@User it seems the charge conserving capacitors (Cxxx n1 n2 q={expr} ) used in sky130 models are being implemented in ngspice. Will try to make some tests...

Hi Stefan,
I uploaded a first attempt for a non-linear capacitor with charge formulation in pre-master branch. It is similar then the existing capacitor model with capacitance formulation.
I made checks with different non-linear examples in time domain and with the both sky130 varicaps too.
Please make a check and give me your opinion.
Dietmar