Power needed for remote wireless signal repeater

Posted by Eric Fletcher on July 08, 2008, 10:55:44 AM

Power needed for remote wireless signal repeater
I need to provide power for a remote wireless antenna setup and am completely new at this. Here's what I know and think I need; I'd appreciate any advice.

-- 2 radios draw 400mw at 12V
-- Monitoring camera (optional) draws 5V at 1A (Linksys WVC54GCA)
-- Mean daily insolation at my location (45° 34'N): annual 4.4 kWh/m2; November worst case 2.5 kWh/m2

I would like to get the power from a solar array but am not sure how to calculate what wattage would be needed and what components I would need for the system. The radios would be on 24/7 and I would like to have sufficient power from batteries to last at least 3 days without any sun.

The site is located on a rock outcrop and weather conditions can be extreme (+40C to -40C). I can build a weatherproof container to protect batteries and electronic equipment as needed. The camera is optional in part because I'm not sure it can withstand the conditions. The site is ~600m from my house with the only other access from a wilderness park area so I would like some sort of security.

Wind is an option but I haven't got data about it for the location; however, a wind generator would be more visible and thus less secure.

Background: Although I cannot get a decent signal at my house, my property has a ridge where we were able to get a good signal from hot spots of a local wireless provider. He is willing to put up the tower and antennas if I can provide the power. This will give me high speed access and give him the ability to provide service to a number of other nearby locations.
 

Posted by Eric F on July 09, 2008, 12:05:16 PM

Re: Power needed for remote wireless signal repeater (Reply #1)
Well, here goes a try at being of some help.

First, lets just work with the two radios.  Together they need 800mw at 12v.  That's 19.2wh per day or 576wh per month or only a little more than a half a kilowatt-hr per month.  Use .6 to up to 1KW-hr per month and use the "off-grid" calculator under the "learn" tab of this website.

If we add in the power for the camera using 5V and 1w. That comes to (1w x 24 hr x 30 days) = 720wh per month.  Again, add the 576wh-month from above and you get to 1,296wh per month.  Go to the calculator and enter about 1.5KWH/month.

I'm unsure how this will work out since using a 5v camera with a (most likely) 12v system isn't going to work.  A 12v camera would make things simpler for you.

Hope this helps or at least spurs some clarifying discussions.

Eric
 

Posted by Eric F on July 09, 2008, 12:18:35 PM

Re: Power needed for remote wireless signal repeater (Reply #2)
Quote from: Eric Frantz on July 09, 2008, 12:05:16 PM
Well, here goes a try at being of some help.

First, lets just work with the two radios.  Together they need 800mw at 12v.  That's 19.2wh per day or 576wh per month or only a little more than a half a kilowatt-hr per month.  Use .6 to up to 1KW-hr per month and use the "off-grid" calculator under the "learn" tab of this website.

If we add in the power for the camera using 5V and 1w. That comes to (1w x 24 hr x 30 days) = 720wh per month.  Again, add the 576wh-month from above and you get to 1,296wh per month.  Go to the calculator and enter about 1.5KWH/month.

I'm unsure how this will work out since using a 5v camera with a (most likely) 12v system isn't going to work.  A 12v camera would make things simpler for you.

Hope this helps or at least spurs some clarifying discussions.

Eric

My bad!  The camera uses 5 watts per hour or 120 watts per day or 3600 watts per month.  The camera really changes things!  Again use the calculators you will find on this website.

Eric
 

Posted by Eric Fletcher on July 09, 2008, 01:41:18 PM

Re: Power needed for remote wireless signal repeater (Reply #3)
Thanks for the response Eric. I'd tried the calculator but couldn't get it to work -- until I tried it again today with IE instead of Firefox. (In Firefox, it doesn't do any calculations and hangs up for some reason.)

Since there are no sun hour figures for where I live (Quebec, Canada), I guessed that we'd be similar to Rochester NY. With that, it looks like I could manage with a single 30 watt panel if I don't include the camera.

I can see that if the camera really does add 3600 watts per month, my panel requirement goes up to 126 watts which would add quite a bit to the cost. I'll reassess the options in that regard.

However, your followup notes that the camera would use 5 watts per hour but wouldn't it be less if the voltage is only 5V? The camera I looked at comes with one of those 120V converters but the technical specs say 5V so I assume I would be able to provide the power directly from a DC source. Is there some way to tap off just 5V from a 12V source? (This is where I feel like I'm getting over my head!)

The other item that came up from using the calculator relates to the lowest temperature the battery would experience. When I put a value of <0, the calculator displays a capacity of "NaN" but if I used 1, it displays "187" amp hrs at 12V.

Is this just a function of the calculator, or do I need special batteries to withstand very low temperatures? I assume people in places like ND or AK would encounter similar cold -- and certainly my tractor and car manage with their batteries in temperatures that go much lower than 0°F.

Eric
 

Posted by Eric F on July 09, 2008, 02:34:24 PM

Re: Power needed for remote wireless signal repeater (Reply #4)
In simple terms the voltage time the amperage equals the wattage.  Or, V x I = P.  Therefore, 5 volts times 1 amp = 5 watts for the camera.

Yeah, I believe the on-line calculator doesn't like low temperatures.  Look at the "Batteries" section of this site to see the different types and their characteristics.  Looks to me like you could easily use AGM batteries and allow for less drawdown (discharge) and be just fine.  Instead of allowing for 50% discharge, plan on using only about 20% and calculate your battery needs accordingly.  These "less drawdown" percentages will give you longer battery life, too.

Camera at 5V versus system at 12 volts.  Yep, this is a problem.  With AC power it is a matter of using a step down transformer, but in DC that isn't the case.  You would need a resistive load (in series)that would dissapate about 7 volts to get this to work.  That's a lot of load just thrown away, so to speak. 

Consider, for example, if you put a lightbulb rated for 5 volts DC in a 12 volt circuit with a battery, the bulb will most likely be very bright for a very short time (seconds) and burn out.  If you did the same thing with an 18 volt rated lightbulb, it will glow dimly but for a very long time.  Your camera will work best if set up in a circuit that provides the correct voltage.  12volts may very well ruin it in a "flash!"

Eric
 
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