Collie

Hi,

I am curious to learn about the technical side of solar power.

are there any electronics masters out there that could help answer my quesions?

I know the following:
V = I x R
P = I x V

where V is Voltage in Volts, I is current in Amps, R is Resistance in Ohms, and P is Power in Watts.

So using my notebook (the most power hungry device that I have) as an example of a device that I need charged, here are the power requirements (based on the output of the wall transformer/adapter/charger):

19 volts DC; 3.42 Amps; 65 Watts

So this all makes sense using P = I x R, 64.98W = 19V x 3.42A, and 19V = 3.42A x 5.556 Ohms.

So if I were to use a solar charger like this Silva Solar II 12 V charger, by my calculations (CORRECT ME IF I AM WRONG!) and using the following solar panels power specifications:

Voltage: 13.6 V
Power: 4.75 W

using P = I x V, and therefore I = P / V, 4.75 / 13.6 = 0.35A

Current: 0.35A

So my assumptions are: based on good sunlight, one can output 0.35A;13.6V;4.75W per HOUR.

So if my notebook power requirements are 19 volts DC; 3.42 Amps; 65 Watts.
(Yet the battery says it has a rating of 11.1V;4A - giving a power rating of 11.1 x 4 = 44 Watts per HOUR)

then I assume that if I connected the solar panel to the notebook battery, that it would take
65/4.75 = maximum of 14 hours of good sunlight (65W is based on the wall charger rating)

44/4.75 = maximum of 10 hours of good sunlight (44W is based on the battery rating)

So in conclusion, my estimate is that it will take anywhere from 10 to 15 hours of direct sunlight to charge my notebook using the silva solar II 12V solar charging device. am I correct???

I wonder would this system work in a trickle charger manner??? i.e. slowly top up the battery over a period of time???

I am very interested to learn more on this subject so any/all replies would be great!

Collie

mtnroads

You've basically got the formulas correct, but there are two areas of concern. The first is whether you can actually trickle charge that battery - is it a Li-ion or NiMh? I think 12v will not take the charge to full. Remember on a trickle charger the voltage is high, they just dribble off the amps as it reaches full charge. In your case it is a 19v battery so you need more voltage to "push" the electrons in there. And there is no way to get that without going through the 110v wall charger transformer which has all kinds of losses. Some laptops have 12v adapters I believe, but again they are not very efficient. Of course you could look for a 24v panel (larger sizes) or hook two of those little guys in series - that would do it I suppose, but again I'm not sure of the battery constraints.

The other thing is solar panels only deliver full current when the angle to the sun is within a few degrees of directly perpendicular, and it drops off fairly fast when it isn't. So getting that 15 hrs of "good" sunlight will take at least 2 days mid summer, and you will end up fussing with it constantly, especially if you are on the move. Forget it in winter. There are some flexible solar panels made by Uni-Solar or someone like that that are sold at RV and boating supply houses - you might look at one of those - far better output although a bit larger.

gcl8a

Aside from whether or not this panel would work (I doubt it, since the MAX voltage, in perfect conditions, is lower than the adaptor's), these statements and calculations are meaningless.

Power (Watts) is the rate of energy transfer. Saying 'Watts per hour' is akin to saying I rode my bike at '20mph per hour'.

Collie

THE BATTERY IS LITHIUM-ION


ACTUALLY THE WALL CHARGER TRANSFORMER IS 19VOLTS BUT THE BATTERY IS RATED AT 11.1VOLTS; 4AMPS

Collie

hi,

why do you say that these statements and calculations are meaningless??? (thank you for the correction on the watts per hour bit!). It turns out that the voltage is not lower than the battery.

i won't be using the adaptor, but i would wish to charge the notebook battery directly from the solar panel, therefore replacing the job of the wall adaptor.

i have been looking at sierrasolar.com and they have The Notepower 15 Watt laptop charger.

so assuming i connect a 12 Volt cigarette lighter adaptor between the notebook and the solar panel the system should work.
my notebook battery has a rating of 11.1volts and 4amps => 11.1 x 4 = 44.4 watts (is this meaningless???)

Then using the Notepower 15 Watt solar panel i guess that it will take (44.4/15 = 2.96) a minimum of 3 hours to charge the notebook battery.(again is this meaningless???)

cdstg

Hi you would not be able to run the laptop but you could charge the battery
eg if you used a car adapter you could do that no problem

The problem that you will run into is the time its takes as you will never at that 15W output form the solar panel unless all conditions are meet angle to the sun temperature time of year etc

but it will charge it but would more likely take at least 6 hours or more

gcl8a

Yes, it's meaningless. Not trying to be harsh, but my point in the earlier post is that your units don't match up. They still don't:

44.4W/15W= 2,96 of something unitless. If you knew it took 3 hours to charge your battery with your charger, then you could maybe say it would take 3 hours * 44.4W/15W = ~9 hours (note the units), but not knowing how chargers work (I doubt they spit out constant wattage), even this is suspect.

