I have a MacBook Pro 15″ Mid-2015 Retina. It uses a 85W MagSafe 2 Power Adapter, which is emitting max 4.25 Ampere at 20 Volts.

According to

https://omnicharge.zendesk.com/hc/en-us/articles/115000623608-Step-3-How-to-use-DC-output, that means,
I’ll have to configure my OmniCharge to 20V DC Output

To do this:

• Turn the Omnicharge on
• Double Click the Power button, Menu appears
• Click the Power Button to enter DC Configuration
• Use the UP-DOWN Arrows to select “Presets”, press Power Button to enter
• Select 20V, use Power Button to select
• Change to “Yes”, use Power Button to confirm
• Done