Electronic – Which output terminals to use in parallel connection

^{simulate this circuit – Schematic created using CircuitLab}

Here's one circuit you could use to switch from series to parallel. Check that both switches are break-before-make or there could be sparks. The diode protects the charger if the batteries are switched into series. This will waste a little power, but it should still leave about 4.3V which is plenty for charging the cells in parallel. You could get a bit fancier with a 5V relay if you want, which would mean no external switch is required at all. I wouldn't bother with mosfets because they would increase the component count unnecessarily and require a higher gate voltage than you have at hand for high-side switching.

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.