If the capacitor is backwards it would explode. It should be hooked directly across the relay coil in proper polarity. 6800 uf if charged should hold the relay for at least 5 seconds if it charges . Not sure of the circuit used by the relay in question. If there is a bridge rectifier in front of the coil it should not bleed back. You can use an additional relay and timer module and delay it as long as you wish.
Dale H
