Hoover batteries, sensors and charging
It's common for chargers/batteries to have temperature control, this one is an "NTC" or "Negative Thermal Coefficient" because when it is colder, the resistance increases, and when it's hotter, the resistance decreases (thus the "negative" part when relating to the behaviour during an increase in temp)

They are simply NTC resistors and the intelligent charge controller will be able to use it to help it's charging profile and protection circuits.

The robot's power management ICs or controllers will be able to monitor the battery voltage using a wide range of techniques - the most common I use in my designs is a simple resistor divider straight off the battery positive terminal, taking the voltage range for minimum and maximum expected charge to something safe for my microcontroller, and simply reading the voltage using an ADC (Analogue to Digital Converter) on-board the microcontroller to read the battery state, and this may trigger a "We need to charge our battery!" state within the robot's brain. 

Some battery management ICs will have serial or other forms of communication to a host microcontroller for the same kind of battery voltage reporting, and also indicating the state of the charge cycle once it begins. They might do it internally in the chip as part of the charge control anyway, so it's handy information. The IC designers may make use of resistor dividers or more likely some kind of difference amplifier or more complex op-amp setups.