There comes a time when on/off control of power to a load needs to be accomplished with one pushbutton switch. Drag out the junque box! All you'll need are two DPDT relays, two resistors, and an N.O. pushbutton switch

Parts

R  see text

S1  SPDT N.O.

When S1 is closed, K1 is connected directly across the voltage source and pulls in. Because its coil is rated at half the source, the switch closure must be momentary. K1A closes and pulls in K2, and the coils are now in series. Contacts K1B are connected to the load circuit, which can be an entirely different circuit from the relay source. When S1 is pressed again, it shunts the coil of K2, which drops out and the load is disconnected.

Most reasonable-quality relays can withstand twice their coil voltage for short times. However, if you want  high-reliability operation, insert a resistor in each coil circuit as shown. The resistor value should be chosen on the basis of relay pull-in voltage and coil current. One desirable feature of a relay is that it will remain pulled in at considerably less than rated coil voltage.

This has worked for me:

     1. With a variable voltage source, determine the lowest pull-in voltage V1. 

     2. Add about 20% of V1 and call it "on".

     3. With the relay pulled in, slowly lower the voltage to determine drop-out voltage V2.

     4. Add about 20% of V2 and call it "off".

     5. The idle, or holding, voltage would be halfway between V1 and V2.

     6. Calculate value and power rating based on holding voltage and current.

                  73 de WB4FXD