A Cosmic Muon Veto Detector (CMVD) is being built around the mini-Iron Calorimeter (mini-ICAL) detector at the transit campus of the India based Neutrino Observatory, Madurai. The CMV detector will be made using extruded plastic scintillators with embedded wavelength shifting (WLS) fibres which propagate re-emitted photons of longer wavelengths to silicon photo-multipliers (SiPMs). The SiPMs detect these scintillation photons, producing electronic signals. The design goal for the cosmic muon veto efficiency of the CMV is 99.99% and fake veto rate less than 10. A testing system was developed, using an LED driver, to measure the noise rate and gain of each SiPM, and thus determine its overvoltage (). This paper describes the test results and the analysed characteristics of about 3.5 k SiPMs.