Q: When 2 devices with USB-C ports are connected together, which device is charging the other? How is the direction of charging determined?
A: When utilizing USB-C PD charging with the ZMI PowerPack 20000, the battery pack will always try to initiate a request to the other device: "Let me be the power supply."
For example, here are three cases.
Case #1, the other device is a power supply like an AC adapter or wall plug. The request will be rejected. Through the ensuing negotiation, the battery pack will eventually come around and accept its role as a device receiving the charging.
Case #2, the other device is a smartphone or laptop (assuming it's enabled for getting charged via USB-C). The initial request will be granted. The battery pack will proceed to charge the other device.
Case #3, the other device is another battery pack (of the same model or not). In this case, both battery packs will attempt to be the power supply, but only one will succeed. The outcome is random.