Author Topic: Power system thoughts?  (Read 10694 times)

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Offline Flag

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Power system thoughts?
« on: March 16, 2011, 04:26:29 AM »
Many systems (especially Davis) are 5-6 volts yet most solar systems and especially most solar regulators are 12 volt.

So what are the preferred options?

1) 12V system / 12 volt battery and DC/DC convertors to 5-6 volts
2) 12V system / 6 volt paired batteries with 6 volt feeds direct from 6 volt battery

The main issue I see are systems with no Low Voltage Disconnect (LVD) which have the possibility of completely stuffing the battery during times of malfunction or lack of sunlight.     

Does anybody know of a solar manufacturer that makes a 6 volt regulator? 

Offline DanS

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Re: Power system thoughts?
« Reply #1 on: March 16, 2011, 05:08:13 AM »
At the output of the 12 volt solar regulator pop one of these in to regulate down to +6 volts (or a 7805 for +5 volts). Current handling is sufficient. Or you could use a LM317 (adj. version) if you need another voltage (lower than 12v).



You would have to build a little circuit similar to this on a matchbook size board.



Common Radio Shack type parts too.
« Last Edit: March 16, 2011, 05:42:55 AM by DanS »

Offline johnd

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Re: Power system thoughts?
« Reply #2 on: March 16, 2011, 07:15:22 AM »
So what are the preferred options?

1) 12V system / 12 volt battery and DC/DC convertors to 5-6 volts
2) 12V system / 6 volt paired batteries with 6 volt feeds direct from 6 volt battery
Either is viable but personally I prefer [1] because it's a lot simpler to add a DC/DC converter into a 12v solar PSU than install a separate 6v solar PSU.

Quote
The main issue I see are systems with no Low Voltage Disconnect (LVD) which have the possibility of completely stuffing the battery during times of malfunction or lack of sunlight.
It's not difficult to install your own LVD. Actually I've built a little box that has two separate LVDs, one for the 12v supply and one for the 5v supply. The idea is that the 12v one which powers a cellular modem and is the power hog cuts out before the 5v one and allows the logger to keep logging even if the cellular data link is allowed to drop out. Apart from anything else I prefer the ability to define my own LVD voltage levels and not rely on the often vague specs that the regulator makers provide. There's also often too much hysteresis built into the regulator settings IMO.

If you want to email me (support<at>prodata.co.uk) I can let you know at least one source for a reasonably cheap 12v LVD board.

Quote
Does anybody know of a solar manufacturer that makes a 6 volt regulator? 
I do remember seeing some when I was looking around previously, so they do exist, but can't remember any details right now.
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Offline DanS

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Re: Power system thoughts?
« Reply #3 on: March 16, 2011, 07:38:40 AM »
You can cut out all the conversion stuff if you don't need 12v - http://www.google.com/products/catalog?q=6+volt+solar+panel&hl=en&um=1&ie=UTF-8&cid=7218398043270871512&sa=X&ei=iaCATZOeOM_NrQf3zMGmBw&ved=0CGoQ8wIwADgU#  but of course you'll get longer run times using 12v equipment.
« Last Edit: March 16, 2011, 07:47:57 AM by DanS »

Offline Flag

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Re: Power system thoughts?
« Reply #4 on: March 16, 2011, 07:49:06 AM »
6 volt panels are not generally the problem, it's the 6 volt solar regulators that aren't all that common

Offline DanS

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Re: Power system thoughts?
« Reply #5 on: March 16, 2011, 08:39:08 AM »

Offline SLOweather

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Re: Power system thoughts?
« Reply #6 on: March 16, 2011, 10:35:59 AM »
I built 2 of these with the 5 volt regulator for the consoles at Condor Lookout, to run them off the solar system and save the C cells. The consoles just don't draw that much current unless the backlight is on, so the inefficiencies of the linear regulator don't really matter to me. The regulator does get a little warm when the back light is on, so I used the metal cover of the Radio Shack project enclosure as a heat sink.



At the output of the 12 volt solar regulator pop one of these in to regulate down to +6 volts (or a 7805 for +5 volts). Current handling is sufficient. Or you could use a LM317 (adj. version) if you need another voltage (lower than 12v).

