From Technologic Systems Manuals

The TS-8551 implements a M41T00S STMicro Battery Backed RTC using an external and replaceable coin cell battery. This RTC is connected to the CPU via I2C and is handled by the kernel and is presented as a standard RTC device in linux.

For proper operation of the RTC, a non-rechargeable CR1632 coin cell must be set in the battery holder on the TS-8551. This will allow the RTC to maintain time when the system is powered off.