Honda EU30is Owner's Manual page 41

DC Application
The DC receptacle may be used for charging 12 volt automotive-type batteries
only.
NOTE:
In DC operation, turn the Eco Throttle switch to the OFF position.
1. Connect the charging cable to the DC receptacle of the generator and then to
the battery terminals.
DC CIRCUIT PROTECTOR
ON
OFF
38
CHARGING CABLE
(SOLD SEPARATELY)