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Old 02-16-19, 04:47 PM   #21
Robaroni
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Well, a 75% cutoff would then require (4) 24 series batteries. That is too many to safely wire in parallel, in my opinion. I agree that the battery will last longer, but suggest using batteries designed for the job, not car batteries. If he can live with 5 days of autonomy, then he could use two batteries set up for a 75% cutoff. It is my understanding that these are security lights, so it is important that they stay on at night. As I'm sure that you know, a big bird splat will all but shut off the output of a small panel, so that would give him 5 days to find it and squeegee it off before his security lights no longer work.

For what it is worth, my array has not produced more than 8 or 9 Watt Hours per day for the last 11 days according to its monitoring system, and I don't expect the snow to melt off of the array for at least another week, and probably two or three weeks.
First off, he lives in Ca, a very sunny place.

Secondly, I have no idea how big or small your array is. My systems, I have two, an intertie/off line that sends excess to the grid and backs up the house in case of failures and a strictly intertie that just feeds the grid. Total array is 10.4 Kw.
I don't run off grid because it's not a good idea, I only advise running off grid when individuals don't have the option. Why run through battery charge discharge cycles when you can simply have battery back up only at times when the grid is down? I'm several hundred dollars ahead on my Electric bill and haven't paid for electricity in over ten years and that's with charging my BEV, I don't pay for gas either. I also live in a very low sun area on the 42 latitude.

But back to Poor Man's request. He's only charging one 5 watt LED, should he be spending $500 plus to do that? So, being application specific, I took into account his needs and what would be feasible financially in his location. Even if we up his battery to 12v at 30 aH he still has 360 x2.5 hours a day or 900 wH. He probably has easily twice that where he lives, so I'm being very conservative. 25% of 900 wH will give him 225 wH for his 5 watt load which is over 4 days without charging.
If we use your two 75aH batteries we have (150a x 12v) x 0.25 = 450 wH. That's 9 days to 25% discharge. The battery cost is $90 for a deep cycle AGM 30 aH battery and $215 x 2 or $430 for two 75 aH deep cycle batteries. The difference is $340 more for 4 and a half days more(I used the same brand for both calculations). It's just not worth it.

https://www.ebay.com/itm/VMAX800-Gol...frcectupt=true

https://www.ebay.com/itm/VMAX-XTR34-...frcectupt=true

"Bird splat"?

https://www.ebay.com/itm/ACOPOWER-60...frcectupt=true

Here's a 60 watt module. It's 25" x 25", that must be some big bird! The price is under $100. The total cost of the solution I gave with charger, battery, module and LED is under $200, $140 less than just your battery choice. You still need modules, a bigger charger and a lamp.

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Old 02-16-19, 08:23 PM   #22
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Even if we up his battery to 12v at 30 aH he still has 360 x2.5 hours a day or 900 wH. He probably has easily twice that where he lives, so I'm being very conservative. 25% of 900 wH will give him 225 wH for his 5 watt load which is over 4 days without charging.
You lost me with the math. With using your recommended 25% of capacity of a 12V 30Ah battery, I get 90 Wh, only 12.8 hours for the 7 watt LED that he wants to use.
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Old 02-17-19, 07:34 AM   #23
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You lost me with the math. With using your recommended 25% of capacity of a 12V 30Ah battery, I get 90 Wh, only 12.8 hours for the 7 watt LED that he wants to use.
(12*30)*0.25=90

"There is an average of 3470 hours of sunlight per year (of a possible 4383) with an average of 9:30 of sunlight per day."

http://www.sacramento.climatemps.com/sunlight.php

This is for Sacramento which is more northern, not southern Ca. which has more avg sunlight.

So let's say there is substantial sun 7 hours out of that 9:30.

60w (Module size) = 420 wH per day avg. More than enough.

"The longest day of the year is 14:41 long and the shortest day is 9:18 long."

source:ibid

Using the shortest day which is dark 14.82 hours at 7w that's 103.74 wH required.

The percentage of our battery used is:

103.74/(12*30) = 28.8%
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Old 02-18-19, 07:08 AM   #24
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Good. We agree on one thing. The 30 Ah battery, utilizing 25% of its charge, is 90 Wh, not 225 as previously stated. 90 Wh is only enough for one single night, and will fail if there is a cloudy day.

I'm not as optimistic on the panel output. From PVWatts (a free online calculator from NREL), a December day will average 141 Wh, and a July day will average 346 Wh. Yes, it is enough to charge the battery on a perfect day for one single night of use, but just barely, and will fail to provide a single night of charge to the battery in December if there is a less-than-perfect day, or if the panel gets dirty. This assumes an MPPT charge controller, because a PWM controller would provide quite a bit less capacity.

I'm not intending for this to be a pissing match, but I'd hate for the original poster to purchase the system based on incorrect assumptions or wrong math, and then have to buy it again. It is always less expensive and more satisfying to do it right the first time.
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Old 02-18-19, 09:24 AM   #25
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Good. We agree on one thing. The 30 Ah battery, utilizing 25% of its charge, is 90 Wh, not 225 as previously stated. 90 Wh is only enough for one single night, and will fail if there is a cloudy day.

