Since I was gifted a new Raspberry Pi this Xmas, I've found myself becoming much more interested in the details of computer hardware than I've previously been. Among the first thing that I've wanted to do with my Pi is build an on/off switch - Pi are very bare bones, and require you to shutdown or reboot using software. Cold booting happens immediately after plugging in a power cord. This sort of setup is less than ideal for a huge number of reasons - there is little to no in-built hardware to protect my Pi from a power surge, and I have a lot of uses in mind for this and future Pis that make an external surge protector unrealistic. Even for home/office use where the Pi is connected to a stable power source, I'd like something akin to the power button that comes with desktops & laptops that can send an ACPI signal which I can in turn manage a bit using /etc/acpi/powerbtn.sh.
Anyway, I have quite a bit to learn in this area. I've worked with power, but its almost always been external / data center-scale power. Onboard power is less sexy but no less interesting and substantially more helpful to my own individual hacking efforts. This new-found interest has lead me in all sorts of directions. I've found myself testing laptop batteries instead of reflexively replacing them when they start acting funny. The video below has been quite helpful to me - testing the actual capacity of a laptop battery can be a bit complicated, and measuring the voltage of a battery requires you to run to ground (which in turn requires you to figure out which terminals the ground *is*). Check it out: