It’s no exaggeration to say that the development of cheap rechargeable lithium-ion batteries has changed the world. Enabling everything from smartphones to electric cars, their ability to pack an incredible amount of energy into a lightweight package has been absolutely transformative over the last several decades. But like all technologies, there are downsides to consider — specifically, the need for careful monitoring during charging and discharging.

As hardware hackers, we naturally want to harness this technology for our own purposes. But many are uncomfortable about dealing with these high-powered batteries, especially when they’ve been salvaged or come from some otherwise questionable origin. Which is precisely what the Smart Multipurpose Battery Tester from [Open Green Energy] is hoping to address.

Based on community feedback, this latest version of the tester focuses primarily on the convenient 18650 cell — these are easily sourced from old battery packs, and the first step in reusing them in your own projects is determining how much life they still have left. By charging the battery up to the target voltage and then discharging it down to safe minimum, the tester is able to calculate its capacity.

It can also measure the cell’s internal resistance (IR), which can be a useful metric to compare cell health. Generally speaking, the lower the IR, the better condition the battery is likely to be in. That said, there’s really no magic number you’re looking for — a cell with a high IR could still do useful work in a less demanding application, such as powering a remote sensor.

If you’re not using 18650s, don’t worry. There’s a JST connector on the side of the device where you can connect other types of cells, such as the common “pouch” style batteries.

The open source hardware (OSHW) device is controlled by the Seeed Studio XIAO ESP32S3, which has been combined with the LP4060 charger IC and a AP6685 for battery protection. The user interface is implemented on the common 0.96 inch 128X64 OLED, with three buttons for navigation. The documentation and circuit schematics are particularly nice, and even if you’re not looking to build one of these testers yourself, there’s a good chance you could lift the circuit for a particular sub-system for your own purposes.

Of course, testing and charging these cells is only part of the solution. If you want to safely use lithium-ion batteries in your own home-built devices, there’s a few things you’ll need to learn about. Luckily, [Arya Voronova] has been working on a series of posts that covers how hackers can put this incredible technology to work.