Switch from solar to battery

[ioma]

I have fan (12v -0.9a) which use power from a solar panel.The obvious problem is that when is dark or there are clouds the fan stop.I want to make a circuit with relay which when there is no power from the panel the circuit switch to battery and the battery give only 6v to fan.Is this possible;
PS Sorry for my bad English

 

[mneary]

You can do this with two diodes.

[Sorry it was too late last night to make the drawing.]

 

[ioma]

Because i am newbie to electronics can you sketch it;

 

[mbarazeen]

how are you going to charge the battery?

to use two diodes, make the common connection your fan negetive terminal battery negetive and solar panel negetive together. put a diode anode side to the bettery positive and the cathode (normally maked in white) to the fans positive, same another from solar panel (+ve) to the fans positive. until the solar panel out put drops bellow your battery volagte it would be powered from the panel and then from the battery.