From what I understand, if two 12v batteries with different charge levels are connected in parallel with both terminals, the charges of the two batteries will even out. The one with the higher voltage will raise the voltage of the lower one and lose voltage in the process.
I'm confused about what would happen if two 12v batteries at different charge levels (or voltages) are connected by just one terminal. I think the confusion stems from the fact that batteries don't just have positive and ground terminals, but they actually have positive and negative terminals. So, if I understand correctly the positive terminal "pushes" positive charge and the negative terminal "pushes" negative charge.
Is the negative terminal of a 12v battery always the same voltage as the positive terminal, regardless of how discharged it is?
Let's say there's two 12v batteries at different voltages. The first battery is discharged to 12.5v. The second battery is discharged to 11.9v. They each have their own loads attached. In my case the load is a power inverter for first battery and lighting for the second battery.
What happens if both positive terminals are shared by each battery at some point? In other words, if the negative is separated but the positive is shared, does the voltage of each battery change? What happens to each of the positive terminal voltages? What happens to each of the negative terminal voltages?
What if both negative terminals are shared by the batteries? Does it have the same effect as sharing the positive terminals? Is it because of the negative charge "push" effect? What happens at each negative terminal in this case? Do they become balanced out at the negative terminal, causing the positive terminals to each have different voltages but the same negative voltage?
Lastly, what if the batteries share their negative terminal by a common ground? Do the negative terminals even out their voltage? Would the ground voltage change in any way? Would the ground voltage change to the negative voltage of the most charged battery? Would the ground voltage change to the negative voltage of the least charged battery?
Sorry for so many questions. Maybe the answer is simple and explains all of them. Thank you so much for your answer.
EDIT: When I refer to sharing only one terminal on each battery, it should be assumed that that each terminal is also still connected in a way that completes the circuit. So when both positives are shared, the positives are also still connected to each load circuit and each load circuit still ends at the corresponding negative terminal. When both negatives are shared, the positive terminals are each connected to their load circuits and the load circuits are still connected to each corresponding negative terminal. I'm not sure if that changes anything.
Best Answer
A 12 volt battery has 12 volts between its terminals. Voltage is always measured between two points - there is no "absolute" voltage.
If you have a 12 volt battery and an 11 volt battery, and connect the negative terminals, the voltage between the positive terminal of the 11 volt battery and the positive terminal of the 12 volt battery will be +1 volt, assuming the negative lead of your meter is on the positive terminal of the 11 volt battery.
If you connect the positive terminals instead, there will be a 1 volt difference between the negative terminals.
The term "Ground" is much misused in the electrical field. Occasionally it does mean a connection to the Earth, but most often it is just the point in a circuit that the designer chose to call "zero volts", and uses as a reference when measuring voltage elsewhere in the circuit.