In case where the battery has less voltage
than the range of the A/D input wire the
battery through a simple op-amp circuit
that multiplies the battery voltage by 3
and then connect the output of the opamp
to the A/D input. Please be aware that
any monitoring circuit that you put on the
battery will be an additional drain on the
battery. So use quite big resistors on the
op-amp to that you minimize the load.

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.

Hope this helps
Mike Karas