The 7084VP+ (orange body) is the MISCO Glycol and Battery Tester. The battery for the light-source on this refractometer is contained in the ViewPoint Illuminator, a hinged coverplate, on the front of the refractometer. The ViewPoint Illuminator is completely sealed and cannot be opened. Although you cannot change the battery itself, you can purchase a replacement ViewPoint Illuminator. Just look for “ViewPoint” in the refractometer accessories category on this site. The replacement kit comes with complete instructions on how to replace the ViewPoint.
ViewPoint Illuminator for Traditional Analog Refractometer – Replacement Assembly