The Cosmic Ray Energetics And Mass experiment for the International Space Station (ISS-CREAM) is scheduled to be launched and installed on the ISS in August 2017, and will carry out a measurement of the energy and composition of energetic cosmic rays in space. The Silicon Charge Detector (SCD) will identify the charge of through-going cosmic rays. It consists of four layers, each with 2688 silicon pixels and associated electronics. The ISS-CREAM payload was delivered to the launch site, Kennedy Space Center, in August 2015 after the successful completion of integration and space environment tests. A heavy-ion beam, required to verify the capability of precision charge measurement of the SCD, became available at the European Organization for Nuclear Research (CERN) in November 2016. A prototype instrument using the same types of silicon pixel sensors and electronics installed in the SCD was placed in a heavy-ion beam composed of secondary ions ranging from helium to zinc. We present the charge resolution for each ion as a function of the number of layers used for charge measurement so that the improvement in charge resolution is clearly demonstrated as the number of layers for charge measurement increases.