[Cerowrt-devel] solar wifi ap designs?

Mon Jun 5 14:21:57 EDT 2017

On Mon, Jun 5, 2017 at 2:01 PM, <dpreed at reed.com> wrote:

> It doesn't jump to mind, but a radio carrying bits near the edge probably
> won't be used near capacity most of the 24 hours it is operating. Just as
> Iridium was designed to quiesce most of its electronics on the dark side of
> the earth, extending its battery life, you can probably assume that a radio
> in a tree won't be heavily used most of the hours of a 24 hour cycle.
>

Turns out that (at least in OLPC days), the signal processing in the WiFi
module dominated power consumption (relative to the radios themselves). At
the time, power consumption was of order 1 watt and the radios themselves
consuming only a fraction of that.

I don't know what the current chips/modules consume, however.

Another big consumer turns out to sometimes be ethernet chips; gigabit
nic's often take/took a watt of power.

So your mileage varies, and you cannot easily presume its one thing or the
other, but need to measure (which we did extensively for OLPC, to get its
power consumption down to where it is) and fix lots of software.

Linux itself is pretty decent for power management these days: but all it
takes is a single chip/subsystem or stupid applications to blow that up.
So the whole bus structure has to be understood and properly done, and some
technologies are pretty hopeless.

The details matter.  Unfortunately, home routers have generally been plug
in devices, and the vendors haven't paid much attention.
                                                          - Jim

>
>
>
> On Monday, June 5, 2017 1:52pm, dpreed at reed.com said:
>
> > "Deep discharge" batteries work in LEO satellites for such applications.
> But they
> > are extraordinarily expensive, because the designs are specialized, and
> that use
> > case doesn't have the 2-3 day solar outage problem.
> >
> > You are not going to put a good enough system for an AP up in a tree.
> Maybe on an
> > antenna mast structure with solid base and guy wires. Roofs and ground
> are better
> > choices.
> >
> > But I would wonder whether redesigning the AP itself to be
> power-conserving would
> > be the place to start. They are not designed to be "low power" - they
> are designed
> > to be inexpensive.
> >
> > So, for example: why 12V??? No logic needs 12V. Integrate the battery
> into the AP
> > and run it at 3V, eliminating multiple conversion losses.
> >
> > You can use 12/20 V off the solar panel to charge the 3V battery system
> (high
> > current only while charging).
> >
> > Pay lots of attention to the duty cycle of the radio. If you really
> expect the
> > radio to be on 100% of the time, you may have to power it all the time.
> Otherwise,
> > minimize uptime.  Similarly, the processor need not be on most of the
> time if it
> > is mostly idle while accepting and sending packets from memory. (ARM
> BIG.little
> > might be helpful).
> >
> > Get rid of Linux if possible. Linux is not a low-power OS - needs a lot
> of work in
> > configuring or rewriting drivers to cut power. (there's a need for an LP
> Linux,
> > but like Desktop Linux, Linus, and his coterie, isn't terribly
> interested in
> > fixing his server OS to optimize for non-servers, so "server power
> saving" is the
> > only design point for power).
> >
> >
> >
> >
> > On Monday, June 5, 2017 12:01pm, "Richard Smith" <smithbone at gmail.com>
> said:
> >
> >> On 06/04/2017 08:49 PM, Dave Taht wrote:
> >>> I keep finding nicely integrated solar/battery/camera/wifi designs
> >>>
> >>> https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%
> 3Delectronics&field-keywords=solar+wifi&rh=n%3A172282%2Ck%3Asolar+wifi
> >>>
> >>> But what I want is merely an solar/battery/AP design well supported by
> >>> lede... and either the ath9k or ath10k chipset - or mt72 - that I can
> >>> hang off a couple trees. I've not worked with solar much in the past
> >>> years, and picking the right inverter/panel/etc seems like a pita, but
> >>> perhaps there are ideas out there?
> >>
> >> This is something I was up against constantly when I worked for OLPC.
> >> There's a big gap for products that use more power than a cell phone but
> >> less than an RV or a off-grid cabin.
> >>
> >> For the XO itself we worked around it by designing the front end of the
> >> XO to be able to handle the range of output voltages from "12V" panels
> >> (open circuit voltages up to 20V) and to implement an MPPT algorithim in
> >> the EC firmware.  You can plug up any solar panel with a Voc of 20V or
> >> less to an XO-1.5 to XO-4 and it will DTRT.
> >>
> >> Figuring out what to do with the deployment's APs though was always a
> >> struggle.
> >>
> >> Solutions exist but you need to get a good estimate of what sort of
> >> power budget you need.  It makes a big difference in what equipment you
> >> need.
> >>
> >> Unless its a really low power device the numbers can get large fast.
> >>
> >> My WNDR 3700v2 power supply is rated at 12V 2.5A which is a peak of 30W.
> >>
> >> Lets assume your average is 30% of peak.  That's 9W.  Your 24h energy
> >> requirement is 216Wh.  A reasonable input to usable efficiency for a PV
> >> system is 70%.  Given average 5 hour window of full sun you need a PV
> >> output of at least 62W.  It only goes up from there.
> >>
> >> Realistically you need to survive a 2-3 day period of terrible solar
> >> output.  So your storage requirements should be at least 2-3x that.
> >> When you do get sun again you need excess PV capacity to be able to
> >> recharge your batteries.  You would probably need a PV output in the
> >> 100W-150W range to make a system you could count on to have 100%
> >> availability 24/7.
> >>
> >> That's going to be a pretty big chunk of hardware up in a tree.
> >>
> >> If the average power draw is more in the 3W or 1W range then things look
> >> a lot better.   That starts to get down into the 40 and 20W range.
> >>
> >>> so am I the only one left that likes edison batteries? you don't need
> >>> a charge controller... they last for a hundred years....
> >>> _______________________________________________
> >>
> >> I've never used this battery type but it looks like the resistant to
> >> overcharge assumes you replace the electrolyte.  All the cells I've
> >> looked at on a few sites seem to be flooded which means maintenance.
> >> Are there sealed maintenance free versions?
> >>
> >> For discharge nominal is 1.2V but charging is listed as ~1.6V/cell so
> >> you are going to need 16V to charge.  I don't really see how you can
> >> build a workable system with out some sort of setup that can isolate
> >> your 12V loads from a 16V charge.
> >>
> >> Perhaps undercharge them at a lower voltage and live with the capacity
> >> loss?
> >>
> >> --
> >> Richard A. Smith
> >> _______________________________________________
> >> Cerowrt-devel mailing list
> >> Cerowrt-devel at lists.bufferbloat.net
> >> https://lists.bufferbloat.net/listinfo/cerowrt-devel
> >>
> >
> >
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