Measure from AC ground to the cranking battery negative and positive terminal.
Type: Posts; User: CatFan
Measure from AC ground to the cranking battery negative and positive terminal.
Where was the black lead grounded?
Where you measuring on AC or DC? Where were both leads when you measured voltages?
Breaker likely won’t help here, even a GFCI. Problem is the current is flowing between the electrical ground and the earth.
But any outdoor electrical circuit absolutely should be GFCI protected.
Your problem is likely either bad wiring, or you are plugging in with a long extension cord or in a building with a sub panel that doesn’t have a ground rod.
The second case is really common....