I made a simple circuit to alternate its output between two inputs: a photovoltaic panel and a 12V DC voltage source. The schematic: 
The step source represents the photovoltaic panel and the two resistors represent two bilge pumps of 12 VDC 30 W.
It's done in a way that the normal closed contact is fed by the net. But when there's sun, there is voltage and then the relay (12 V rated) changes its contacts. Pumps are then fed with power coming from the panel.
The problem that I got, was that the release/decoupling voltage (voltage to change the the contacts back to normal) isn't the same as the operating voltage of the relay. The relay comes back to normal when the voltage is approximately 3 V, so when it's going dark and the panel is generating between 3 V and 12 V, my pumps aren't going to work properly or at all. They have to be ON 24hrs/day.
Only when it reaches so dark that the voltage is lower than 3 V do the pumps get fed with power from the net again.
What can I do to solve this problem? Is it possible to keep the relay strategy or do I have to change it to something else?
Thanks in advance.
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