Yes — a USB power bank is generally a practical way to extend runtime on the Sony A7R II for timelapse. Community answers indicate the camera can be connected to a USB battery bank and can charge while shooting, unlike some other Sony models.

Also, the quoted “300 shots” battery figure is only a rough rating based on typical still-photo use, not a direct measure of timelapse endurance. Actual runtime in timelapse depends on interval length, screen use, focusing behavior, and overall session duration.

One thing to keep in mind is heat: during long continuous shooting sessions, overheating can become a limiting factor even if external power lasts long enough. So an external USB battery can help with power, but it may not guarantee unlimited timelapse length.

In short, if you want to save money versus proprietary accessories, a standard USB power bank is a sensible option for the A7R II.