Hi!
If you know the voltage, and the current draw of your robotic car, you need to consider the constant current draw, and then see which batteries actually have a good mA/h rating to match.
Example (from memory, excuse any errors, but roughly correct): I made a naff LED torch that can draw from 20mA to about 200mA or so on some settings.
My preference, due to size, was a standard rectangular 9V battery but it would have lasted 30 minutes at most, 12V (3 x 4.5V) battery instead should last 30 hours, so I opted for those - bulky, but longer lasting. 1.5V batteries AA or AAA have a better mA/h value than 9V batteries, which I imagine is why they are in so many battery operated devices. A tiny camera battery would last a few seconds...
It's good to browse online shops which provide datasheets for the batteries, or manufacturers, or check battery comparison webs, to get a better idea. In my experience it was not always easy to find what should be one of the first things a person would want to know about any battery.
Sorry I can't remember the formula for calculating A/h or mA/h. If I can find it I'll add it later.