Hi @omi236,
Here's a [https://www.allaboutcircuits.com/technical-articles/introduction-to-usb-type-c-which-pins-power-delivery-data-transfer/|link] that shows the pinout for a USB-C connector.
There's communication between the host and the device when using USB-C so that the host knows how much power it can supply, if the device can accept it that is, otherwise it defaults to the standard USB 5V value.
The 2.9V reading is too low. Is this measured directly between batt and ground output of charger i.e. open circuit voltage? It should be a minimum of 5V.
The D+ and D- are the data lines.