(Pic and schematic deleted for space...)

Offline johnd

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Re: Power system thoughts?
« Reply #7 on: March 16, 2011, 11:30:58 AM »
The consoles just don't draw that much current unless the backlight is on, so the inefficiencies of the linear regulator don't really matter to me.
Certainly true with the serial/USB loggers, but the IP logger obviously takes more like 100mA so a bit more of an issue unless you have a generously-sized solar PSU.
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Offline SLOweather

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Re: Power system thoughts?
« Reply #8 on: March 16, 2011, 01:33:40 PM »
The consoles just don't draw that much current unless the backlight is on, so the inefficiencies of the linear regulator don't really matter to me.
Certainly true with the serial/USB loggers, but the IP logger obviously takes more like 100mA so a bit more of an issue unless you have a generously-sized solar PSU.

Yeah. Instead of the WLIP, I use the serial WeatherLink and my own outboard Serial/Ethernet data hub to send the data from the stations to my server. It draws a little more than a WLIP (165ma at 12 VDC). But, it's a lot more configurable (even remotely from the server) and doesn't heat up the console sensors. The whole solar site (console, interface, and cell modem lives on 7 AH per day at 12VDC at one minute updates.

Offline johnd

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Re: Power system thoughts?
« Reply #9 on: March 16, 2011, 02:10:34 PM »
The whole solar site (console, interface, and cell modem lives on 7 AH per day at 12VDC at one minute updates.
OK, thanks - that's interesting. Couple of questions if I may:

How do you arrive at the 7Ah figure? So the 0.165A draw is from what you're calling the serial/Ethernet hub (is that a device server or something else?) So that's 4Ah. So does the cell modem live on just 3Ah? Thats an average draw of 0.125A or 1.5W @ 12v, which seems quite good. Is the cell modem powered up continuously in the sense of not sleeping between transmits? But presumably the current draw does fluctuate to some extent when an active transmission is under way? So how do you measure the average draw in practice?

And, second, have you ever calculated how many Wh of sunshine you need to generate your 7Ah (Did you say that your total panel power was 150W?) I guess this is only going to be very rough comparison because it will depend on the panel angle. I'm thinking that I should have my panels more vertical than horizontal because you need to maximise the winter power generation (at 52N) - I don't really care about summer power generation because I've got to size the panel array to get through the winter - summer generation is almost irrelevant as long as it exceeds winter. (This sort of argument tends to throw the folk installing panels for power generation because they're keen to maximise summer or year-round generation whereas all I want to do is to get through the winter reliably.) Actually what I would like to know is how to calculate the optimum panel angle to maximise midwinter power generation
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Offline SLOweather

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Re: Power system thoughts?
« Reply #10 on: March 16, 2011, 04:06:24 PM »

OK, thanks - that's interesting. Couple of questions if I may:

How do you arrive at the 7Ah figure? So the 0.165A draw is from what you're calling the serial/Ethernet hub (is that a device server or something else?) So that's 4Ah. So does the cell modem live on just 3Ah? Thats an average draw of 0.125A or 1.5W @ 12v, which seems quite good. Is the cell modem powered up continuously in the sense of not sleeping between transmits? But presumably the current draw does fluctuate to some extent when an active transmission is under way? So how do you measure the average draw in practice?

Rather than trust the spec sheets, I set everything up on the bench and measured the current individually for the hub, and for the router both in transmit and in receive, and measured the transmit time for one transaction. Then I used that to calculate the total current draw per day.

I was very careful about this, as the site already had its solar power system in place, 3 or 4 panels on the roof and 400 AH of storage. I wanted to make sure I wasn't tapping too much power out of the battery bank.

Quote
And, second, have you ever calculated how many Wh of sunshine you need to generate your 7Ah (Did you say that your total panel power was 150W?) I guess this is only going to be very rough comparison because it will depend on the panel angle. I'm thinking that I should have my panels more vertical than horizontal because you need to maximise the winter power generation (at 52N) - I don't really care about summer power generation because I've got to size the panel array to get through the winter - summer generation is almost irrelevant as long as it exceeds winter. (This sort of argument tends to throw the folk installing panels for power generation because they're keen to maximise summer or year-round generation whereas all I want to do is to get through the winter reliably.) Actually what I would like to know is how to calculate the optimum panel angle to maximise midwinter power generation

Yes, I used to do this a lot at my previous job. Let me see if I can remember everything.

