Is it safe to swap my battery-powered LED strips over to a USB plug?

The red and black wires are exactly what you need. Inside a standard USB cable, the red is your 5V positive and the black is your ground. The other two wires, usally green and white, are for data transfer and you can just snip those off or tuck them away. Since three AA batteries provide 4.5V, jumping to the 5V provided by a USB port is well within the tolerance for those cheap LED strips. You won't melt anything as long as your polarity is correct. I did somethig similar recently when my wife got a Crosley record player for her birthday. It was a gift from a coworker and she found the perfect spot for it, but there weren't any outlets nearby. I checked the wall wart and noticed the output was 5V at 1A. Instead of running an extension cord across the room where our nephews woud trip on it, I just chopped a spare USB cable and wired it to a portable battery bank I had lying around. It works perfectly and now the whole setup is mobile. If you're worried about the soldering or the splice looking messy, you can acutally buy **battery eliminators** online. They are basicly dummy plastic batteries with a thin ribbon cable that leads to a USB plug. You just pop them into the existing battery compartment and it bypasses the need for disposables entirely without you having to cut into the device's original wiring. It's a cleaner look if the battery door is visible. Another option if you don't want to tether it to a wall is using rechargeable batteries with built-in USB ports. But for garage lights that stay in one place, the hardwired USB route is the way to go. Just make sure your connections are insulated with a bit of electrical tape or heat shrink tubing so the positive and negative don't touch. Even with the low voltage, a short coud still trip the protection circuit in your USB brick.

While the 0.5V differential is indeed negligible for most consumer-grade integrated circuits, you shold carefully examine wheather those specific under-cabinet units utilize a dedicated current-limiting resistor or if they simply rely upon the internal resistance of the alkaline cells to prevent the diodes from drawing excessive current. If there is no such resistor present on the PCB, the consistent 5V potential from a regulated wall adapter might eventually cause the LEDs to exceed their thermal threshold, so I would suggest measuring the current draw with a multimeter before you finalize the enclosure, expecially since cheap motion sensors often lack robust over-current protection.

This is so cool!! I've been looking for a project like this for my tool bench. If I switch all my garage lights over to USB power like you guys are talking about, does anyone know if those motion sensors still work the same way? Also, totally off-topic but since we're talking about garage upgrades—has anyone here ever tried using a pressure washer to strip oil stains off an asphalt driveway, or does that totally ruin the seal?

Quick update—I got one light working using the red and black wires. Thanks for clearing up the data wire thing! However, someone mentioned resistors and internal resistance, and now I’m a bit paranoid. The LEDs are noticeably brighter now, though the motion sensor still seems to trigger fine. I’m just worried they might burn out or get too hot. shoud I add a small resistor to the circuit?