The negative battery terminal isn’t grounded because it is attached directly to the car’s chassis. Many cars have an electrical system that uses negative ground, and grounding would cause a short circuit, making the situation even more dangerous than just having ungrounded electronics.
Usually the negative side of a battery is attached to that. But, there are many circuits that work differently. Some circuits need a negative voltage, so the positive side of a battery would be "ground".
Some circuits need a negative voltage, so the positive side of a battery would be "ground". Some circuits need positive and negative voltages, in which case there could be two batteries, one with the negative side attached to ground, and the other with the positive side attached to ground. This works because voltages are relative.
Eventually all of the excess electrons have been moved off into the ground. The negative terminal is actually at 0V, just like the Earth. Now, because of the chemicals in the battery, we still find the positive terminal is 12V higher than the negative terminal, at 12V.
The positive terminal doesn't have to be positive against ground necessarily because the physical Earth is not a reference point for the battery, only it's negative terminal is. That being said there is one additional factor , namely capacitance, every object has capacitance.
Bingo, that's it. In the circuit below, no current flows out of the plus side of the battery unless an equal current flows into the minus side. At the grounding point d, the current c-d exactly matches the current d-a. In other words, add the ground, or remove the ground and nothing changes.
If you connect the - terminal of your battery to the box, then the - bus of your circuit will be more coupled to the earth than other nodes in the circuit. So, signals originating outside your circuit that get in ( radio waves, for example, or 60 Hz …