The confusion here is from the initial poor description of how a battery works.
A battery consists of three things: a positive electrode, a negative electrode, and an electrolyte in between. The electrodes are made of materials that strongly want to react with each other; they are kept apart by the electrolyte.
The electrolyte acts like a filter that blocks the flow of electrons, but allows ions (positively charged atoms from the electrodes) to pass through. If the battery is not connected to anything, the chemical force is pulling on the ions, trying to draw them across the electrolyte to complete the reaction, but this is balanced by the electrostatic force-- the voltage between the electrodes. Remember-- a voltage between two points means there is an electric field between those points which pushes charged particles in one direction.
When you add a wire between the ends of the batteries, electrons can pass through the wire, driven by the voltage. This reduces the electrostatic force, so ions can pass through the electrolyte. As the battery is discharged, ions move from one electrode to the other, and the chemical reaction proceeds until one of the electrodes is used up.
Thinking about two batteries next to each other, linked by one wire-- there is no voltage between the two batteries, so there is no force to drive electrons. In each battery, the electrostatic force balances the chemical force, and the battery stays at steady state.
(I kind of glossed over what it means for two materials to "want" to react with each other. Google "Gibbs free energy" for more details on that. You might also google "Nernst equation.")
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.
Best Answer
The convention for current flow was established before the electron was discovered (by J.J.Thomson in 1897). The convention is that current flows from + to -. We now know that, in fact, the mobile charges are electrons and flow from - to + but the convention has endured and we all use + to - flow but keep the reality in the back of our minds.
As is universal practice in maths, physics and general engineering, + is higher than - so we refer to the positive terminal of a battery or power supply as having higher potential. (Conventional) current will flow from the higher potential to the lower, i.e., from + to -.
simulate this circuit – Schematic created using CircuitLab
The magnitude of the current will, for a resistance load, be proportional to the voltage or potential difference between the terminals and inversely proportional to the resistance between the terminals. This is succinctly stated in Ohm's law.
$$ V = I \cdot R$$
which can be written as
$$ I = \frac {V}{R}$$