Also insure your subnets are different on the ethernet interface and the wireless interface. If they are the same, that causes routing problems in the laptop.

It appears it is not getting this include. You may want to add this include to your library. I would try putting it in SIM900.h if that file exists.

Code:

#include <HardwareSerial.h>

The compiler treats libraries and sketches a little different.

Do all the other lines in that file that display that message refer to "Serial."?

It may help if you would post the entire error displayed with that message.

OK. Then we will start with the stuff that causes interference like that with the w5100 chip. The first is the microSD SPI interface.

Code:

void setup(){

  // disable SD SPI interface
  pinMode(4,OUTPUT);
  digitalWrite(4,HIGH);

  Ethernet.begin(mac, ip);
  //Ethernet.begin(mac, ip, subnet, gateway);
  Serial.begin(9600);
  Serial.println("Better client test 12/01/11"); // so I can keep track of what is loaded
  Serial.println("Send an e in serial monitor to test"); // what to do to test
}

Try that first, even tho I think this is going to be the second thing I recommend.
How does that do? Still garbage out the serial monitor?

Add: May as well make it one response. Here is another topic with the second suggestion. Read reply#6.
http://arduino.cc/forum/index.php/topic,86729.0.html

Hi zoomkat, unfortunately your code doesn't work in my installation

"doesn't work" is still vague.

Doesn't compile?
Doesn't run?
Where does zoomkat's code stop in "your installation"?
What is the last message?

What I normally recommend is using internet connection sharing on the laptop/tablet. With Windoze, the ethernet port is assigned 192.168.0.1/24 by default. All computers/devices on the ethernet port must be in the 192.168.0.x range.

The laptop/tablet will perform a NAT on that localnet to the ip the wireless router assigned to your tablet.

The only challenge is if your WISP uses 192.168.0.0/24 on the wireless network. Then you will have a routing challenge. Most WISPs do not use that subnet (or 192.168.1.0/24) on the wireless localnet for that reason.

Google search "Windows internet connection sharing" for details/instructions.

When you want to use the microSD, then enable it. Then it won't interfere with the w5100.

Code:

void setup()
{
  // start microSD
  SD.begin(4);

  // start the server
  Ethernet.begin(mac, ip);
  // disable w5100 SPI
  digitalWrite(10,HIGH);
  server.begin();
}

Now you can use both w5100 and microSD.

I had problems like that. Mine was due to the microSD SPI interface. It would run fine for a while, then garbage.
You might want to try this change.

Code:

void setup()
{
  // disable microSD SPI
  pinMode(4,OUTPUT);
  digitalWrite(4,HIGH);

  // start the server
  Ethernet.begin(mac, ip);
  server.begin();
}

If serverName contains the url, then this should work.

Code:

    client.println("GET /search?q=arduino HTTP/1.0");
    // send host url in header for virtual hosting servers
    client.print("Host: ");
    client.println(serverName);
    // send blank line (end of header)
    client.println();

May I suggest a lower current, switching-type charger. You might take a look at that other charger posted above by bHogan.

I just gave a 6V wall wart a try. The input voltage was steady at 6.24V with or without a load on the battery and the charge rate was about 80Mah with and without a load on the battery. So by all accounts the charger worked properly with a wall wart.

Then in the now almost famous words of Tim Taylor, "MORE POWER!"

Solution: Bigger solar panel.

I am not using a regulator

The battery charger is a regulator of a sort. I should have use battery charger instead.

In that case it may be some type of low voltage or overheat problem. The datasheet does mention on overheat, it will taper off the charge rate current. On low voltage (3.5v) it will shut off until the input goes above 3.9v.

How does that charger do if you power it with a 5v-6v wall wart?

Edit: My apology. I was interrupted by work.

If the voltage from the solar panel drops below its V_mp (voltage at maximum power), then you are probably using more power than the solar panel is capable of producing. But I can't see the "big picture" from here.

Sounds like the regulator is not functioning as you expect. The info you gave me helps.

The solar panels are constant current. If the panel produces 17ma at 5v, then it should produce about 18ma short circuit, and about that current at every voltage in between.

But as you can calculate, the power it produces depends on the output voltage of the panel. At 5v output, that would be 85mw. At 1 volt output, that would be 17mw.

Since the panel output voltage is increasing when you add a load, I would think the challenge is in the battery charger, not the panel.

@bHogan: Good article on MPPT.

"does not work" is very vague. I recommend trying the example in the IDE Examples folder. Under Ethernet, there is a web client. Probably more help available for that code.