Bloomberg’s Mark Gurman revealed that Apple thought of adding an internal battery for its HomePod device.
The HomePod mini and HomePod currently are made for indoor use, with a cable and power plug requirement. Apple apparently thought of considering a portable version and worked on it internally. A prototype was made but then the idea fizzled out. Gurman said that if it did come to fruition it would be unlikely to be launched under Apple branding.

The HomePod mini does sport a USB-C port for connecting to a charger, and if the user has a battery pack with USB-C then it’s possible to carry the smart home speaker around.
The rumor apparently surfaced during the time Beats Pill+ was discontinued. The portable Bluetooth speaker was the last Beats-sold device, with the firm apparently refocusing on other products.
The closest thing to a portable HomePod mini would be the iPod Hi-Fi boombox to make the iPod louder, but it was discontinued in just over a year.