[Moved] scale down voltage for PIC A/D input

Hi,

I have a design problem: I have a PIC microcontroller (16F690) and I want to measure the voltages of 3 Battery Cells in series (each one having 3.6V). My PIC can only measure 0-5V so the first cell I could measure directly, but already the second one would be out of range (my PIC only measures in respect to one GND). I thought using a Differential amplifier (I just read about it, but know just very little about OpAmps), but I saw that if the Cells are connected to the GND through resistors, they will loose charge in time!

Does anyone know how to measure all these Voltages with a circuit without discharging the Cells (in long term)??? I just want to measure the difference of each + and - (so always 3.6...not 3.6 and 7.2...)
Could anyone provide a circuit to acchieve this?

Thank you a lot!!!!
Greets,
Jorge

Use 3 A/D channels.
The first channel across the first cell to measure 3.6 Volt.
The second channel to the second cell to measure the 7.2 volt level, use a resistor divider to scale it for the 0-5 volt range.
The third channel to the third cell to measure the 10.8 volt level, use a resistor divider to scale it for the 0-5 volt range.

You can then calculate the voltages across each cell.

You can use a solid state switch to connect the channels.