This has been a known issue for some years, going back to at least 2014. The problem is the Ambient Mode in Chromecast, which displays artwork and images when nothing is being streamed to the Chromecast. And to be sure, the images it displays are attractive and fun to look at. But because they're so high-resolution, the data consumed by Ambient Mode is noticeable.
The problem is exacerbated by the increase in data usage from newer smart home devices like doorbell cameras and security cameras, which tend to upload large amounts of data to the cloud for AI features like facial recognition. Combined with the high data usage of Chromecast Ambient Mode, this puts you at risk of exceeding the data caps from your ISP - and getting an unexpectedly large bill.
In the past, tips such as plugging the Chromecast's USB power cord into the display's USB ports (so that it's unpowered when the display is off) are helpful, but that still means the Chromecast is using data any time you're using the display for any reason. During a long weekend of DVD watching the Chromecast will still consume data as it downloads high-res photos. A couple of years ago, someone had the idea to post two small 16 x 9 pixel images of a solid color in Google Photos, and then set the Chromecast to display only that image folder, but for some reason that's no longer working. Another tip is to set the Chromecast to cycle images only every 10 minutes, which helps but not very much.
Chromecast now has an Experimental Mode for Ambient, which now includes a Low-Bandwidth setting. But if you're operating on a very limited data plan, even this may be too much. So what to do?
