I am working on a vehicle which has electronics equipment inside it. A neutral electrical charge of the ground connection is desired.
When I measured the negative terminal, which is connected to chassis and batteries negative, compared to the ground/earth wire in the wall socket, the voltmeter read ~ +6V DC. I also tried to measure the current from chassis to wall socket earth. The (cheap) voltmeter showed ~40 mA for a very brief moment, then zero milliamps, and the voltage difference was neutralized. Immediately after removing the current meter, the voltage difference would return to ~ +6V.
The system is: vehicle, batteries, solar charger, inverter. (and appliances but those were disconnected)
Sometimes I feel a spark when touching the vehicle while grounded.
The system is 12V. I'm not quite sure where the 6V comes from.
a) Why isn't chassis ground at zero potential? Isn't it being earthed via the rubber tires?
b) Is there a way to achieve a better earthing, e.g. a loose wire that hangs and touches the ground?
c) Is there something wrong with my configuration, or is this completely normal?