Battery Module 6ES7291-8BA20-0XA0 6ES7 291-8BA20-0XA0 6ES7291-8BA2O-OXAO

6ES7291-8BA20-0XA0 | SIMATIC S7-200 Battery Module BC 291 — Long-Term Data Buffering / Memory Module Slot / S7-22X CPU 

What Data the Battery Protects — and Why Duration Matters

The SIMATIC S7-200 CPU retains certain data types in RAM: data block (DB) contents, time-of-day clock values, retentive bit memory, retentive timers, and retentive counters. All of these depend on power to survive. During normal operation, a built-in supercapacitor in the S7-22X CPU maintains this data through short interruptions — typically for a few days at room temperature.

For longer power-off periods — planned shutdowns, seasonal outages, extended maintenance windows, equipment stored as cold spare — the supercapacitor's charge is not sufficient. The BC 291 battery module extends the data retention period from days to months. While the battery is installed and the CPU is powered off, all designated retentive data remains intact regardless of the outage duration.

This is not about programme retention. The user programme in the S7-22X CPU is stored in EEPROM and survives power loss indefinitely without a battery. The BC 291 specifically protects the retentive run-time data that resides in RAM.

Key Specifications

Application Scenarios

Seasonal production line: A packaging machine runs seasonally — production stops for three months during the off-season. The BC 291 maintains all retentive data (batch counters, process parameters stored in retentive data blocks) through the shutdown. When production resumes, the machine restarts with its data intact.

Cold spare CPU management: A maintenance department keeps a pre-configured S7-224 CPU as a cold spare. The BC 291 maintains the retentive data in the spare unit while it sits in storage, so it can be fitted to the machine without requiring a full data reload from the SCADA or programming PC.

Extended plant turnaround: A process plant performs a 6-week annual maintenance shutdown with complete electrical isolation of control panels. The BC 291 preserves all retentive process data through the shutdown period.

FAQ

Q1: What is the battery chemistry and shelf life of the BC 291?

The BC 291 uses lithium-based battery chemistry, classified under REACH Article 33 regulation for the presence of a controlled organic solvent in concentrations exceeding 0.1% by weight. The battery's service life depends on ambient temperature during storage and operation. Check the production date on the module label; modules stored for several years before installation may have reduced remaining capacity. Install a fresh BC 291 at each planned maintenance interval if long-term data retention is critical.

Q2: Does the BC 291 also retain the user programme?

No. The user ladder or FBD programme in the S7-22X CPU is stored in EEPROM and is non-volatile — it is retained indefinitely without any power source. The BC 291 only extends the retention of retentive RAM data: retentive bit memory, data block values, retentive timers and counters, and the real-time clock. Separately, an optional EEPROM memory cartridge (6ES7291-8GC00-0XA0 or similar) provides an additional programme backup layer but occupies the same slot as the BC 291.

Q3: How can the CPU's retentive data be confirmed after BC 291 installation?

After fitting the BC 291 and cycling power, use STEP 7 Micro/WIN (or STEP 7-MicroWIN SMART for SMART variants) to read back the CPU's data block and retentive memory contents via the programming cable. Compare with the expected values before shutdown. This confirms that the battery maintained the data during the interruption.

Q4: Can the BC 291 be hot-swapped with the CPU powered on?

The recommended procedure is to install or remove the BC 291 with the CPU powered off. Inserting or removing the battery with power on is not recommended — the battery slot shares the housing opening that provides access to the CPU's internal circuits, and the handling risk to adjacent circuitry outweighs the benefit of a hot-swap.

Q5: What replaces the BC 291 for new S7-200 systems?

The 6ES7291-8BA20-0XA0 was discontinued in April 2021, in line with the overall end-of-life of the SIMATIC S7-200 platform. For new machine designs, the S7-200 SMART and SIMATIC S7-1200 platforms have their own data retention mechanisms — SMART CPUs use a built-in capacitor and optional memory cartridges, while S7-1200 CPUs use retentive memory with load memory card backup. Existing S7-200 installations can source BC 291 modules from industrial spare parts channels.