You aren't doing the conversion properly. The Arduino uses twos complement arithmetic so there's no need to convert. Also, the fraction is part of the twos complement number but you are treating it separately. I use this code:
The datasheet says that the EOSC bit is set to zero on power up so writing zero to the register will have no effect. Are you sure the battery is the correct voltage (check it on a multimeter) and is it in the right way round?