How does the contact pad used with Duracell myGrid mat work?
I have not seen the circuitry used (or even seen a Mygrid device) but it almost certainly works by having two diodes from each Mygrid contact point to the internal power circuit in the device being charged.


One diode per contact connects with its Anode to the contact and its Cathode to the accessory "+ve" connection  (ie the diode "arrow" points from contact to accessory +) . This diode conducts when a Mygrid +ve terminal is encountered. 
One diode per contact connects with its Cathode connected to the Mygrid contact and its Anode to accessory "-ve" connection (ie the diode "arrow" points from accessory towards the Mygrid device. This diode conducts when a Mygrid -ve terminal is encountered. 


This way, whatever polarity is on the contact will be correctly routed to the accesory.