Galvanic cell under external voltage

[akaSidewinder]

Hi! I am new in fe modelling. I modell galvanic cells with comsol multiphysics. i use the ac/dc module. I have an electrolyte surrounded by boundaries. most of them insulated, others with current-voltage- relationships. I simulate the voltage- and currentdistribution over the electrolyte, depending on different current-voltage-relationships. This works out fine so far. what i want to do now is to apply a fixed voltage between several boundaries. fixed potentials or currents as bc do not fulfil my needs, because the currentoutput over the boundary will not be constant.
i hope i discribed my problem clearly and please excuse my bad english.

Thanks for any kind of help!
greetings Chris