In certain PBP on this forum, GMNiles had fantastic idea- to let have techno-oriented PC scrapyard bot as "pet" and have him upgraded through mechanical parts. I'm very keen to do something similar in my game, so I ask about assistance in this case- anyone has an idea hot to do this with mechanics, apart from Leadership feat ?
You could treat the robot as a hireling that is paid in batteries.
For example, suppose that the robot is as useful as a footman, who charges 5 sp per day. A battey costs 100 gp and holds 10 charges. 100 gp would buy 200 days of service from a footman. Declare that the robot consumes one charge every 20 days, so that the robot can run for 200 days on a single battery.
When the party upgrades the robot, you can also increase the power the robot consumes. A robot valet would consume one charge every 10 days. A 3rd-level expert charges 1 gp per day, the same as a valet. A 5th-level expert charges 10 gp per day, so a robot with equivalent abilities would consume one charge per day.