"In case where the battery has less voltage
than the range of the A/D input wire ... "

With my 8051 (C515 from Infineon) you can program the range to measure, so I don't think that I need the op-amp.


"So use quite big resistors on the op-amp
to that you minimize the load."

What do mean by "quite big"? Is 100K enough?


"You also need to look very carefully at any
circuit paths from the battery to inputs of
chips that have input protection diodes
(most chips) because when the chip is un-
powered these inputs look like a load from
the battery circuit to virtually ground
through the input protection diode and out
the Vcc pin of the chip. This could be your
opamp or the 8051 A/D input."

Thanks, I didn't know that.