Voltage level when connectiong two power sources
Apart from wiring non idealities, what you are missing is output resistance.

Each non-ideal voltage (current) source has a series (parallel) output resistor, which is ideally zero (infinite).

When you parallel two power sources the voltage you measure across them depends on each output resistance, even neglecting wire resistance.

As an example, have a look at this:







simulate this circuit &ndash; Schematic created using CircuitLab

Let's suppose you measure voltage where the arrows are, plus terminal on top, minus on bottom.

Total voltage across resistors is 1V, current is then 6.6mA, the voltage you measure is about 1.67V.

As you can see the voltage depends on the sources voltage and their output resistance. If your wires are non ideal you will experience a voltage drop because of this 'fight current' that is flowing because the sources fight to 
set their voltage.

But what if you connect two ideal voltage sources in parallel? Well, you can't, and our way of modelling electronic circuits does not care about describing the impossible.

There is actually a catch: some devices can not be modeled as voltage/current sources, as your smart charger. First of all, most voltage (current) sources have a maximum output current (voltage). Up to the maximum the output is guaranteed to be stable, after that it drops. Moreover, some devices vary their output and can even switch between voltage source and current source mode.

I am sure you now understand that such a connection, which is usually undesirable, must be analyzed carefully, and the outcome might be very different depending on the nature of the devices.