The small black gauge connector is for charging the battery pack, the bigger yellow/orange is for discharging it.

The second battery has the same connectors, one for charge one for discharge, the are squeezed into the gap under the top of the chassis. Disconnect them both.

Once the batteries are fully disconnected, turn GR16 on, to discharge the on board capacitor fully.

Then reconnect the second, and the first battery pack. Expect some sparks when the yellow connectors are connected. Don't forget to connect both battery packs.

Turn on GR16 to check if it is working. Reassemble and fix the rubber cover with duck tapes :) or glue them on.