Need advice on building a simple solar battery backup system

Likesspace · September 2009

Hi guys....
First of all, I'm glad to be a part of this forum.
A few years ago we added solar pool heating panels at our home and it has certainly made me a beliver in the power of solar energy.
As for my question today.....
I have a home weather station that uses a 12 volt DC aspirated fan system for the temperature system.
I am currently running this system by plugging a 12 volt DC converter into a 115 volt AC outlet, but I'd like to switch the system over to solar if possible.
What I want to do is to use a 12 volt solar panel to charge rechargeable batteries that will supply 12 volts to the fan when solar energy is not available.
The design of this weather system needs 24 hours of energy available to operate properly.
Please understand that I am totally new to solar usage other than our pool heater, so please speak to me in clear and understandable language.
In short, I need a panel that will charge rechargeable batteries that will put out 12 volts of power. I also need this to be a "self switching system" meaning that when the sun is not shining the batteries will automatically kick in.
If anyone can tell me the items I will need to build a DIY kit or a ready made commercial system I would really appreciate it.
Thanks in advance,
Dave

dwh · September 2009

Re: Need advice on building a simple solar battery backup system

Looks like this:

PV (solar panel) -> charge controller -> battery -> inverter (for 120v loads, omit for 12v loads)

Each of the "->" represent wire runs and each run needs fuse protection.

Figure out how many watts per day you need, and how many hours of *full, direct sunlight* (PVs put out almost nothing without full direct sunlight) per day.

Say you need 500 watts/day and you figure you can get 5 hours of full direct sunlight. Then you would need a 100w solar panel.

But you better bump that up some, since no system will be 100% efficient.

Then, if you are making a backup system, you should probably add a decent AC powered battery charger as well, which will keep the battery topped up when there isn't any sun but there is grid power.

As for self-switching...you just hook it up so it runs off the battery all the time, and then see that the battery is kept charged with either the solar charge controller or the AC powered battery charger.

But you (or we) can NOT spec out any system without knowing the true wattage of the loads that will be hooked up to the system.

niel · September 2009

Re: Need advice on building a simple solar battery backup system

what do the specs say for the wall adapter (wall wart)? watts? volts dc? current dc?
better yet, if you know what current is required by the weather station then we can better figure this from that info as wall warts are usually rated higher power than is normally drawn.

mike95490 · September 2009

Re: Need advice on building a simple solar battery backup system

First, we need a idea of how much power the fan draws. Then we can plan what size panel and size of battery you need.
Count on about spending
$600 panel
$120 charge controller
$100 battery

BB. · September 2009

Re: Need advice on building a simple solar battery backup system

By the way--it is Watt*Hours per day...

Say a 10 watt fan * 24 hours per day = 240 Watt*Hours per day

Say in winter you average 4 hours of sun per day, and you have flooded cell battery and a small inverter... Overall, end to end efficiency (from panel marketing rating to power out the AC socket is around 50%):

240 Watt*Hours * 1/4 hours of sun per day * 1/50% efficiency = 120 Watt Solar panel

Battery size--3 days without sun; 50% maximum discharge, 12 volt battery:

240 Watt*Hours * 1/0.80 batt eff * 1/0.85 inverter eff * 1/12 volt batt = 29 Amp*Hour battery

Anyway--rough idea of where to start. Issues of deep winter, week or more of no-sun kills battery, etc... Off-grid solar power is not cheap to do right.

Obviously, minimizing power consumption and losses is a good start (smallest fan possible, using AGM batteries vs flooded cell, efficient AC inverter--or no inverter if you can use less power from 12volts direct, etc.) all help.

However, there is an issue with using 12 volts direct... The actual voltage range can be from 10.5 to 15.5 volts... Many 12 volt devices (usually designed for use in a car) will stop working below 12 volts or get damaged if operated at 15+ volts.

-Bill

tallgirl · September 2009

Re: Need advice on building a simple solar battery backup system

I'd work on getting rid of the need for the fan in the first place.

As others have pointed out, this can get very spendy very fast. But also, they are grossly over-designing the system.

We're talking about FAN in a WEATHER STATION. Not a HOUSE.

I assume the fan is to make sure air flows over a temperature sensor inside of a Stevenson Screen weather station enclosure? How many cubic feet on the inside? Where's the air come from? Can you reduce volume INSIDE the enclosure? How many CFM are you wanting?

