Smooth switching between 12 V batteries, with a toggle switch
You could add two Schottky diodes to the switch, allowing either battery to power the load via a diode. During switchover the voltage will drop 0.35&nbsp;V (1N5817 @200&nbsp;mA) below the voltage of the battery with the most charge, and it avoids the current surge caused by adding a capacitor. You could even remove the switch if a 3% power loss is acceptable.







simulate this circuit &ndash; Schematic created using CircuitLab