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[mahlness]

I'm a teacher in a third grade classroom in Seattle, and our goal is to run 2 of our 4 XO's through the year using only solar to power them. Figured it would be a timely learning lesson for me and the kids - but a solar/battery issue is driving me crazy.

The two solar panels (14v/10w) that we've been using work great - but.... When I plug in the panel to the XO to do more charging, I have to reseat the battery briefly. If I don't, the battery will not charge (no orange charging light showing) - and at times the xo will not even start up, unless I reseat the battery.

Am wondering what's going on - like why should I have to reseat the battery all the time? I'm wondering if there is some polarity issue at work here. Is the panel messing up something in the battery?

If anybody else is having this problem - or has ideas to share about what the problem might be, or how to fix it, I'd very much appreciate hearing. Many thanks - Mark

http://ahlness.com
http://roomtwelve.com

[Dinky]

I am curious.

Does the problem occur if the solar panel is covered when first plugged into XO (no power) ?

[mahlness]

Yes, it occurs if they are plugged in and there is no power coming from the panels (dark, cloudy mornings)

[wolf9545]

What happens when you use the normal AC adapter?  Do you still have to reseat the battery?  Also, does the machine start up fine if the battery is charged and you are running it off the battery (no external power source)?

[Dinky]

For starters, the schematic is at : http://wiki.laptop.org/go/Hardware_Power_Domains

Since no one has responded, here is my guess (without documentation of the actual charger chip) and something to try.  This is from an instrument repair person.

Conjecture:

The solar panel under no load will drift to a much higher voltage.  When it is plugged into the XO, a high voltage transient results that causes the protection components of the charger chip to shut itself down.   But, the solar panel is now under a small load and hence has a voltage near to its normal operating voltage.  Removing and replacing the battery resets the charger chip.

The first thing I would try is a resistor (shunting) on the output of the solar panel, say 330 or 470 ohms (see documentation for solar panel). This could be remove after it is plugged into the XO  (a simple switch with the resistor inline between the positive and negative leads).  So, there would be a load to load transition rather than no load to load.

Hope you get an answer from someone who "just knows" the solution from experience.

Edit:  Resistors 2 watt or greater.

[mahlness]

Thanks very much! Your conjecture sounds like a good guess. I wonder if there were some way I could replicate resetting the charger chip without reseating the battery - or installing an inline switch... hmmm - way over my head for sure now 

thanks for the link! - Mark

[Dinky]

Does the solar panel supplier have a suggestion?  They should be getting some feedback by now about this new application.

If the connector is not molded onto the cable, it is an easy access point to temporarily clip in the resistor as a test (cover may unscrew).   If that works, a resistor can sometimes can be hidden in the connector.  This would require a small resistor (1/4 watt) which would, in turn, require a larger resistance (about 5000 ohm), which might still work and could be left in circuit (again, consult solar panel documentation).   Access may also be possible at the solar panel end of the cable.   

A final choice is a small adapter box with an in and out connector, if  neither end of the cable is accessible.

[Richard Smith]

For starters, the schematic is at : http://wiki.laptop.org/go/Hardware_Power_Domains

Since no one has responded, here is my guess (without documentation of the actual charger chip) and something to try.  This is from an instrument repair person.

Conjecture:

The solar panel under no load will drift to a much higher voltage.  When it is plugged into the XO, a high voltage transient results that causes the protection components of the charger chip to shut itself down.   But, the solar panel is now under a small load and hence has a voltage near to its normal operating voltage.  Removing and replacing the battery resets the charger chip.
Edit:  Resistors 2 watt or greater.

Mark,

Someone sent a link to your post my way...  I'm a hardware guy at OLPC.

Indeed.  You are most likely hitting the over voltage cutoff.  The XO protection circuits will kick in sometime after 18V.  Since you list a 14V/10W panel the open circuit voltage (ie when you plug it in) on a good sunny day will probably be up in the 20V range.  Other people using various solar panels have reported similar experiences.

I'm not exactly sure what sequence of events is happening with you having to pull the battery but removing the battery will change the load on the PV so the  voltage would change.  Someone would need to put a volt meter on your panel and watch whats going on.

This same problem exists in solar setups that have a battery attached.  The output of the PV array can be higher than the batteries are happy with.  So most solar setups need a charge controller which is a DC/DC converter that will match your PV cell voltage to a specific output voltage.  The also handle charging the batteries to make sure they don't get overcharged. Which is bad for lead acid cells.  Most of the charge controllers have a separate output for directly powering DC devices.  You could use something like this.  I've seen charge contollers for $30 up to several thosand depending on the load needed.

The next step up would be to use a MPPT or Maximum Power Point Tracker.  Its much fancier solutions (and more $$).  In addtion to matching the output voltage of your PV array to the voltage of your load a MPPT allows your solar array to operate in its maximum power range even when you are not under full sunlight.  Various vendor claims say 30% - 40% more usable power from your PV when you are not in full sunlight.  I see small MPPTs for about $100.  Dedicated googleing will probably turn up some for cheaper.  Go hit up your local RV shop and see if they will donate some for the cause.  RV shops often have small PV kits.

There are also many designs for MPPT's on the web and they are not that complex but not trivial.  If you know someone who likes electronics building one should not be too hard.   Although projects like this seldom end up costing less than a commercial one unless they are really simple.

If you want to try something thats on-the-cheap then the load resistor idea suggested previously will work but you will have to futz with connecting the load and then disconnecting after the charging starts.   Depending on how much load it takes to pull the voltage down to a level the XO can deal with you could loose a watt or 2.  You don't want the resistor sucking up your power if you can help it.

I have a suggestion for a slightly modified version of the resistor idea involving a zener diode.  Rather than a shunt resistor if you were to us an 18V zener then its only going to conduct when the voltage is above 18V.  Depending on the load curve of the PV panel and how beefy your zener is you might get a way with just a zener.  If not then a small resistor can be added to limit the current.  The zener won't start conducting until the voltage gets above 18V so in your normal operating range the loss will be tiny.  You should be able to leave this connected all the time.

Hope this helps.
Richard.

[Dinky]

Other people using various solar panels have reported similar experiences.

What did they find as the best (simple) solution  for the XO ?

The zener is a better solution -- did not want to get involved in selecting zener until the overvoltage was found to be the problem (KISS principle).

Before any modifications, I would also want to try just covering the solar panel (discharge with resistor perhaps) before plugging it in to XO. (Per earlier reply).

Great to get an answer straight from the "horse's mouth" !

[mahlness]

Some info on the solar panel in question...

Manufacturer website: http://pvmaster.com/en/default.asp

Panel specs listed there: http://pvmaster.com/en/specification.asp?value=10

Here's a picture of what's on the back of one of those panels:

[mahlness]

Just started another question in a different direction - are my XO's actually LOSING power at times when hooked up to solar panels?  http://olpcnews.com/forum/index.php?topic=3957.msg26847#msg26847

[AuntiMame]

mahlness,
I just put the question to GPSolar and will report back ASAP.

Aunti

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