Trying to Understand the Basics

[thebrez1]

Yep, it's true. I have been reading this forum for a while and lots of other info on solar for a while now and thought it was time to join in.

I have done ZERO solar projects but I have tried to educate myself before driving everyone crazy with the same old questions. However, I still do have questions. Of course I started by thinking of grid tie-in and am now, realistically down to charging a 400 Ahr battery bank to keep my CPAP machines running for a week or so. Solar does not appear to e very cost effective, but I like having options.

Here is my problem. I am not a number person, I like and understand theories and concepts so here goes. If I have a 210 watt panel, 12 volt, 6.68 Amp panel and a 200 ah battery bank, do I understand correctly the following? If i discharge 100 amps (50 DOD) it would take about 4 days to recharge if I was not pulling any power out of the batteries and I had 5 hrs of good sun every day? I know there is loss and inefficiencies with wiring and equipment, but in a perfect world does 6 amp charge current mean the same thing as 6 amps per hour or 30 amps a day?

I look forward to pestering you all with more questions in the future 

And thanks!

[Sunking]

Here is my problem. I am not a number person, I like and understand theories and concepts so here goes. If I have a 210 watt panel, 12 volt, 6.68 Amp panel and a 200 ah battery bank, do I understand correctly the following?

You will get the same answer here as you did on the other forum. You have every thing all mixed up. First a 210 watt panel with a Vmp of 18 volts produces 210 watts / 18 volts = 11.66 amps. If using a MPPT charge controller delivers a 210 watt / 12 volts = 17.5 amps to a 12 volt battery. More on that later.

Amp Hours is a very technical math formula of Amp Hours = Amps x Hours or AH = A x H. So with that being a true statement means A = AH/H, and H = AH/A.

So you have a 200 AH battery with a 6.68 amp load the battery will be fully discharged in theory in 200 AH / 6.68 amps = 29.94 hours.

If i discharge 100 amps (50 DOD) it would take about 4 days to recharge if I was not pulling any power out of the batteries and I had 5 hrs of good sun every day?

No sir 200 AH / 100 amps = 2 hours. In rreality much less time because of the Peukert factor, realistically about 1 hour.

I know there is loss and inefficiencies with wiring and equipment, but in a perfect world does 6 amp charge current mean the same thing as 6 amps per hour or 30 amps a day?

Again Amps x Hours = Amp hours. So if you have 6 amps x 5 hours = 30 AH

Now to really confuse you. Quit using amp hours, use watt hours to figure out what size everything needs to be.

[thebrez1]

Now, maybe I was not clear on my original post. I have 200 amp battery pack. I drain 100 amps and disconnect the load. Panel gets 5 hours of good light. It should recharge 7amps X 5 hours, or 35 amps correct? so the battery would reach full charge on the 3rd day, 35 amps x 3 = 105 amps, assuming each day gave 5 hours of max rate charge and no additional drain on the battery during that time period.

The basic figures I am working with in real life is a 125 amp hr, deep cycle battery with approx. 3.5 amps per hour drain (2 CPAP machines) so I am looking at 3.5 x 8hr or 28 amps per night which gives me basically 2 nights before 50% DOD, 3 nights in a pinch with damage to the battery likely. Seems like a very short time for such a large battery. I have used jump starts for camping and get 1 to 1.5 nights out of a portable charger (of course that was for one machine). What am I doing wrong? Using this scenario, I would need 4 hours a day with the above panel to bring the battery back to 100% each day if everything is working optimally. Dang small load for needing 2 Batteries for a fudge factor of a few days of rain and 2 panels to be able o bring the batteries back from deep discharge in a day.

I know I a not getting this right and I am getting twisted in knots here. I know solar is not the most cost effective thing out there by a long shot, but a total of close to $850 (plus and inverter which should be pure sine wave although I have used modified with success) for two batteries and two panels makes for a pretty expensive battery charger back up!

If will look up the watts can I figure things more accurately or am I already close to realistic expectations regarding battery life and charging time?

Slap me around and set me straight. Da** newbies! Sorry all, but I am really trying to get this straight. I don't plan on putting in a 5KW system, but I still want to understand how this all works.

[Sunking]

Now, maybe I was not clear on my original post. I have 200 amp battery pack. I drain 100 amps and disconnect the load.

Do you mean you you have a 200 AH battery and use 100 AH? Amps is just a microsecond measurement. 200 and 100 amp is meaningless in the context you are using it.

You need to be working in watt hours, it makes life a lot easier. One mistake I can tell you are making is assuming you get a 5 Sun Hour Day. Unless you live in Tuscon AZ you get no where close to a 5 Sun Hour Day this time of year.

