[Cerowrt-devel] solar wifi ap designs?

[dpreed at reed.com]

"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
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