Author Topic: My calcs related to Super cap, Solar panel and Battery sizings  (Read 559 times)

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Offline AA1ZA

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My calcs related to Super cap, Solar panel and Battery sizings
« on: October 10, 2018, 01:12:56 PM »
I'm a newbie here.  After buying a used Vantage Pro 2 on Craigslist that needed some restoration (note, this is my first ever PWS), I read a number of forum posts here related to the ISS' super cap, battery and solar panel.   Some posts suggested - when considering replacement of the super cap - to install one with higher Farad value or to install a second one in parallel.  Another post I read suggested that increasing the solar output potential (by adding a second solar panel, for example) may be of greater impact than increasing the capacitor storage, etc. etc. etc.

Since I was at that juncture on how to proceed, I did a little research of my own to understand whether an increase in capacitor storage or solar output potential will be of significant benefit (let alone concerns of how such upgrades may affect the optimal functioning i.t.o. battery recharge rate, battery life, etc.). 

The solar panel that comes with the unit produces an output of 0.5W as per Davis.   In North Texas where I live, we have an average of 5 hours sunlight a day.   That would mean I could expect 0.5W x 5hrs = 2.5 W.h per day from my solar panel - give or take.

The 10F super capacitor that comes with the unit can store up (0.5 x 10F x (2.7V)^2) / 3600sec = 0.01 W.h when fully charged.

Davis' specs claim that the ISS draws 0.14mA on average at a Voltage ranging between 4V and 6V (so I used the maximum of 6V in my calculations).   Assuming a 12-hour night, this means a nightly draw of 14mA x 6V x 12hrs = 0.01 W.h.    Where did we see that number before?  :-)   

Lastly, Davis claims that a fully charged CR123 battery should last about 8 months in no sunlight.   A CR123 delivers about 1,550mA.h.   When converted to W.h it becomes 3V x 1.55A = 4.65 W.h.   Since the nightly consumption is 0.01 W.h, a 24-hour day's consumption of the ISS would be double that, i.e. 0.02 W.h.   This means that a fully charged CR123 should provide about 4.65W.h / 0.02W.h = 230.6 days of standby in no sunlight, which is about 7.7 months.

In summary then...

1)  The charged super cap seems sized adequate to power the ISS on its own for a full night (in the area that I live in, anyway). 
2)  The battery - if it is in good shape and fully charged - should be able to keep the ISS going uninterrupted as claimed for +/- 8 months if there was no sunshine.   
3)  The little solar panel seems adequately over sized to power the ISS during daytime and to replenish capacitor and battery capacities when needed.

Hence, no need for me to fiddle with different caps and/or additional solar panels, it seems.  :-)

Davis VP2 > Envoy+USB logger > Meteobridge;

Offline johnd

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Re: My calcs related to Super cap, Solar panel and Battery sizings
« Reply #1 on: October 10, 2018, 01:32:00 PM »
My personal view: You're overthinking this - the standard VP2 power set-up works fine and needs no tweaking. If the normal battery life starts to get significantly shorter than the typical 18-24 months then maybe change the supercap.
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Offline azchrisf

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Re: My calcs related to Super cap, Solar panel and Battery sizings
« Reply #2 on: October 20, 2018, 11:22:00 AM »
Generally true, however, the place where a larger solar panel and, especially, supercap comes into play is in areas (actually, any area) that has an extended time of no sunshine or very little sunshine that would not fully recharge the supercap. If you don't want to be relying on the battery to power it during these circumstances, it's a good idea. Otherwise like you said it's engineered properly.
« Last Edit: October 20, 2018, 11:23:44 AM by azchrisf »