“While most PCs use a RTC/CMOS chip that has an external battery, many industrial x86 computers use RTC/NVRAM modules. They’re convenient when new, since the battery and RTC crystal are encapsulated into the module. The module’s potting keeps the lithium cell from drying out as quickly in high temperatures, extending its life. Since the cell is potted, in the unlikely event of a cell rupture or leak, the system won’t be destroyed by battery electrolyte. The downside is, the battery eventually dies, and often the NVRAM is used for storing some critical parameter that keeps you from booting. This was the case with the Multitech industrial 486 single-board computer on my bench today: it uses a Dallas DS1387 RTC/NVRAM module, which is similar to the DS1287 but with more NVRAM (4K vs. 114 bytes). Unfortunately, the DS1387 has no currently available replacement. The chip used in the module was produced as a bare IC, requiring external battery and crystal, but that part is also no longer available.”