RepOne has USB-C built in. USB-C is great because it's a reversible port, and the included battery management chip supports fast charge. UCB-C also supports a lot more advanced features that we don't need or want, like the ability for RepOne to charge other USB-C devices. This means more advanced implementations of USB-C won't charge RepOne.
The simple way to go about charging your RepOne Sensor is to follow our golden rule:
Use the included charging cable.
The included cable is USB-A (charging brick side) to USB-C (RepOne Sensor side).
The most complex chargers are USB-C to USB-C, like most new Apple chargers, and those won't work to charge your RepOne Sensor. Those also aren't compatible with the included charging cable, so no confusion there as long as you stick to the golden rule. There are some rare old school rectangle USB charging bricks (USB-A) that are compatible with the included cable, and if you happen to have one of those and your RepOne Sensor isn't charging, try another one.
In firmware version 0.5.X, the only indication that your device is charging is when it wakes from dead. That will change this September when the new 0.6.X firmware launches.