Is the Google Play Services component always determining the user's current activity, or only when an app is connected to receive activity updates? The documentation isn't really clear about this, it just says that it has a minimal battery impact in general. I'd like to know if an app constantly querying the user's current activity (with a 2-minute interval, for example) would put any additional strain on the user's battery or not.
The battery cost is based on the interval the app specifies. So if you request a 2 minute interval, the device will sleep for 2 minutes, wake-up for a few seconds to do an activity recognition and then go back to sleep for another 2 minutes. Intervals of 2 or 3 minutes should have a minimal battery impact.