If you’re anything like every other human being on the planet, you have several cordless power tools and their batteries are all dead. You never remember to put the batteries on their chargers until you need the tool for a job only to realize that it won’t turn on. Discipline is difficult, so Lance of the Sparks and Code YouTube channel decided to build a robot that swaps out the batteries on the charger for him.

This robot does still require some attention: Lance has to place a stack of batteries into the robot’s hopper to get the process started. But after that, the robot will set each battery in the charger, wait until it is full of juice, move the battery out of the way, then repeat with the next battery. The batteries all have to be identical (or at least share a charger), but it is a good idea to keep all of your cordless power tools within the same ecosystem anyway.

For this to work, the robot needs to know the status of each battery. While the batteries have internal electronics, interfacing with those would have taken some reverse engineering skill. Instead, Lance chose a more practical solution: using a light sensor to determine when a battery’s “charged” status indicator lights up. An Arduino monitors that sensor and also controls the robot’s motors through a CNC shield. There are two motors: one to push the sled that grabs batteries from the hopper and another that lowers the charger down onto the current battery, which is necessary because the batteries vary in height.

Now Lance has a convenient way to charge up all of his power tool batteries — if he can remember to place them in the hopper.