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Getting the Most Out of Your Solar Panels

The following are questions an answers from Richard Smith OLPC Solar specialist, and questions from participants in April 4th Sunday Forum.
Sunday April 4th Presentation
RE: Solar Input and managing charge levels for XO-1 and XO-1.5 Discussion with guests Richard Smith and Robert Pilawa. When using solar panels to charge XOs, the output of the panel varies according to the amount and intensity of the sun. This means that sometimes the voltage spikes up and even exceeds the limit of the XO controller, and can trigger a shutdown of the charge.
The guest specialists discussed the design of the XO input circuit, and a new device under development by Robert Pilawa that can control the input voltage cheaply.
Solar Presentation Agenda from Apr 4:
Robert Pilawa's Slides:
Pilawa-Podgurski, MIT Mathematics department
Robert Pilawa & Brooke Pilawa-Podgurski, MIT grad students, Cambridge, Massachusetts/MA, USA]' to 'Low cost solar charger for the developing world: low cost solar charger with built-in power tracker, adheres to USB charging standards, can charge most cell phones, portable lights, lead-acid batteries. . It can produce up to 35Watts in full sunlight.

Problem: Hot sun issues in Kenya etc creating unexpected conditions. Everything charges fine and then things mysteriously stop.
Richard Smith Richard went back to manufacturer (GoldPeak) to clamp voltage at ~18V to prevent this problem. Using a small circuit added to the solar panel.Samples of this solar panel were built.Testers all said it worked great. But then Richard himself tested it and discovered another small problem with even this new solar panel.This should now be corrected (this $2 added circuit helps) and is now going into production.
XO Power Design
Charging chip is the entire power source of the laptop. 1 input chip creates power rails for all devices on motherboard. Separate power supplies for each of those devices.

RS: XO 1.5 Changes to Support Solar Charging
XO-1.5 overvoltage at 25V instead of 18V for XO-1.XO-1.5 can limit the power it draws.
If you want to set the XO-1.5 to use no more than 10W, you can use battery as buffer.

Allows you to use panel at more optimum range.(Power draw of XO above 10W!) XO-1 sucks up to 17W; XO1.5 sucks up to 20-25W)

Control duty cycle of charging chip "turning it off for x-amount of time"
  • EG. Turn it off 50% of time.. you get 12.5W
  • EG. Turn it off 0% of time.. you get 25W
  • How does the user in the field accomplish the change?
  • Power rail 7.5 - 8V

Question? what is oscillation and is there a fix? Answer: Richard Smith: you'll hear a "screaming" noise Output voltage of panel drops / rises / drops.

Q? There has been talk about oscillation issues at low input voltage/power. have these been fixed already? A: 1-2 kHz range is in audible range.Inductors / capacitors.On XO-1.5 it's MUCH louder. Running change Richard will be ordering to production. Richard has attempted to make on power-limit implementation in field automatic. Is this change firmware-only or an ECO? Both.. (must be XO-1.5)

Q? Is oscillationfix HW? A: Richard: yes Easy modif.. resistor changes.Tiny resistors, beyond normal soldering.Microscope required..All that's coming down the pike..
Other ways of Charging
  • standalone charger.
  • Problem: MBC (multi-battery charger) too expensive.
  • MOQ (Min Order Quantity) too high, perhaps 1000 units.
  • New concept: single battery charger?
'Q:?' How about a kit for do-it-yourselfers at a much smaller cost? 'A:' "Leave a Little Room" in Ethiopia / Tanzania requesting 200.NS How about always ordering 2 batteries per laptop? Richard: yes!Mulitple Battery Chargers was about removing wires from the classroom..But now a different driving requirement?Workflows already set up so MBC hasn't caught on so far.Several deployments build their own charging racks..

Locked down sometimes for security.But we try to encourage kids to take XOs home. So 2 batteries would make sense. Nancie: Roger in Thailand discussed battery failures "due to improper shutdown" ?

