are just in the process of installing PV and a very large "48V" (52V in practice) battery bank (24kWh initially for 3days' power, 52.8kWh in a future upgrade for 6days all assuming no changes in behaviour in event of a power cut). Keeping as much on DC as possible and avoiding the losses associated with repeated conversion feels like a no-brainer, but as you've noted, the devil is very much in the quality details!
Nothing yet. I think it's more likely to find quality "48v" (as you noted, higher in practice, and even higher charging!) to 12v/5v/24v DC-DC units.
I've been distracted by other projects :)
 

armandh

New Member
Nov 21, 2020
11
1
3
St louis county mo
Power quit during a storm 3 days ago.
Double conversion [AC-DC-AC] 1KW Liebert UPS [purchased used] held just fine while I hooked up the [large barely portable] generator.
Cable/internet went out with the power for 9 hours. The AC-DC-AC held through the 2 second brown outs when the 5 ton cooling cycled on..
Our second generator use in 8 years at this home. The UPS has connection for optional battery packs. Ive not felt the need.
My home brew XigmaNAS servers have, in the past, been tolerant of power disconnection. For each I have the embedded Xi OS on a SSD.
Those who use a USB stick for this often find they have failed while the OS ran merrily from the RAM-disk. [USB sticks are NG for a swap file]
This tolerance was last tested just B4 replacing the 7 YO UPS batteries. [4 12V in 24V series sets paralleled, each set can be changed live]
Ive never done it that way as I can shut down the servers,

to be sure the gasoline generator would be ready for its next use I ran the cool down on [corn free] low lead 100 Aviation gas then ran it dry.
 
Last edited:

Mithril

Active Member
Sep 13, 2019
354
106
43
Take a look as this: GitHub - t123yh/atx48

This might be a perfect solution to power your PC by 48V.
Interesting, I couldn't find price for the main module (I assume thats at least 50% of the BOM right there). Any idea on cost for the parts and board if someone was looking to build one or a few? This might be overkill for some of the machines I'd want to use it on be if the price is right.

FWIW my current thinking is a quality 48V to "12V" (with an offset/adjustment so its about 12.2 +/-1 0.1v for line drop etc) "perfect diode" (mosfet modules used in solar, good to ~15amps) OR-ed with an AC powered 12V supply. I don't currently need to keep my nas or file server running super long, they can just gracefully shutdown same as now.

I likely am going to get an inverter for AC loads still, but A) that's still an extra efficiency loss (DC - AC - DC) for computers and B) the larger the capacity the more expensive and higher the standby power (generally). Plus if I have any inductive loads (like a fridge) I'm worried about how "clean" the power stays for sensitive loads.