Re: [Cerowrt-devel] solar wifi ap designs?

[Richard Smith <smithbone@gmail.com>]

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