Next you would never want to intentionally discharge a battery 50%. That is the max limit and it will shorten battery life. Max per day discharge is 20%, and the solar panels must replace each and every day. In winter that is the months of December and January where you likely recieve only 2 to 3 Sun Hours per day, not 5.

I am not trying to be cynical, I am trying to lead you to giving us the correct information to help you.

So here is what we need.:

Watt Hours per day. To determine this how much power does the CPAP use measured in watts, and how many hours per day does it use. For example if it uses 12 volts and 6.68 amps is 80 watts. If it runs 8 hours each night it uses 80 watts x 8 hours = 640 watt hours.

Location, where do you live like a city

Time of year use? All year?

Then we can tell you what you need.

[thebrez1]

Ok I will do that. I am not necessarily trying to get an answer as much as I am trying to understand the process (even though this is a real problem i am workig on). I understand I will not get 5 hours of sun, I was using that as a constant so i could concentrate on the problem. Same with the 50% DOD, just using it as a constant. I do undertand that 50% would shorten the life of the battery. I would love to stay at 30 or above, but the more I learn, the less viable solar seems. Of course I am smart enough to know I dont know much

Don't worry about hurting my feelings, I have thick skin and an even thicker skull. I am excited about having a source where I can get some guidance form pepole that actually know what they are talking about. Keep slapping me until I get it and I will appreciate it!

Now, machine 1 is 1 amp max at 12 volt so 12 watts x 8 hrs = 96 amps?
Machine 2, 110 vots 2.5 amps max = 275 watts X 8 = 2296 amps??

OH WAIT, I THINK I GET IT. Since these kinds af numbers are obviously wrong I reread we are not talking amps, we are talikng WATTS so I have a total of 2296 WATT HOURS NOT AMPS!! Please tell me this is correct! (and how do I get from watt hrs to amps?)

Also i live in Oklahoma and this will only be a stand by system for use when the power goes out.

WEEEEEE THIS IS FUN

[Mike90250]

Volts x amps = watts (AC or DC, doesn't matter for our purposes)

My CPAP (Resperonics M) with humidifier at full heat, draws 52 watts.

over 8 hours, thats 416 watt/hours.

A deep cycle Costco marine battery @ 12V 90Ah holds 1080wh, half of which is usable (50% DoD for a short battery lifetime. 2 years of camping every other weekend is still 100 cycles and it should last that long)

If you have a 12V DC input and a car cord for the CPAP, you are all set, but if you have to use the AC adapter (85% efficient), and a decent pure sine inverter ( another 85% efficient gizmo) you will end up burning another 30% (125 wh) of power which will take a 90ah battery right to the 50% mark @ 541 wh.

For recharge, the first 80% of recharge can go pretty fast, but then as the battery fills up, it goes much slower, so you could go form 50-80% in 1 hour, but the 80-95% would take 2hours, or more, depending on the Absorb time profile. So say you have a magic panel that is actually putting out 200W of power @ 15V (needed to recharge a battery) you will need 2.5 hours to recharge a perfect battery. But with the absorb time being longer than the BULK time, 4 hours looks more realistic.

to get 200W, you will need about 250W of well aimed panels.

So all YOUR variables (power draw, battery size, PV array size, inverter or none) will have to factor in this for YOU.

And have a regular 3 stage charger to use on cloudy weekends, and the day before & after to top off the battery so it will last the longest.

AGM batteries cost a lot more, but are no-spill and have a better recharge efficiency, which could affect your time/power requirements in a positive way.

And I hope I made no blunders or stoopid math mistakes in my frantic typeing.

[thebrez1]

CPAP #1 is 110-240 volts but operates on 12v and does have a dc cord/plug. I could run it off an inverter but I assumed it would be more efficient straight DC because of the inverter loss. I plan on running machine 2 off the inverter which is a modified sine 300 watt unit I have used before with no problem. I have not planned on using the heaters.

So my battery has 12 volts X 125 amps or 1500 watt hours? Is this correct? If so, that gives me 750 (max) usable watts and with both machines using 2296 watts every 8 hours I can only run both machines for a couple of hours? I really am missing a step or two here aren't I?

[Sunking]

So my battery has 12 volts X 125 amps or 1500 watt hours? Is this correct?

Almost correct except one nick pick it is 12 volts x 125 AMP HOURS = 1500 wh. 12 volts x 125 amps = 1500 watts. Watts like amps is a measurement at a specific moment, there is no time.

