control system for battery

I'm working on grid level electrical storage. The modelling process is being performed in simulink and i began using for the first time this week. Ver R2010a is being used. The storage model control system is meant to check the network voltage (p.u) and then act according to one of the 3 ways; 1] discharge the battery if the voltage is low i.e. below a specified limit of say 0.94 or 2] charge the battery if the voltage is high i.e. above a specified limit of say 0.97 so as to restore the voltage between the predefined limits or 3] take no action if the voltage is within limits

Could i get some help in designing the system.

thanx