Ignore that. After reading mtnroads' response, I see what you're trying to do:

You're saying your notebook uses 44W. That's power. So, if you used it for one hour, then that would use 44Watt-hours (energy). If all of your solar power went to charging the battery, then it would take:

44Watt-hours/15Watts = 3 hours of charging for each hour of laptop use to charge it. Assuming ideal conditions, and that the voltage of the solar panel is enough. This is why the units matter. If you used it two hours, thats 88Watt-hours, and six hours of charging.

Maybe you understand this, but it wasn't clear from your post. Does this make sense?

As to the Notepower 15, do they give some specs? All I found was a picture and the dimensions. Hardly enough to think about dropping close to $300. Are there any reviews of its effectiveness?

mtnroads

Yes, if your laptop has an efficient 12v adapter that you can plug into the solar module it will recharge the battery. The adapter may have some losses and you may not get the full 1A current, so I would concur with cdstq that 6 hrs should do it, maybe 4 if conditions are optimal, ie. you stop for a siesta, the sun is directly overhead, the solar array is flat and perpendicular to the suns rays. You might be able to find a way to rig it up across the top of your rear panniers and charge while you ride.

Might work out pretty well. Then again, you could carry a 110v charger and charge it at coffee or lunch breaks every other day also, assuming you are near civilization. If you are going into the outback, the solar charger is a splendid idea.

mtnroads

Most laptops draw 20-30 watts, depending on size, screen brightness, tasks running, etc. I plugged my VAIO into a kill-o-watt once and it was drawing only 18w, my Thinkpad with 15" is probably double that. If you use it for an hour that is roughly 30 watt-hours, which is of course what the OP is thinking of when he is doing his math. I agree that using the rated voltage of the charger is innacurate, but it serves as a max limit. There are plenty of ways to recharge that type of load with a small solar panel and maybe even run it. In fact, there is some good info and products for doing just what you want at this site:

https://www.ecomall.com/biz/sunwatt.htm

Have fun!

seeker333

Your calculations accurately predict that it takes an inordinate amount of time at peak solar conditions to charge a laptop.

You're probably gonna lug around your charger anyways, so whats the point of the solar panel? You can just plug into AC in many truckstops, gas statons, restarants and motels.

Then again, whats the point of a laptop. Do you really wanna lug that thing around, and constantly worry about it breaking or theft. Seems like a liability to me.

Do you know about pocketmail? It weighs about the same as your laptop charger (8 oz).

https://www.pocketmail.com/us/

cdstg

Hi there is a differance between running the laptop and charging the battery my compaq lasts about 2 to 3 hours per charge

Looking at the battery it is rated at 4400 mAh = 4.4Ah
so that is the total that is can deliver so for 3 hours running that means it takes 4.4/3
which is 1,46 A to run it for an hour

so that would mean if I could charge it at 1 amp it would take 4 .4 hours to charge eg 4 hours 24 minutes
that would assume 100% efficency which is not going to be the case

so its 12volts at 4.4Ah this makes it 52.8Wh eg 12 * 4.4

so now for the solar panel its 15 watts and for 4 hours of sun ( or whatever you daily sun average is ) x 0.85 this is a good derating rule of thumb for a solar panel then your solar panel can deliver 15 X 0.85 X 4 = 56.1

so the good news is that your battery would be charged well at least my one would be so I would so that its worth it but the weight of the laptop and solar charger may not be so a PDA or something like that may be much better

Collie

great. thankyou, that all makes perfect sense.

I am a programmer by trade and I am planning to tour South Korea and then move on to other Asian countries, so bringing the notebook with me is a must. I plan to tour/work as I go depending on whatever.

I have configured my notebook so that the battery can power the system for anything from 3.5 - 5.5 hours.

I don't need to power the notebook directly, I would just like to charge the battery.

AND FOR ALL THE NAGGING COMMENTS ABOUT WEIGHT/MODERN LIVING/ELECTRONICS ON TOUR/MEANINGLESS CALCULATIONS ETC....
WHEN SOMEONE ASKS A QUESTION ON A FORUM, WOULD IT NOT MAKE SENSE JUST TO EITHER ANSWER THE ACTUAL QUESTION OR IF YOU DON'T KNOW THE ANSWER THEN JUST SAY NOTHING?!!!

THE WHOLE "LEAVE IT(THE NOTEBOOK) AT HOME" IS NOT EXACTLY A SHOCKINGLY INTELLIGENT/GROUNDBREAKING SUGGESTION LADS!!! I'M SURE ALL 99% OF FOLKS PREPARING FOR A TOUR ASK THEMSELVES MANY TIMES IF THEY REALLY NEED THE VARIOUS ITEMS ON TOUR.

