RepOne Sensor can be charged via its USB-C port on the top side of the device. Make sure only to use a USB-A to USB-C cable to charge, like the one shipped with your order. Modern USB-C to USB-C cables use something called USB PD, or USB Power Delivery. With USB PD, the cable facilitates a handshake between the client device, and the device serving the power (the power brick). This allows things like high wattage charging, trickle charging, etc. RepOne, along with many non-smartphone devices, doesn't support USB PD.
Since firmware version 0.6.12, RepOne Sensor has the ability to display % battery remaining, and indicate whether or not it's successfully charging.
On startup, if the battery fuel gauge hasn't had a chance to take a good power measurement, it might show "--%" in place of an actual reading. If this is the case it will likely only take a few more minutes before a battery measurement is acquired. For similar reasons, there are situations where RepOne Sensor can display a battery % value that may be quite old. For example, when RepOne is left on overnight and doesn't completely die. When the tether is pulled it may display its last read battery level before displaying a new value.
The battery fuel gauge takes some time to learn the battery in your device. If you're seeing fluctuations in remaining battery %, give it a few full charge-discharge cycles and the system will characterize the discharge curve and get better.
Just before your RepOne Sensor is fully drained, it will briefly display a "Low Battery" screen on a tether pull. It then shuts back down to conserve battery if left on. Please don't leave your device on!
This screen will only disappear when the battery charge increases above 3%. This means the screen won't necessarily disappear immediately when plugged in.