I have a RAVPower 32000 mAh power bank as a backup for my Hubitat Elevation hub. If the Hubitat hub loses power, the database can become corrupted, causing my lights, locks, routines, and schedules to flake out. It has been connected in series with the hub for about 6 months. I can verify it supplies voltage while accepting voltage and the hub stays powered when input voltage is lost. I graphed the output with a USB meter. The voltage did not waver a bit when power was removed and reconnected.
Bought mine on Amazon but they had a falling out.