Weeding time vs. Charging time
Tertill spends more time resting than running--this is expected behavior due to the amount of energy it can gather from the sun. In practice, it works out to about an hour or two of runtime per typically sunny day. Note that the robot is always charging via its solar panel, even when it's running.
Once the robot's battery charge has dropped below a predetermined level, it will turn itself off to conserve energy. Every 20 minutes, it will wake and see if the battery charge level has risen above that level. If so, it will run for at least a minute. if not, it will turn itself off for another 20 minutes.
This charge behavior of short cycles (instead of running until the battery is depleted, then waiting until it is fully charged) is beneficial for a couple of reasons, including a longer life out of the battery, more frequent motion (reducing the likelihood of the robot ending up in a shady spot and waiting a long time to get enough sunlight to run again), and more reserve energy for extended rainy/cloudy conditions.