Add up all the amperages of the stuff you are trying to run and multiply x 24 to find out how many amp-hours (AH) you need per day.

(1) Let's say our system draws an average of 0.3 amps x 24 hours = 7.2 AH.

For 35 degree  N latitude and this central California coast climate, in the winter, we have at most 5 hours of good sun on panels that are not tracked.

So, I need enough current from the panels to supply 7.2 AH plus 5 hours x 0.3 A = 1.5 AH (to run the equipment during sunny hours/charging), or 8.7 AH.

8.7 AH / 5 hours = a panel that can deliver 1.74 peak amps, or about a 20 watt 12 volt panel (12 x 1.74 = 20.88).

Note: This is all "perfect world" design. Round everything UP and add fudge factors. :)

For your 52N lat you may need to double that. There are charts that show things like this on the net, but like your experience with solar installers, everything I can find is related to that sort of work, not this.

Then, the battery bank. Lead acids of any sort last longest if never discharged below half their capacity.

Therefore, in a perfect world, your battery should be at least 2x the size of the AH you calculated in (1), or 17 AH.

To allow for decrease in cold weather capacity, and for operation in extended overcast, I'd make the battery 4-5 x bigger.

The tilt angle rule for the panel is usually to set it up from flat to an angle equal to your lat, pointed due south in the northern hemisphere, so I'd try setting your panel up at 52 degrees to start.

So, my conservative design for this system would be a 2x bigger panel (40 watts), and a battery 70 AH or bigger.

YMMV... ;)
« Last Edit: March 16, 2011, 05:21:57 PM by SLOweather »

Offline Flag

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Re: Power system thoughts?
« Reply #11 on: March 16, 2011, 07:14:37 PM »
The trick is to setup the power so that you can tell you have a primary power failure long before all power fails all together. The thing with the WLIP is that you don't have much of a window once the primary power fails to get the problem fixed. I think more emphasis needs to go into the backup power then the primary power.

If the VP2 backup power works similar to the WMII then it requires a proper voltage difference so as to know when to switch? New backup batteries normally sit at around 4.7-4.8V which is just that little bit less than the 5 or 6 volt input.

What would occur if the backup voltage was 6 volts and the external power supply 4.5 volts? Where would it take power from?

There must also be a top end voltage limiter as some 6v solar systems can output around 7 volts else the console/envoy has a fairly wide voltage acceptance?

Some might also remember some years ago when the factory installed the backup battery wires in the wrong hole which drew power directly from the backup batteries until flat then switched to the external power. This was a bit confusing for a while but did highlight some different options for power/backup.           

Offline SLOweather

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Re: Power system thoughts?
« Reply #12 on: March 17, 2011, 04:17:36 PM »
For a DC system, I'd have "primary" power at a volt or so more than "back-up" power and isolate them with a diode in each line. The higher voltage primary voltage would keep the back-up diode from conducting. When primary volts dropped below back-up, that diode would conduct. The primary diode would keep the back-up source from back-charging the primary source. That would be pretty much zero-time switching.

Of course, if using silicon diodes, you'd lose a half-volt or so across the diode...

Offline johnd

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Re: Power system thoughts?
« Reply #13 on: March 30, 2011, 11:36:54 AM »
The tilt angle rule for the panel is usually to set it up from flat to an angle equal to your lat, pointed due south in the northern hemisphere...

I had meant to follow up on this comment. Do you have any references for this 'tilt angle rule'? (I'm not disagreeing in the slightest, just curious to see a little more discussion of the topic. So often the recommended angle seems to be based on achieving the maximum summer or annual power generation by the panel, whereas what I'm interested in is achieving the maximum midwinter power generation (summer is presumably well able to take care of itself!). Is this what the 'tilt angle rule' actually delivers?)