BB. · September 2009

Re: Need advice on building a simple solar battery backup system

I doubt it is in a Stevenson Screen...

Anyway, here is a 24 hour fan aspirated wireless weather station that looks to use around a 10 watt panel (just a guess).

From the data sheet, it appears the fan runs "full speed" under full sun, and cuts way back when "in dark" (2x 1.2 volt C cell Ni-Cads).

[FONT=Fixedsys][SIZE=1]Current Draw (ISS SIM only) . . . . . . . . . . . . . . . . . . . . . . 0.14 mA (average), 30 mA (peak) at 4 to 6 VDC
Solar Power Panel (ISS SIM / Fan-Aspirated) . . . . . . . . . 0.5 Watts / 0.75 Watts
Battery (ISS SIM /Fan-Aspirated) . . . . . . . . . . . . . . . . . . CR-123 3-Volt Lithium cell / 2 - 1.2 Volt NiCad C-cells
Battery Life (3-Volt Lithium cell). . . . . . . . . . . . . . . . . . . . 8 months without sunlight - greater than 2 years depending on solar
charging
Battery Life (NiCad C-cells) . . . . . . . . . . . . . . . . . . . . . . . 1 year
Fan Aspiration Rate (Fan-Aspirated Only) . . . . . . . . . . . . 190 feet/min. (0.9 m/s) (full sun), 80 feet/min. (0.4 m/s) (battery only)
(intake flow rate) 500 feet/min. (2.5 m/s) (full sun), 280 feet/[/SIZE][/FONT]

Since nobody knows what the wattage requirement is at this time (which we all said)--just giving some basic information. And included minimizing consumption as an important part of the plan--Did not assume that this was a house fan.

-Bill

tallgirl · September 2009

Re: Need advice on building a simple solar battery backup system

BB. wrote: »

I doubt it is in a Stevenson Screen...

That white thing on the bottom is a Stevenson Screen. They aren't all the pretty white louvered boxes I've been too lazy to put mine in. But then, I want my temperature sensor in full-sun on purpose.

(Honest -- I forecast A/C load using full-sun temperatures!)

BB. · September 2009

Re: Need advice on building a simple solar battery backup system

Sorry Tall Girl, you are correct... I was used to them looking more like the foot cube or so whitewashed boxes.

-Bill

tallgirl · September 2009

Re: Need advice on building a simple solar battery backup system

BB. wrote: »

Sorry Tall Girl, you are correct... I was used to them looking more like the foot cube or so whitewashed boxes.

-Bill

Yeah, and I want one of those pretty whitewashed ones in my backyard. But I'm lazy. The fake plastic ones are too ... fake.

Likesspace · September 2009

Re: Need advice on building a simple solar battery backup system

Wow, this is an active forum....
Okay, I guess I should have given a little more information in my first post......
Tallgirl is right that the fan is used to circulate air inside the stevenson screen, in fact my setup is very similar to the photo that was posted in an earlier reply.
The fan that I'm using in my home built system is an old power supply fan out of a personal computer.
I'm sure that the draw is minimal, in fact I've read of others only supplying 4 - 5 volts to one of these fans without issues.
I have not looked at the fan itself to see what the actual draw is, but will try to do so within the next day or so.
Needless to say, I don't want to spend anywhere near what some of the previous posts suggested since I'm sure I'm not spending much at all to power the fan on AC power. I was hoping for a low cost solution, mainly for wireless operation.
Thanks for all of the information so far and I'll post the fan information when I get back home and take a look at the fan.

Dave

BB. · September 2009

Re: Need advice on building a simple solar battery backup system

If you can the fan down to where a pair of "C cell" Ni-Cads will work--

A small solar panel and no more than two Ni-Cads (two Ni-Cads in series should not hurt each other when drained dead--where the "weak cell" is reversed charged by 2 or more other cells that can still drive current current in the forward direction).

And Ni-Cads will cycle down to zero charge without major damage. You may still find you need to replace the set every 1-2 years in normal operation.

NiMH would be more environmentally friendly--and have more capacity... But I don't think they would withstand the "deep cycling" to zero volts as well.

Lastly, as always, the requirements drive the system. If you can live with variable speed fan and occasional outages of no motion--then the power system can be much smaller, cheaper, and simpler.

