I need to register the temp and the voltage of a Sealed lead acid(SLA) battery, and the specification are:

temp: -20 to +70 Celsius with error < 1C
voltaje: +8 to +16 with error < 0.1V

I chose the ADC 0804 and the LM35 for the temperature, and an analog multiplexer between them, but

the sensor need negative voltaje to tell -20C. How can I solve this? I don't want to produce negative voltaje because I only have +12V.
The datasheet says put 2 diodes in series in the GND pin but I don't know how to connect the output to the ADC