If you're doing it to learn about electronics and physics it's a great thing to do.

Tinkering is fulfilling in and of itself.

You may be interested in the Kelvin Water Dropper.

There's actually a decent amount of research into exactly this sort of thing, called, appropriately, "energy harvesting". Depending on the application, it can be fairly effective. I know that vibration energy harvesting has been successfully in machinery monitoring applications, for example. I haven't looked much into stray EM harvesting, but I'm sure it is possible..most likely to supplement a primary energy source rather than as the main source itself. But, yeah, for sure it's an interesting topic with a fairly good amount of study into it...I think the University of Michigan had a few good research programs related to harvesting for ultra low power applications. You night check out some of what they've done

The best advice I’ve seen online (ok, it was ChatGPT) is that it’s just not worth it to work with such small amounts of electricity, because the equipment required is too expensive and sophisticated (e.g, devices to read the charge of a capacitor without discharging it) to make anything that’s efficient enough to be worthwhile.

I guess ChatGPT has never heard of passive RFID tags? LLMs have some good uses, but they're not great at a lot of things. You can't really advance science and engineering by strictly regurgitating scraped text.

There are reasons to grab small amount of electricity from the environment. Why have a battery in a pacemaker if you can generate power via mechanical forces? It really just depends on the use case on how practical and feasible it is.

Depends on the use case. It is a very good idea to harvest small amounts of energy for example to use it in a calculator or a clock or a remote control or button or light switch. This way you never need to replace batteries or have them leak and destroy the thing.

Apart from that. There aren't many use cases for those very small amounts of energy. You have to ask yourself what you're going to use that small amount of energy for. Because batteries and wires are way cheaper. And they store amounts of energy you'd need 20 years of harvesting with equipment that costs a lot more. It just depends on the use case. And for little amounts of energy, the use-cases are severely limited.

You're allowed to do this as a hobby, however ;-)

ChatGPT is not a relevant or a reliable source.

https://www.ti.com/video/3878027903001
https://www.ti.com/product/BQ25570
https://www.ti.com/tool/TIDA-00242
https://www.ti.com/lit/ug/tiduc93/tiduc93.pdf?ts=1692465536615
https://www.ti.com/lit/ug/tiduew2/tiduew2.pdf?ts=1692465539544
https://www.ti.com/lit/ug/tidub22b/tidub22b.pdf?ts=1692465541647
https://www.ti.com/lit/wp/sszy004/sszy004.pdf?ts=1692465544492

It's a fun engineering challenge. Weird energy harvesting tech mostly has applications for sensor networks. Some of the new generation of bluetooth chips have ridiculously low power consumption -- so being able to deploy them without a battery somewhere without maintenance is occasionally useful.

Some currently used technology are piezo energy harvesting from mechanical vibration, low-light solar, and thermocouples.

My approach is usually to think around the TPL5110 and a pseudocapacitor. The TPL5110 is a timer that has a current consumption of 35 nA and can operate down to 1.8V. Every 2 hours, it would activate an ATtiny10 that can operate in the microampere range. That chip (very quickly) measures the voltage on the capacitor relative to a reference and decides whether it has enough power to "do the thing". If it does not, it signals to the TPL5110 to turn itself off for another 2 hours to let more charge build up.

If it does have enough power, the ATTiny10 either "does the thing" itself or switches a MOSFET to activate another system or whatever. The "thing" can be to use the power stored in the pseudocapacitor to charge a battery for a short time (e.g. around a second), if you want. Afterward, the system goes back to sleep until it has a relevant amount of power again. However it's often a battle to outpace the self-discharge of a lithium cell, so having the system "do a thing" without a battery present is often better.

This does result in practical stuff sometimes, especially when using low-light solar. Besides sensor networks, you can for example manufacture replacements for tritium indicator lights this way that only activate on at night. In my experience, an SMT indicator LED is quite visible at night with under 10 uA of current. I have a series of ridiculously overengineered indicator lights that stick to the top of doorframes so I don't hit my head on them at night (I am quite tall, and live in a traditional home in Asia).

Incidentally, I tried building a resonant circuit at 60Hz and was able to pick up a few mV from nearby fluorescent lights -- not enough to use. I used a ridiculously large coil of wire that I happen to have lying around. A more fun trick is to use LEDs as their own power source -- during the day they work as tiny solar cells, and that lets them flash occasionally at night :D

Kelvin Water Dropper

I can't find it now, but i'm pretty sure there exists smart home light switches that are powered by you pressing it.

IIRC they run Zigbee to communicate with the smart home and had a bit more resistance when pressing (= more energy).

Maybe take a look at BEAM robotics. Specially Pummer circuits.

The idea of Pummer circuits is to store energy from a small solar panel during the day and flash an LED at night. Energy is normally stored in super capacitors or NiCd batteries.

Might not be exactly what you are looking for, but it can give you some ideas to experiment with.

Some cool stuff I saw when I worked in a lab in college working on wearables. Movement, heat, etc.

https://www.iloq.com/ for example does a lot of parasitic energy stuff. Most of the advertised things now seem to be RFID, but I think they also had locks that are powered by the key insertion.

There's plenty of places where infrastructure power is not available and a flat battery would be a disaster. Door lock is one mundane one. Space is one more extreme one.

You likely aren't going to get enough energy to make up for the losses incurred when boosting voltage to 4.2 volts or whatever your battery requires. There's tons and tons of scam devices out there in the world that attempt to convince people these devices make sense, but they really aren't usable for anything meaningful.

Charging a battery with a couple microamps per hour. Would probably negates things like self-discharge? But certainly wouldn't recharge a battery that you have in use with a device. And if that device has radio or storage attached to it, you definitely aren't gaining enough electricity.