-Bill

PS: By the way, you can put more Ni-Cads in series--but they would then need to be "matched cells"... So that they all lose power at the same time (because they have been matched by capacity). Radio Control guys will match cells in their battery packs. And some battery chargers include the option to measure cell capacity.

tallgirl · September 2009

Re: Need advice on building a simple solar battery backup system

BB. wrote: »

NiMH would be more environmentally friendly--and have more capacity... But I don't think they would withstand the "deep cycling" to zero volts as well.
-Bill

They do it a lot better than NiCd does, in part because the added capacity means they are less likely to do so.

Brock · September 2009

Re: Need advice on building a simple solar battery backup system

Out of curiosity what is the weather station? Doesn't Davis have a fan aspirated unit that just has a solar panel built in? Correct me if I am wrong but if you don’t have direct sunlight you don't need to fan aspirate anyway?

So wouldn't a simple fan connected to a solar panel do the trick?

BB. · September 2009

Re: Need advice on building a simple solar battery backup system

Brock,

Check the link under post #7 for a Davis fan aspirated unit...

It appears the fan runs at "100% air velocity" in sunlight, and 40% under battery power (I wonder how accurate the numbers are) being powered by two Ni-Cad C cells.

The air movement appears to increase accuracy even when the sun is not out.

-Bill

Likesspace · September 2009

Re: Need advice on building a simple solar battery backup system

Hi everyone....
Okay, I finally had a chance to look at the fan and the only information I could find said:
12 V -0.15 A
I would assume that to mean 12 volts and 0.15 amps but you all are the experts.
As the last post mentioned the Davis units run at 100% during the daylight hours and 40% during the evening hours and I'd like to get my unit set up to operate the same way.
When the air temerature falls at night it seems that my readings are just about a half to one degree lower than another sensor that I have on my property. The two sensors are very close to each other and do give a reading within a tenth of a degree during the daytime.
As I said initially I'd like to have this unit operate on solar when possible and on stored battery power at night or on cloudy days.
If anyone has advice on how to go about this I would appreciate hearing it.
I'm sure that I will not understand most of what you have to say but I can always forward the information to my brother who is an electrical tech.
Also, if you have links to places where I can purchase the needed supplies that would be appreciated as well.
Thanks in advance.

Dave

Likesspace · September 2009

Re: Need advice on building a simple solar battery backup system

Oh, and Brock...
I use a Fine Offset weather station that I've done several modifications to.
I am trying to get the most accurate readings possible with this unit, even though it is a low cost alternative to the Davis VantagePro.
I do have the sensor in direct sunlight which will give the most accurate readings as long as the shield/screen functions properly.
I've tried both home made and commercial radiation shields and still experienced solar heating at certain times during the day.
I now have the sensor enclosed in a piece of 3" PVC pipe which is reduced to 1.5" and made into a U-bend. I then have the fan mounted into a 3" adapter fitting which draws ambient air across the sensor and vents at the fan.
So far this has worked almost perfectly and I get zero solar heating during the day.
My next step will be to piece together a very nice station out of various professional components that can be found online. Since this station will be quite expensive I want to have all of my ducks in a row before just jumping into it head first.
If anyone is interested, here's a simple website design I've been working on for when I get the real station up and running. Since I also know next to nothing about building web pages I thought I'd better play around with this part of the equation as well.
So far it's been a hit with close family members and friends who seem to use it daily for their weather information.
Anyway, here's the site: http://sites.google.com/site/middletonstreetweather/
Thanks again to everyone for their advice.

Dave

BB. · September 2009

Re: Need advice on building a simple solar battery backup system

P=I*V=0.15a * 12v=3 watts

Not a big fan (good for solar).

However, I would probably try to go on the "smallest" size possible first--and see how it works (not very expensive).

First handy website that popped up (in Canada, never used vendor--just to look at an array of 5 volt fans):

1"/25mm/5VDC FAN

DELTA ELECTRONICS

1 inch/25mm square fan (10mm thick), model AFB02505MA. 5VDC, 0.1A, short flying red/black leads with miniature 2 pin connector. UL/CSA/CE/VDE rated fan. 5 blade, high velocity (7000rpm) miniature, ball bearing, very low noise (20dBA) fan, 1.2CFM, ideal for spot or chip cooling. Has an adhesive double sided tape dot over the info label, can be used to stick it directly to a chip or device, it has been removed for the picture to show details. Comes in factory trays of 120 fans.

Looking for a small, low rpm, low power fan... 5 volt 0.1 amps, 0.5 watts.

Run at 5 volts get 1.2 CFM. 190 Feet/Min flow rate though sensor--

1.2 CFM / 190 LFM = 0.006 square foot aperture

sqrt (0.006sqft)=0.08 foot throat
0.08 ft * 12in/ft = 0.95" square aperture

So, small fan, low CFM seems to be capable of same linear ft/min.

5 volt fan running on "panel" meets requirement (maybe).

Now the question is what is the Voltage/Airflow power curve... I would be testing running on a pair of 1.2 volt C cell Ni-Cads.

Assuming power is reduced by 80LFM/190LFM or to 40%... Gives motor power to 0.5w*0.40=0.2watts.

Current at low voltage would estimate to be:

I=P/V=0.2 watts / 2.4 volts (2x C cells) = 0.08 Amps

Assume a "typical" C cell Nicad has 3.00 AH rating (just a google result).

Two C NiCads in series, 3.00 amp*hours:

3AH / 0.083 amps = 36 hours of "no sun" operation--so that makes sense...

Install fan so it "draws air" past the sensor (so "fan heat" does not raise local temp even a little bit).

I don't know--but I would spend $10-$20 for the 5 volt fan (or equivalent listed above) and get a couple C Cell NiCads for $4-$6 and see if you can build/modify your system to run "accurately". You can also try substituting a pair of good quality NiMH AA batteries (2.00 AH or larger) for testing too.

Panel size:

0.083 amps * 24 hours = 1.99 Amp*Hours of power per 24 hours (at low power)
0.2 amps

To recharge in 4 hours of sun:

1.99AH / 4 hours of sun = 0.50 amps of solar panel for 4 hours
0.5 amps charging + 0.1 amps for fan in full sun = 0.6 amp rated panel

If you can find a 6 volt (or 5 volt) panel:

6 volt * 0.6 amps = 3.6 watts minimum

Now, there is no regulator here... So, you may want to put a 5 volt clamp (like a 3T regulator) to prevent over voltage on the fan. And, you could put a diode in series with the "charging to the battery to lessen battery over charging a bit (not sure it is really needed). And a blocking diode between the solar panel and the "loads" to prevent back-feeding the panel at night (if it does not have its own blocking diode).

Anyway my 2 cents on how I would proceed. You can change the assumptions to meet your needs.

-Bill

Likesspace · September 2009

Re: Need advice on building a simple solar battery backup system

Thanks a lot for the suggestions.....
Surprisingly enough, I did understand quite a lot of what you said, and I'm sure that my brother can fill in the "blank" areas. :-)
I totally re-designed my fan system this afternoon and now I'm ready to tackle the solar project.
If nothing else, it sounds as if it won't cost that much money and will be a fun project to work on.
In short, your advice is exactly what I was looking for.
The way I see it is if I can make a $160.00 weather station perform as well as a $550.00 Davis then I have a good handle on what needs to be done to get the most accurate results possible in a backyard environment. Well I've already accomplished getting my readings as good as the Davis so now I'm looking to improve upon my design to make it as good as it can possibly be.
I really do apprecaite all of the time and effort everyone has spent on this.
I'll be sure to give an update once I get everything worked out.

Dave

Just Me · September 2009

Re: Need advice on building a simple solar battery backup system

Or, you could move to a house that is next door to or behind a person that already has a Davis and just buy the console.

tallgirl · September 2009

Re: Need advice on building a simple solar battery backup system

Just Me wrote: »

Or, you could move to a house that is next door to or behind a person that already has a Davis and just buy the console.

He might have to be REALLY REALLY close -- I have a neighbor with a wireless Lacrosse system (what I have) and I only get ONE of their sensors.

A better solution is to convince the neighbor to publish their data on Weather Underground

Brock · September 2009

Re: Need advice on building a simple solar battery backup system

Before we moved we had our wireless Davis station up and my dad thought it was really neat. They lived next door at the time so I bought him a console. His receiver was at least 500 feet, but line of sight from the station and worked fine. I have never tried walking away with the console to see how far away it can still receive data, but then again our station is on the Weatherlink (Davis) site
http://www.weatherlink.com/user/nevermab/
and the weather underground
http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KWIGREEN9

I just wised the weather link updated more often. Hey is there a way to get weatherlink to send wind gust's over to WU?

Need advice on building a simple solar battery backup system

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