How do I combine 3 batteries in series and parallel with different outputs?
Here's how to do it (but it's still a bad idea).







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. 12 - 24 V switching.

As others have pointed out this is not a good engineering solution to whatever problem you are trying to solve.


In 12 V mode all batteries will discharge at the same rate. Since they're 12 V I'll assume that they're lead-acid and so you'll be recharging them too. This is fine in parallel mode.
In 24 V mode BAT1 will discharge at a current I but BAT2 and BAT3 will discharge at I/2 so they will be unevenly discharged and this will cause problems when switched back in parallel as their voltages will be different and high currents may flow.