Devices that report their battery life such as controllers or Vive trackers can send this battery info to the SlimeVR Server - Charging status will update the battery voltage in the SlimeVR Server to display whether it is charging or not - Battery level is a range from 0-1 in SteamVR, however the SlimeVR Server expects a range of 0-100 https://github.com/SlimeVR/SlimeVR-OpenVR-Driver/pull/41 includes an example with the HMD sending battery messages to the SlimeVR Server if it reports battery level.