With the release of firmware version 0.6, charging your RepOne Sensor became a lot simpler. If your battery is below 3% capacity you'll see a "Low Battery" screen. This screen displays for a few seconds when you attempt to pull a rep, then the device shuts back down into a low power state. If the cord you plug into your RepOne Sensor is compatible, within a second or two you'll see a charge indicator on the top right of your screen. If you plug a compatible cord into your RepOne Sensor while the "Low Battery" screen is showing, a charge indicator won't display until you're above 3%. This shouldn't take much longer than 5-10 minutes. A future firmware update will show the charge indicator on the "Low Battery" screen.
Charging the battery takes approximately 1.5 hours from dead to full. You can get enough charge from a dead unit for one session in about 15 minutes.
RepOne Sensor uses the same chip you'll find in many common consumer electronic devices, like the Bose QC35 noise cancelling headphones. Like those headphones, during and immediately after charging, the chip will read artificially higher % charge. When the battery is in a 'relaxed' state, a few minutes after it stopped charging, it'll display a more accurate fuel gauge reading.
Some chargers aren't compatible with RepOne Sensor, even though they might use USB-C. This is because RepOne Sensor doesn't support USB PD, a charging standard that requires the device and the charger to communicate with each other about how they should be charged. Your RepOne Sensor comes with a USB-C (small side) to USB-A (big side) charging cable. Most USB-A chargers will work to charge your RepOne Sensor. Some smart devices like computers and fast-chargers might have a USB-A port but can refuse to charge devices, so make sure the adapter you use is just a simple 5v wall adapter.