I have a small Wayne 12 volt battery backup pump. The wires coming from the switch had a float which I removed. When those 2 wires are touched together it activates the pump. I would like to wire the wires from the switch and the 12 volt charger to a relay or solenoid so that when power is off from the charger the circuit completes. and activates the pump. Can you help me?
This is quite straightforward, and requires a 12-volt, SPDT relay with contacts adequate to handle the pump current.
As long as the charger is providing voltage, the common (the upper in the diagram) lead contacts the NO (normally open, on the left) contact, while the NC (normally closed) is not connected. When the charger shuts off, the relay relaxes, and the common connects to the NC contact, turning on the pump.