I AM SIMPLY INTERESTED IN THE IDEA OF SOLAR POWER (AMONGST OTHER THINGS) AND IT'S POTENTIAL USE/LIABILITY WHILE ON TOUR.

Again, thank you all for helping me uderstand the solution better

Collie

Collie

*** FOR THOSE INTERESTED ***

here is a reply to my query, from SundanceSolar


Hi Collie,
Thanks for the numbers. Unfortunately, laptop charging appears to be a little more complicated than just hooking a panel up to the laptop - even when people use the DC adaptor made for their laptop, the results are unreliable. It is not so much the rate of charging - some laptops just don't seem to charge at all. We are in the process of designing a package specifically for this. The manufacturer of our SunLinq panels now recommends charging a Xantrex PowerPack https://store.sundancesolarcorp.com/xapo.html with a 25W panel, then charging the laptop (day or night) from the Xantrex. You may be able to get away with a 12W panel if you aren't that concerned with how fast it charges. I would not recommend a 5 or 6.5W panel.

FYI, I have only two corrections to your numbers, both having to do with "ideal" vs. "actual" charging - it doesn't really work to go by the listed power of the panel (e.g. 6.5W, 12W) to size systems as these are ideal numbers - we go by the current output at max power, multiplied by voltage to get a "real" power - then you have to take into account about a 15% loss due to charging inefficiencies. Also, I believe you meant that the battery has a rating of 11.1 volts, 4 Amp-hours (not 4 amps). Based on that, the numbers are:
The 6.5 Watt panel has a current of 433mA, multiply by 12V to get 5.20 Watt-hours output per hour of sun on the panel. Now, your battery is 11.1V X 4 amp-hours (I believe) which gives 44.4 Watt-hours required input to fully charge it. 44.4 divided by 5.2 = 8.5 hours. Now add about 15%, for a total charge time of about 10 hours IF IT WORKED.
The 12 watt panel has a current of 800 mA, that would not quite halve the charging time.

Please take a look at the Xantrex units, we will be getting a smaller 150W unit that's not on the website yet. If you aren't in a hurry, you might try buying the panel, see if it works, and if not get the Xantrex for your trip.

Please feel free to get back to me with any further questions you may have.

Steph Baldwin
Sundance Solar Products, Inc.
603-456-2020 v.
603-456-3298 f.
www.SundanceSolar.com

velo2000

Speaking of Sundance Solar, I ordered a folding 6.5 watt panel with AA/AAA battery charger from them recently. (It was on backorder, but should ship any day.)

I had been thinking about bringing a small laptop on my upcoming 3.5 month tour, but I finally decided against it due to concerns about being able to charge it via solar power (and I didn't want the extra weight of a laptop). I'm planning to bring a PDA or smartphone which has wifi access and will use that for keeping a journal and updating my blog when I can. Some of the current crop of smart phones can do almost anything a laptop can. Check out the Nokia N95 or E61i to see what I mean. Good luck!

Lt.Gustl

I remember older laptops with greyscale screens that used C cells I think, it was a glorified word processor that a buddy use a spreadsheet in DOS to keep track of his inventory when out and about.

The PDA concept is a good one, with a folding keyboard you wouldn't lose many of the basic functions needed as well as not having to fight the tiny buttons.

Also there are lightbulb to outlet adaptors that could be made to work in just about any public place.

It's possible one with enough tinkering time could hook a tranformer up to a dynohub for 12V charging, that's not free energy though.

Good luck and let us know if you arrice at anything.

Robert C

I remember a lot of those simple laptops, like the Tandy 100. I think that they would fit my needs while touring also.

I also had a "GRID" laptop at one time (I think I still have it somewhere) it ran on 12 Volts. I was able to, directly, plug it into my work truck by running leads to the battery connectors on the inside of the laptop. At that point I had no power trouble at all. One could do the same (it seems) with any 12V Laptop. However, the solar array would need the power to run the laptop dirrectly at that point.

Certainly contacting the manufacturer, like you did, is a good way to get an answer. It is obvious, from thier ansrer, that they do not want to say that it will work because they do not want someone saying that they said it would. If there is a 12V travel addaptor for your laptop then it is clearly the way to go (this assumes that you are using a 12V pannel with enough current, information that you can get from your current [using "current" to mean something different than the last time I used it] power adaptor; as example, mine says 24V @ 2A).


On another note, I have one of those PDA keyboards, a Belkin G700 for my Toshiba 755. I can not recomend it. I use it because it is small and lite. However, the software for the keyboard is buggy (ie, sometimes you loose control of the cursor arrow keys, to get it back I need to insert a space and then a backspace to delete it, then I have my arrows back). It also isn't a good keyboard. A lot of times it fails to register keystrokes The space bar is a particular problem, it has to be hit right in the center.