"I think that precaution is redundant... when you start your robot there is ripple when start the ON switch, and your uP works after that. When the charger goes on, I dont think that you'll have a ripple more dangerous than when you started the robot..."

I think not, power on causes a reset, changing power source should not.

The easy way is to use a diode bridge, a diode from the raw DC and a diode from the battery, both feeding the regulator. That way the battery will only feed when power is off ( raw DC voltage must be a tad higher than battery voltage) and the only change seen by the regulator on switch to batter is a small drop in input voltage

Erik

Erik