Macbook Pro (Retina, 15", Mid 2015) USB ports won't charge an iPhone or iPad; they go ding ding ding as it starts to charge and obviously can't get enough current, so stops charging, then tries to charge again....
MacBook Pro 15", macOS 10.14
If your Macbook has a USB 3.0 port, it should charge your iPhone well enough. But it won't have enough power to charge an iPad. It will trickle charge, but that's too slow for any useful power transfer to an iPad.
Are you using Apple cables for your iPhone? I wouldn't suggest even trying to charge your iPad from your Macbook.
This link might help you: If iTunes doesn't recognize your iPhone, iPad, or iPod - Apple Support
The problem is not only charging, but also to synchronize and backup the iPad. With Apple cables never works, and with third party cables sometimes it works.
Macbook pro won't charge iPad (iPad is constantly dinging when plugged)
