Reusing Old Phone Batteries for Low-Power Devices at Home
I always keep my old phone batteries that I replace, in order to protect the environment. Most of these batteries still have about 70% to 85% of their capacity left, but they have to be replaced so the phone doesn’t suddenly shut down during daily use. However, if they are used for other low-power devices in the house, they are still quite good. I have reused them in a few cases:
- Installed one for my grandma’s radio so that she only needs to plug it in to charge and can use it for a whole week. Previously, we had to constantly replace 1A batteries, which was quite costly.
- Presence sensors consume a lot of battery; replacing two AAA batteries every week is rather painful.
- The digital scale uses three AAA batteries, and I have to replace them once a month (probably because I bought low-quality batteries).
Each AAA or AA battery has a voltage of V = 1.5 V. Usually these devices use 2 or more batteries, so the power circuit will use about V ≈ 3 V. Phone batteries have V = 3.7 V, and I tried installing them and most devices have been working stably for the past year.
The issue with using phone batteries is handling the charging circuit. Here I chose to buy a step-down module from 5 V to 3 V to charge these batteries; just connect them in parallel and it works.
Purchase link:
If you want to ensure safety, you can solder an additional capacitor to the battery to lower the voltage to match the power requirements. However, from my tests it doesn’t seem very necessary, so I skipped it.