Charging issue: iPad charging the wrong device

If you are using on iPad with a USB-C connector to charge another iPad, the order that you connect the USB-C cable determines the direction that power will flow.

Connect the USB-C cable first to the iPad that will supply power.

When connecting to a USB Type-A connection, power can only flow from USB Type-A to the connected device.

Most models of iPad require a Power Adapter rated at 12W (i.e., 2.4A @5V) or greater to reliably charge. 5W Power Adapters (1.0A @5V) are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow. Many recent models of iPad are supplied with an Apple 20W USB-C Power Adapter that supports USB PD (Power Delivery).

Some third-party USB-C power adapters (and portable PowerBanks) may not support USB PD. Power Delivery mutually negotiates the charging voltage; the source, load and cable are all elements involved in the negotiation protocol. Unless the Power Adapter explicitly supports USB PD, it will almost certainly fail to charge the iPad; proprietary charging standards, such as QC (Quick Charge), are not supported.

Many computer USB ports also cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, computer USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).