Mophie battery case on iPhone8

On my iPhone6 there was a switch to activate the battery in the case. The iPhone8 does not have this switch but nobody can tell me when the case battery is supposed to kick in. For the first week it kept the phone at 100% which I didn’t think was a good idea. Just today it started dropping and is down to 90%. I am hoping it kicks in around 30 or 40%. I am purposely not going to plug the phone in to house power so I can see what happens. If it goes as low as 25% I’m going to charge it up manually and call Apple, unless you people can help me on this issue before that.