Hi..

A very low cost thing that can help you is a LM741,
which you can connect to a refrence voltage, such as botton
battery, and measure the supply (main battery backup)
Connect it as SHMIDT configuration.
Whenever it is lower than some value, it will change the
output level... for your 8051.
also
you can use A2D and constantly read its value.

goodluck.
OK