In this particular day and age, with climate change and global warming being plastered across the news and media, Nokia couldn’t have chosen a better time to let slip details about a self-charging mobile phone.
How it actually works is fairly straight forward. Energy from other components in the phone such as the radio transmitter create energy, which is collected and stored in the battery within the phone. This stops energy being wasted and makes a fairly efficient phone. Further details become slightly harder to understand but is related to piezoelectric elements, adding more energy to the battery after collecting energy on rails within the phone that hit a capacitor.