The 15598 powers the clock and SRAM when the primary 12 Vdc power supply is not connected. This 3 V lithium coin cell battery needs to be periodically replaced. Campbell Scientific recommends replacing the battery every five years.
The 27307 is a service for upgrading a CR200 datalogger to a CR200X. Customers with a CR200 datalogger that has a serial number of 5405 or lower will also need to purchase the 19345 4M Memory Upgrade (see below).