Purchasing More Batteries:
Note: A Battery size changed in XO-1 version, but all batteries work? $25 batteries available at [1]. $28 panels avail now for iLoveMyXO to buy (100 MOQ)
Robert Pilawa’s Low-cost solar controller and why!
1. charge controller basics
2. max power point tracking
3. cost
4. prototype
Note: Batteries commonly damaged in the developing world, because people buy the cheapest etc. Overvoltage protection critical.Similarly at night, if you charge the battery dry (too low voltage) you will trash your battery, sometimes if you do this even twice.
Q? MPPT in charge controller?? A: Is it only for XO-1 because xo-1.5 has it built-in? or has doing it in the charge controller some advantage for xo-1.5 as well?
'A:' 12V panel has an open circuit voltage of 23-27V? but MPPT of 17V? Think of it as gear box, of car of bike. If you don't have gearbox, you can't get up to 55MPH.So you will be running in 1st gear, and wasting your power. You need DC-DC power converter.. staying at Maximum Power Point (MPP).MPP depends on the temperature and sun. You end up wasting up to 30% of avail power of panel. Solar panel manufacturers keep pressing price down and down.Same with lead-acid batteries.You can take advantage in the developing world.Price pressure coming from household uses.Developing countries can take advantage.But now fancy electronics in the middle hasn't yet had these economies of scale yet.Needs to happen.
  • Targetting $0.25 / Watt?
  • Low-cost Maximum Power Point Tracker.
  • Target: 100W tracker $25
  • In the US a household version today can be $20.
  • Low cost power electronics help..
  • High-volume parts..
  • New Target Application: XO / off-grid applications..
  • Innovative encapsulation techniques (for Robustness)To recharge lead-acid (car) and cellphone batteries.Put it in between your solar panel & XO.Get the real 10W instead of 7W.As Richard was alluding to.Do this on the XO itself would be nice, sure, but we need external circuity in real world here.Many XO installation?Let's say you have 10 of these old Peak Panels.Daytime charge a lead-acid car/marine battery.Then at night charge the XO's / batteries from the lead acid battery.
Q? Cost consideration?
'A: SJ: typically you can capture maybe 30% or more of avail power not used if you have a low power panel... with less than a 50W panel it wouldn't make economic sense, if you can get a 20% increase in power but could just buy another panel for $4/W. (you'd probably pay $6/W for a panel for consumer work) 20% extra power should be used. This makes sense if the additional cost of the fancy controller is less than $0.80/W, otherwise you'd just buy more panels.
'A:' Robert Pilawa : Current Technology and Costs: Todays' designs areabout $1/W commercially. We are targeting something like $0.25/W. this should be available for testing early summer. We'd love to get people with solar installations to have general field testing on what is important feedback.
Q?Why does cost scale with power? 'A:' Robert Pilawa: panels' costs scale with power.Imagine 20% surplus power coming from 10W converter (eg. 12W). From an installation perspective you should look at it as $'s per Watt. Today's designs cost $1 / Watt
'Q?'SJ :should folks use small panels or large panels.Blankets of panels? EG. 500 square feet of panels? 'Q? ' What if big client arrives? "A:'SJ: rule of thumb too simple 10W panel or 1000W panels very different. Metric we should use $/Watt..
'A:' Robert: that's a market analysis, yes
'A:' Nancie: on Vietnam, she had written primer on designing solar/battery setup.