[thebrez1]

Ok so I am getting hung up or confused with hours. So can I always just use watts and amps and forget about hours? Am I also correct that I would only be able to run both machines at the same time for 2 or 3 hours? That just seem right. Is my machine (12 volt) really that different from a 110 machine? What a power hog! 96 watts vs, 2200 watts?!

[Sunking]

we are talikng WATTS so I have a total of 2296 WATT HOURS NOT AMPS!! Please tell me this is correct! (and how do I get from watt hrs to amps?

OK machine # 1 is doable on solar, machine 2 is going to break the bank. I will run a design example for you using # 2 with a daily usage of 3000 watt hours used in Kansas City

For the battery multiply the daily usage by 5 so you have 5 day reserve capacity and discharge 20% per day. 5 x 3000 wh = 15,000 Watt Hours. To find the battery capacity in Amp Hours just divide by battery voltage. So let's use 12 volts, 15,000 / 12 volts = 1250 Amp Hours. That battery cost $2100, and weighs 900 pounds

For the solar panel we make a adjustment to compensate for overall efficiency. Using an efficient MPPT charge controller the adjustment factor is 1.5. So take the daily watt hour usage and multiply by 1.5, so 3000 wh x 1.5 = 4500 wh. Ok to find the solar panel wattage we factor out the hours so all we have left is watts. This is where you need your solar insolation Sun Hour number. In KCY December is 2.9 hours. So 4500 wh / 2.9 hours = 1551 watt solar panel. Round that up to 1600 watts. That is about $4000 worth.

I will stop there to let you get over sticker shock, because there are still some expensive things needed like a huge expensive charge controller.

[thebrez1]

OMG! You have got to be kidding! For her machine I need EIGHT 200 W panels plus a forklift battery pack, plus inverter, charge controler ect!!!!! How the heck is solar even considered!

So I was right that I could not run both machines for more than a couple of hours on a 125 amp battery,and yet I could run mine for almost a week (to 50%)? Even that is not long for such a huge battery! How then, could I run for almost two full nights off a small sealed battery in a portable jump start power pack? This is not making any sense to me. My new battery weighs close to 40 lbs! The little one was not even 5! Everything is getting dark.............

Guess the wife better find another machine or learn how to go a long time without sleep! Geeeeze, I can run mine for a week to 50% DOD,but can't make it one night (to 100% DOD) with hers going too? Crazy, glad I have the generator. Sounds like I have no choice but to use it. Bummer

[Sunking]

Your #1 machine is real doable, as it only uses 100 wh per night based on what you told me. For that all you need is 50 AH battery and 75 watt solar panel with a small charge controller.

Not sure I understand why the #2 machine uses so much power. Does it have a heater/ humidifier or something?

[thebrez1]

It does have a humidifier. I will look at the manual and see if that includes the humidifier draw. But , agian, if I have this correct, it is 110volts, so even if it only pulls 1 amp that is still nearly 900 watts per night correct? Is 110 just not doable on solar? I don't see how something so small could pull so much current to spin a fan. How the heck can you power fridges and other things?

[thebrez1]

Also, i wanted to thank everyone who has helped me understand this. This is a challenge to me and I actually think this is fun. Your paticience is greatly appreciated since I know you tend to get the same questions over and over again. I have someone from the power company coming out to the house to talk with me about rebates, credits, ect. I would relly like to find a way to work at least some solar into my life but I have allready learned that will not be because of saving money!

I need to learn a lot more about charging times ect. but thak you all!

[Sunking]

It does have a humidifier. I will look at the manual and see if that includes the humidifier draw. But , agian, if I have this correct, it is 110volts, so even if it only pulls 1 amp that is still nearly 900 watts per night correct? Is 110 just not doable on solar?

OK you are giving some conflicting information. earlier you stated:
[QUOTE]Machine 2, 110 vots 2.5 amps max = 275 watts X 8 = 2296 amps??{/QUOTE]That is a whole lot of different that 110 volts at 1 amp.

Power or Watts = volts x amps, and Energy or Watt Hours = Watts x Hours.
What I am driving at is you have to know exactly what you need to generate. Take the two extremes.

1. If you design and build to use 3000 watt hours per day, but only use 1000 watt hours you tripled your cost and wasted $6000.

2. If you designed and built for 1000 watt hours, but use 3000 watt hours you go dark and destroy your batteries

But for battery systems the cost depend on your area due to Solar Insolation, and cost run $3000 to $5000 per 1000 watt hours or 1 Kwh. FWIW if buying power from the utility average cost is 12 cents per 1 Kwh. Average solar battery cost are $1.60 per Kwh. You do not have to be a rocket scientist to understand the cost. Only a politician can spin those numbers to say solar battery is good for your wallet.