Look at the power converter cable for your laptop. The boxy part should say what DC voltage it puts out. You'll want a voltage regulator rated at that voltage. Get a solar panel that gets really close to that voltage and a lot fo current, and connect it to the voltage regulator, and the regulator to the laptop. As long as your laptop has that correct voltage going into it, it will use its internal charging circuits to properly charge its batteries.
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Figure out the charging voltage needed. Figure out the amps needed or fraction thereof.<br /><br />Find a solar cell that will provide that amount of power.<br /><br />Design a regulating system that will protect the laptop. Make sure that the polarity of the charger matches the needs of the battery. Make sure that you've made arrangements for any sensing wires etc.<br /><br />Put it out in the sun.<br /><br />If your battery is a lipo watch this <a href="http://www.youtube.com/watch?v=QJCZ4ayioCU&feature=PlayList&p=2C047A93F664A2FF&playnext=1&playnext_from=PL&index=11">link</a> and forget all about building your own charger. <br />