How to properly measure voltage of external supply
Since the MCP3001 doesn't have an internal reference voltage source, you will have to supply an external reference voltage that your input voltage will be measured against.

Depending on the measurement accuracy required, you can either use your ADC's supply voltage or a dedicated voltage reference like these. Either way, the accuracy of your measurements will depend on both the ADC's accuracy and that of your reference voltage.

Also remember to adjust your input voltage divider to scale the battery voltage to the range between 0 V and Vref (which might be lower than 5 V).