What if teachers may grab power for fan & other uses
'A:' Robert: extra fans!Richard: there's never extra power!
'A:'Nancie: adults' cellphones will be charged. there will be issues. cellphones are 1/15th the power perhaps, on the bright side.(1/15th the power of XOs?)
'A:'Nancie: power diagrams on the above page [[2]]
Q? what are most people using in the field?
'Answers:'Richard: 10W olpc recmomended panels. Goldpeak will make... other wattages. We have a 10W b/c its a nice number, they will make panels for you in whatever size. The Solomons will use 15W panels because that's a primary vehicle for them... and the extra 5W makes a diff.
Why and how Regulators or Controllers?
'A:' lot of people try to use these other panels, but: the goldpeak panels is suited... the low-peak voltage is lower than most other panels and most others out there trip the voltage limit quickly(which keeps them from charging an XO) Others with a more advanced voltage regulators are (comparatively) expensive.
'A:' If you have a battery in the system, lead acid or otherwise, you absolutely need a controller. You can think of the XO as being a battery in the system; its controller is on-board inside the XO.
Q? How to manage or view the solar input levels?
'A:'Is the application at the panel, or b/t the building and the charge controller? Where is the controller / device? 'A:' the limiter is a stopgap; this would make it(charge controller) you'd save a small amount with the limiter Richard: this is much better than limiter (charge controller) which will no longer be neccesary.
Q: What is the Cost? A: (maybe $2)
I should say that I hope to be able to get robert's design implemented as an option you can purchase with the panels.then you could order an olpc solar panel and have the circuit built in.
Q: How long does it take to charge an xo from a Solar source?
'A:' 30W hours with a limiter or what have you?
'A:'Empty to full, with a 10W panel, that's 3 hours. That is max uninterrupted output in reality it's somewhat longer, seldom do you get peak output unless you are in a sunny tropical area.the 10W panel also. It's not that 10W is a limit.
Q: How do you evaluate the required input for charging?
'A:' If you're in Kenya, you can get strong sun and more than 10W output. That's why you use the W-hr number to estimate time to charge.
Q:You're saying robert's device will help that?
'A:'no, if the device is doing max power output it won't

help.'A:' robert: right now you'd pull voltage out at 11 or 12V, right? 'A:' richard: yes, but then the XO gets real hot and it averages out...

Q:How can people visualize how efficient their panels are being?
'A:' Suggestion: the charge-controller could have a display.richard: you could put something on the working on learning activities to do that. Comment: that would be specially good on the 1.5 where you can get realtime data from the ec.
'A:'Richard: you could get a good running average on the 1.0 (in real time)

Q?: in consumer's tech what are people getting for 'that extra $5' if that's what this controller would cost?

'A:'richard: superior, stable performance in low-light conditions; and would be able to charge devices they wouldn't noramlly be able to (at different voltages:cell phones, lead-acid batteries, etc; (there are additional smarts in there)

Q?: Suggestion Learning Activity NS: As you think about an activity to monitor power coming in, it would be useful to monitor / log each week over the course of the year the gradient, so they know what to expect the following year. SJ suggestion - and also let them record the angle at which they support the panels for optimal charging!

'A:' Activity could include info on the course of the sun around the earth at different lat/longs.(related to the optimal angle, over the course of the day, for each day of the year) and then you can include data on the physics of solar panels what their output is related to solar illumination. How power output changes with a sunnier day, a cloudy day with mirrors reflecting sun onto the panel from many places (embedding each panel in a parabolic mirror?)

Q? What about controlling from a interface

'A:' on the xo-1 it is v easy to understand such things; log data is already supported in the file system. OLPC PowerLog records this information. On the 1.5 and new f11 for XO builds
Discussion on Solar intensity and Solar Panel Optimal use
COMMENT: I have heard from people in Peru that elevation (like in the

Andes) interferes with solar panels. Doesn't seem logical. Is it true?

Q? Some people have said that elevation make solar panels work more poorly
'A:that didn't make sense to me: ' no, it works better at elevation earlier.'A:' if you keep adding solar power, you can get more and more power out of a panel until it burns out.
Q?what data sheets can we get about how the panels work? their max/min power output at different input illuminance, standard deviations,etc.
'A:' They are all on the wiki and Serendipity at

Free Charge Controller project: open-source maximum power point tracking (MPPT) for solar panel, wind turbine, etc.
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