What is the purpose of a battery backup in a PLC?

• A. A battery that allows the memory to retain its values when PLC is turned off
• B. A device that stores the program on external disk
• C. A device that increases PLC speed
• D. A power supply for continuous operation

Answer: (B)

The main idea is that a battery backup keeps the PLC’s memory alive when the main power is lost, so important data isn’t lost during a power outage. This backup powers the volatile memory long enough to retain values like timer counts, counters, and other retentive data, allowing the PLC to resume with its previous state when power returns. The program itself is typically stored in non-volatile memory, not on an external disk, so a battery backup isn’t primarily for storing the program. It also isn’t meant to increase speed or provide constant operation; its job is to preserve memory contents across power interruptions.