6ES7954-8LE03-0AA0 is the S7-400 PLC backup battery module from Siemens. This module is used to store PLC programs and data to maintain data integrity during power outages or maintenance. The backup battery module is usually composed of lithium batteries and connected to the backup battery slot of the PLC. It provides long-lasting power to ensure that the PL can continue to operate even after a power outage, and can protect stored programs and data from loss.