I'm not as optimistic on the panel output. From PVWatts (a free online calculator from NREL), a December day will average 141 Wh, and a July day will average 346 Wh. Yes, it is enough to charge the battery on a perfect day for one single night of use, but just barely, and will fail to provide a single night of charge to the battery in December if there is a less-than-perfect day, or if the panel gets dirty. This assumes an MPPT charge controller, because a PWM controller would provide quite a bit less capacity.

I'm not intending for this to be a pissing match, but I'd hate for the original poster to purchase the system based on incorrect assumptions or wrong math, and then have to buy it again. It is always less expensive and more satisfying to do it right the first time.
141/60=2.35 hours of sun. He lives in Ca. I took the cap. Sacramento because it is further north. Even if there data is wrong at 9 plus hours it will never be 2.35 hours. What is the US location of your data? Where is the link? I supplied a link.

Here's another link to Ca.


"Using a yearly average, there are 5.38 daily peak sun hours across the state of California. This number is calculated for a fixed solar panel."

"California hosts a very favorable environment for solar panels, with high average peak sun hours and a lot of sunny days.'

https://www.turbinegenerator.org/solar/california/

Sacramento:
188 sunny days
partial sunny days 77
total number of days with sun 265

https://www.currentresults.com/Weath...f-sunshine.php

In the winter under worst case conditions, living in northern Ca. he still has plenty of sun.

Your advice is to use two 75 aH batteries:

12*75*2 = 1.8KwH total. At 25% = 450wH. A 7 watt ( I originally used a 5w LED because I eliminated the losses from converting from 120 to DC by using an LED that runs on 12V) will mean that he has 450/7 = 64/14.82 =4.31 days with absolutely zero sun to charge the batteries during the day. Which clearly will never be the case as 265/365= 72.6% of the time there is sun in Sacramento.

So let's look at the cost of your lighting an LED for 4.31 days of zero sunny days.

two 75aH group 24 batteries = $214 *2 = $428

https://www.ebay.com/itm/VMAX-XTR34-...frcectupt=true

To charge 1.8kw*.25 = 450w; 450/5.38 = 83.6w so we need a module that will do that:

A 100w module with a charge controller: $115

https://www.ebay.com/itm/PV-SOLAR-KI...frcectupt=true

115 + 428= $543

As opposed to

12v 30aH battery $89


100w module with charger $115

total = $204

I used same module and battery manufacturer in both assessments.

https://www.ebay.com/itm/VMAX800-Gol...frcectupt=true

Difference is $339 more It's not realistic, he's only lighting a small LED. Will there be the rare time when the system might go two days or about 50% discharge? Maybe.

And what's your solution for the LED? I included an inexpensive circuit to run a 12VDC LED. Are you using an inverter with 120v mains? If you are then you have to add the cost of an inverter and the losses from the double conversion the the $543 it already costs.

Even if I doubled the batteries it only adds another $89 to the total cost and he could do that at a later date so he doesn't have to spend for what he might not need. My design can grow, if yours is overkill what's your recourse? You already spent more than you had to and again we're only lighting a small LED.
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Old 02-18-19, 06:31 PM   #26
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Well, I said it before, but will say it again. The number came from PVWatts. This is the gold standard tool, from the National Renewable Energy Lab, NREL. Since you used Sacramento, so did I. I don't think it is good policy to give a wild guess when someone asks for advice.

100 watts is fine for panel size if using an MPPT charge controller. Using a cheapo is fine, too, but he'll need at least a 130 watt panel for the same charging capacity. If he wants to charge his battery faster, simply pick a larger panel.

The charge controller should also control the lights - on at dusk, off at dawn or some user defined number of run hours. It should have a proper low depth of discharge protection.

Why mess with conjuring some electronic circuit and hacking a light bulb apart, when you can just use a 12 volt bulb?
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Old 02-19-19, 07:58 AM   #27
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Well, I said it before, but will say it again. The number came from PVWatts. This is the gold standard tool, from the National Renewable Energy Lab, NREL. Since you used Sacramento, so did I. I don't think it is good policy to give a wild guess when someone asks for advice.

100 watts is fine for panel size if using an MPPT charge controller. Using a cheapo is fine, too, but he'll need at least a 130 watt panel for the same charging capacity. If he wants to charge his battery faster, simply pick a larger panel.

The charge controller should also control the lights - on at dusk, off at dawn or some user defined number of run hours. It should have a proper low depth of discharge protection.

Why mess with conjuring some electronic circuit and hacking a light bulb apart, when you can just use a 12 volt bulb?
I didn't, I linked a 12V example. He doesn't take anything apart. The original bulb runs to an E26 socket which is 120V, so you need to include an inverter to 120v for your total cost.

NREL shows 5.8 to 6.5 for Sacramento. What did I quote from an alternate site:

"Using a yearly average, there are 5.38 daily peak sun hours across the state of California. This number is calculated for a fixed solar panel."

We can use MPPT but it adds equally to both analysis calculations, I didn't look back but I don't think sun hours is that critical that he needs MPPT but if he does he can substitute the module package with the CC for separates and include a cheap MPPT.

https://www.ebay.com/itm/100A-MPPT-S...DiFg:rk:7:pf:0

Please cite the cost of an MPPT CC that also controls external devices. I have expensive controllers here but they run into the hundreds of dollars.

Also starting small allows him to build to his needs. Over building leaves no reduced cost options. Look at the price your system will cost and now you want to add an MPPT with peripheral control?

He's not running an iron lung to save lives he's lighting porch light! You're up to several hundred bucks.

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