See section 7 of the article on the upgrade ...in our 2019 EII manual it shows it as a Black Charge wire as shown in the wiring diagram and in the PICTURE below.
The reason we disconnected the wire inside the 7 pin cable was to protect the alternator. Lithium Batteries they can "draw" charge current much faster than older AGM or other Lead Acid Batteries...so ultimately they would try to draw way more power than the truck wiring would provide. (To install the DC/DC converter you'll run dedicated lines back to the DC/DC charger.)
Full battery upgrade is here...including the section 7 about disabling the charge wire.
https://4-ever-hitched.com/blog-articles/f/lithium-battery-upgrade
Step 7;
Disable the 7 Pin Charge Line:
We will be installing a new DC/DC charger into our Oliver EII. But before we do that install we need to disable the current charging wire that runs from the 7 pin cable on the tongue of the Oliver. That 7 pin connector is the one that plugs into the back of the truck for lights, brake lights, emergency brake power.
Inside the 7 pin cable there is a BLACK wire that provides 12V (B+ ) from the truck battery/alternator B+ all the way back through the trailer, through a 20A breaker and back to the batteries via the 12V Bus bar in the trailer.
We will not be using that (Black) charge line for the trailer an longer.
We disabled that line on the 7 pin connector by putting heat shrink coating over the screw terminal on the black B+ wire conductor and also taping up the screw lead the body. We reassembled the conductor and tested that no 12V was present on the wires inside the trailer.
Wiring Digram for Oliver EII 7 Pin
To disable the Charge Wire (BLACK) from the 7 pin, remove the cover and disconnect the wire from the connector by taping up the wire and putting it back inside the cover. Don't cut the wire in case you need to use it in the future.