Of course I understand that ideally one might alter the tilt angle with the season, but assuming that it is to be fixed and that you need the maximum possible midwinter power generation...
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Offline SLOweather

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Re: Power system thoughts?
« Reply #14 on: March 30, 2011, 12:02:09 PM »
EDIT! See my next post on this. I changed my thinking...

Ideally, there are trackers that aim the panels directly at the sun all the time. ;)

Seriously, though...

I believed the "angle = latitude" rule was general purpose to maximize output throughout the year. But, after doing the following research...

If you want to do it experimentally, you could wait 'til solar noon on the winter solstice (pray for a clear day) and aim the panel perpendicular to the sun's rays. That should maximize solar output at the worst of the winter, while sacrificing maximum output the rest of the year

Or, if you don't want to wait, use a solar angle calculator like this one
http://www.srrb.noaa.gov/highlights/sunrise/azel.html
or this one
http://www.esrl.noaa.gov/gmd/grad/solcalc/

and see what the sun's elevation angle and azimuth are for solar noon on that day.

In playing with the calculator and my home location, it looks like the  "angle = latitude" rule is pretty close. For winter solstice, the sun at solar noon is at 31 degrees, and our lat is 35 degrees. I don't think 4 degrees is going to make a measurable difference in panel output.

At the summer solstice, the sun at solar noon is at 78 degrees.
« Last Edit: March 30, 2011, 02:46:12 PM by SLOweather »

Offline SLOweather

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Re: Power system thoughts?
« Reply #15 on: March 30, 2011, 02:42:54 PM »
I've had a little time to think about this, and I've changed my mind. It may be coincidental that my numbers came out close enough to muddy my thinking....

The standard way to set a solar panel, as I recall (gotta find a reference...) is to tilt it up from horizontal by the same degrees as your latitude. Now I think that indeed is to maximize annual production.

To maximize the winter output, I think the procedure would be to find the sun elevation angle at local solar noon on the solstice as described above. This is the angle from horizontal at which the sun will be.

Then. tilt the panel back from vertical that amount (or up from horizontal 90 - that amount). However you do it, the panel face should be at right angles to the sun's rays on that date at that time.

Here's a page that discusses this in some detail.
http://www.macslab.com/optsolar.html
I haven't studied it enough yet to see if it makes me look stupid or not. :)
« Last Edit: March 30, 2011, 02:51:10 PM by SLOweather »

Offline Flag

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Re: Power system thoughts?
« Reply #16 on: March 31, 2011, 05:37:00 AM »
For a DC system, I'd have "primary" power at a volt or so more than "back-up" power and isolate them with a diode in each line. The higher voltage primary voltage would keep the back-up diode from conducting. When primary volts dropped below back-up, that diode would conduct. The primary diode would keep the back-up source from back-charging the primary source. That would be pretty much zero-time switching.

Are the diodes necessary with say a Davis Envoy? Does an Envoy have any power intellegence between the primary and secondary backup power?

Offline SLOweather

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Re: Power system thoughts?
« Reply #17 on: March 31, 2011, 11:52:58 AM »
Are the diodes necessary with say a Davis Envoy? Does an Envoy have any power intellegence between the primary and secondary backup power?

Almost certainly, the Envoy uses one or more diodes to isolate the batteries from the DC jack. Without them, the DC supply would try to charge the non-rechargeable batteries, leading to leaking or worse.

A quick inspection of a wired VP2 console reveals 2 diodes which a little DMM probing leads me to believe are for isolation between the jack and batteries. The Envoy should be similar.

Offline johnd

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Re: Power system thoughts?
« Reply #18 on: March 31, 2011, 12:49:31 PM »
The Envoy should be similar.

The standard console and Envoy have identical part numbers for the main PCB assembly (7346.047)
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Offline Flag

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Re: Power system thoughts?
« Reply #19 on: April 01, 2011, 06:14:18 AM »
One (1) volt different?

So how would say 5.1V primary and 5.0V backup go? (0.1 volt different)   

Offline SLOweather

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Re: Power system thoughts?
« Reply #20 on: April 04, 2011, 12:04:38 AM »
One (1) volt different?

So how would say 5.1V primary and 5.0V backup go? (0.1 volt different)   

I dunno. Why don't you try it and let us know?

 

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