Calculating specs of the Li-Ion battery depending on Robot's weight
In order to compute the power source you need to know the maximal and average power consumption of your robot and the maximal working time between the recharges. 

In order to estimate the motor power (everything else will consume a lower power) you will need the maximal acceleration of your robot and how often it will need to accelerate/decelerate. 

Then multiply the working time by the average power and you will get the capacity of the batteries in W.h 

On my estimation, you will need at least 1kW motor and if you want to work 2h, then 2kW.h batteries you will need. If the batteries are 48V then 42A.h capacity you will need. The current draw of the motor will be around 20A on maximal power mode. The acceleration time from 0 to 50km/h will be something like 10s

The efficiency of your mechanics is not calculated here.

Addendum that changes almost everything:

After reading the parameters added in the question, I think such robot can not be designed to work from batteries. For 24h work of 1..2kW motor, your batteries will get huge. 

Better consider using internal combustion engine. The power density of the fuel is the best from the existing energy sources. The ICE will provide enough power for the electronics as well.

If you want to use the electrical motors in order to get better control, you can use a hybrid schematic - a small battery that to power the robot for 1..2h and small diesel or gasoline engine that to charge the batteries from time to time.