I want to charge two series connected lithium phosphate rechargeable cells (IFR18650-Powercell) nominal voltage 6.4 from a silicon panel (about 100ma to 500ma in practical use, about 9V open circuit) and produce 5V to charge a small computer (HP Ipaq). I do not want the cell voltage to drop belpw 5V for the pair so that they do not get damaged. Many voltage regulators drop out at a voltage higher than 5 (eg LM309 drops out at about 7V) and switch mode devices seem to have a high quiescent current (eg DE-SW050 uses about 15ma). Is my task physically possible?

Thanks.

 

You should use a charge control IC designed for Lithium Cells. Otherwise the risk of destroying them/burning/exploding batteries goes up greatly. The IC, along with support components listed in the data sheet will regulate the voltage and current for the proper charging of the batteries.

 

Blocking diodes will serve you. You'll need one for each regulator.