[Cerowrt-devel] Low Power UPSes (Was: Re: [Bloat] Dave Täht quoted in the ACLU blog)

David Lang david at lang.hm
Sun Jun 29 23:45:52 EDT 2014

On Sat, 28 Jun 2014, Joseph Swick wrote:

> On 06/28/2014 12:28 AM, Dave Taht wrote:
>> One thing that does bug me is most UPSes are optimized to deliver a large
>> load over a short time, a UPS capable of driving 5 watts for, say, 3 days is
>> kind of rare.
> I think this is something that's in need of a new approach/disruption.
> For low power devices like NUCs and RasPi servers, running them off of a
> traditional UPS is hugely waste-full, since you're going from your Line
> voltage (120VAC or 240VAC in many places) to 12 or 24VDC (Or 48VDC for a
> bigger UPS).  Then when the UPS has to kick in, it converts the battery
> voltage back to your line voltage.
> A better approach would be to have a UPS that had a good intelligent
> charger for your deep-cycle type battery that coming off the battery,
> you kept it at the correct DC level for your NUC or Raspi.  Which for
> many of these devices is 5 or 12VDC.  So in a sense, it becomes your
> low-power device's power suppy, it just happens to have the added
> benefit of having a built-in backup battery.
> Coming from a Ham Radio perspective, some hams run their base stations
> off of deep-cycle marine batteries with some form of charger keeping
> them topped off.  This way, the radio operator can operate his or her
> station for days just on emergency power.  Since a lot of ham gear is
> designed to operate off of 12VDC (with some notable exceptions like your
> high-power amplifiers).
> It shouldn't be hard to develop a decent grade Low-power UPS for home or
> small office use that can run these low power devices for days at a time
> with out all the inefficiencies of converting VAC to VDC and back again.
> And there's probably a bunch of Raspi (or similar low-power computer
> boards) enthusiasts who already have for their own personal use.

I think a lot of people are just using li battery packs with USB output to run 
their Pi type computers, with a wall charger into the battery pack.

it may not be the best thing for the batteries, but it's off-the-shelf and 

for 12v computers, it's easy to just float a gell-cell on the output of a power 
supply. If you want to be a purist, have some sort of current limiting resister 
so that when the battery is extremely low you don't overload the power supply, 
but in practice, the power supplies are cheap (getting hold of an old PC power 
supply is probably free, and they tend to have a fairly heafty 12v output), and 
gell cells are pretty forgiving of abuse, so you can get away with the 
dirt-simple PS -> battery -> device the vast majority of the time.

It helps that "12v" equipment tends to actually be speced to run off of 
automotive power, which is about the ugliest power source you can deal with.

David Lang

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