Ignition voltage drop on 411 S5

[Thomas591]

I have a problem with too low ideling in City traffic. I found out that when the electric fans in front of the radiator are switched on, the voltage on the ignition coil drops to 9,5 Volt when ideling. The result is a drop of 200 rpm, making the Engine run too Slow.
To solve this problem I am thinking to activate the ignition coil ballast relay every time the fan relay is activated..

Is my idea of a solution practical?
Will I fry the ignition coil on the Long run?
Any other ideas?

Regards

Thomas