You can’t “mess up” the phone’s internal circuitry by clever logic. Either the phone draws power or it doesn’t. You can’t “push” more Amp to a device than it’s ready to handle unless you play around with usb voltage which is very risky.

The robot lawnmowers I know also rely on the boundry-cable system and random direction change. It works surprisingly well and keeps the grass nice and short with only 1-2 hours of activity per day. So regardless of actual performance, the idea for this mower works.