Amir,
The negative of the battery will need to be connected to the buffer ground, possibly through a resistor. The voltages at the inputs to the buffers must be within the buffers' common mode range, which is usuallyapproximately the power supply voltages. Check the data sheet for the amplifiers.
If the battery negative needs to float to a voltage too high for the grounded buffer, then you need a more complex circuit to handle the high common mode voltges.
Lynn
