We discuss the design and properties of a proposed planar cryogenic permanent magnet undulator with 20 mm period length called CPMU-20. The undulator is set to use (Pr,Nd)2Fe14B as permanent magnet material and Permendur poles and is set to be part of the planned SoTeXS beamline at the BESSY-II upgrade which will offer a unique working environment for research into energy-materials – especially energy-storage materials. The CPMU-20 is designed to produce high photon fluxes in the energy range of 0.5 to 5 keV with a maximum K-value of 2.2 which permits research into a wide range of materials used in state of the art batteries. The optimization process that led to the specific device properties like the period length, the width of the poles and the end-magnet configuration – which ensures an aligned electron beam through the device for the whole gap-range from 6 to 22 mm - will be presented in detail. This includes a discussion of the usage of the UNDUMAG and WAVE software written by Michael Scheer for the optimization and simulations.