S7-400 Battery Selector Switch: Positions and Monitoring Setup

Overview of S7-400 Power Module Battery Backup

The Siemens SIMATIC S7-400 PLC retains RAM content, retentive bit memory, internal timers and counters, and the diagnostic buffer through a backup battery system integrated into the PS 405 (AC) or PS 407 (DC) power supply module. The battery compartment accepts up to two 3.6 V lithium cells in parallel. A three-position slide switch on the front of the power supply selects how the internal battery-monitoring circuit operates. Choosing the correct switch position is essential for reliable CPU data retention, predictable fault annunciation via the BAF (Battery Fault) LED, and safe, alarm-free battery replacement.

This article documents the meaning of each switch position, the relationship between switch position and the BAF LED, the diagnostic-buffer entries generated by the power supply, the recommended position for normal operation, and the controlled procedure for hot-swapping batteries without losing the CPU's retentive data. It applies to S7-400 PS 405 4 A, PS 405 10 A, PS 407 4 A, and PS 407 10 A modules that are equipped with a battery compartment and a BAF LED. PS modules without a battery compartment do not have the switch and do not provide backup after line-power loss. For the full module data and wiring, refer to the SIMATIC S7-400 Automation System Manual.

Hardware Reference and Module Variants

The selector switch is fitted only to PS modules that ship with a battery holder. Confirm the variant before assuming backup-battery support. The relevant MLFBs are listed below; the battery holder, the BAF LED, and the selector switch are present in each of the variants shown.

The backup cell itself is a 3.6 V lithium AA-size cell, Siemens MLFB 6ES7971-0BA00 or a manufacturer-equivalent with the same chemistry, voltage, and form factor. The two cells are wired in parallel inside the battery holder; the module does not balance them. The cells begin supplying the CPU RAM-retention current the moment they are inserted, even before the PS module is powered from line. Detailed module data is in the S7-400 Module Data Manual.

The Three-Position Battery Selector Switch

The selector switch is a small three-position slide switch on the front of the power supply, just below the battery holder. It is labelled, in silkscreen, with three positions:

• 1 BATT — the monitoring circuit supervises only the cell in the upper holder (battery 1).
• 2 BATT — the monitoring circuit supervises both cells; the BAF LED is lit if either cell is below the alarm threshold.
• OFF — the monitoring circuit is disabled; the BAF LED is forced off and no battery-diagnostic events are generated.

The switch has no effect on the physical backup function. As long as a healthy cell is installed in either holder, the CPU is held in RAM retention when line power is lost. The switch only changes what the module reports about the cells. This is the single most important concept in the topic: the position you choose does not make the backup better or worse — it changes only the alarm behaviour.

Position: 1 BATT (Single-Cell Monitoring)

In the 1 BATT position, the monitor measures only the cell in the upper holder. If that cell's terminal voltage drops below the alarm threshold of the monitoring circuit, the front-panel BAF LED is lit and a diagnostic event is written to the CPU's diagnostic buffer. The lower cell, if fitted, is still electrically in parallel and will provide backup, but the monitoring circuit never inspects it. A deeply discharged, missing, or reversed lower cell therefore produces no BAF alarm and no diagnostic event.

Use the 1 BATT position when only a single cell is fitted, or when the system designer wants a clean, low-noise alarm path during a period when the lower cell is intentionally empty. It is not the recommended position for a fully populated, two-cell installation: it silently disables half of the alarm function for which the second cell was purchased. Engineers migrating from a single-battery cabinet should leave the switch at 1 BATT only until the second cell is installed, then move it to 2 BATT.

Position: 2 BATT (Dual-Cell Monitoring - Recommended)

In the 2 BATT position, the monitor checks both cells. The BAF LED is lit and a diagnostic event is generated as soon as either cell falls below the alarm threshold. This is the most conservative and the most informative mode of operation, and it is the position Siemens recommends for normal service. With two healthy cells installed, a single cell failure is detected early; the remaining cell continues to hold the CPU in RAM retention, and the maintenance crew has time to schedule a battery replacement before the second cell also discharges.

Two practical rules follow from this:

1. Always fit two cells when the holder accepts two.
2. Replace both cells at the same service interval, and replace both whenever one cell has triggered a BAF alarm. Mixing cells of different ages defeats the redundancy the switch is designed to monitor.

Position: OFF (Monitoring Disabled)

In the OFF position, the monitoring circuit is fully disabled. The BAF LED is not driven and no battery-diagnostic events are written to the diagnostic buffer. The cells, if fitted, still provide backup as long as they are healthy. This position exists for one purpose only: to silence the BAF LED and the diagnostic event that would otherwise fire the moment a cell is half-disconnected during a hot-swap.

Leaving the switch in OFF during normal operation is a maintenance error. The next genuine battery failure will be silent: the CPU will still be held in RAM by the remaining healthy cell, but no annunciation will be generated, no diagnostic event will be written, and the failure will not appear in any maintenance log. After the cell replacement is complete, the switch must be returned to 2 BATT (or 1 BATT, if only one cell is fitted) and the BAF LED verified off.

BAF LED and Diagnostic Buffer Events

The BAF (Battery Fault) LED on the front of the PS module is the only local annunciator. Its behaviour in each switch position is summarised below.

In addition to the local LED, the PS module reports battery status to the CPU over the backplane. The CPU in turn writes an event to its diagnostic buffer. The exact event text and identifier vary between CPU firmware versions, but the typical entries seen in STEP 7 or TIA Portal are:

• "Backup battery exhausted"
• "Battery 1 failure"
• "Battery 2 failure"
• "Backup voltage too low"

On CPUs with an SF (System Fault) LED, a battery fault may also drive the SF LED depending on the CPU firmware. In STEP 7 v5.x, view the diagnostic buffer via PLC › Diagnostics/Settings › Diagnostic Buffer. In TIA Portal, navigate to the online device, then Online & diagnostics › Diagnostics › Diagnostics buffer. The event timestamp is wall-clock from the CPU, not from the PS module.

Hot-Swap Battery Replacement Procedure

The cells are intended to be replaced with the PLC powered, so that the CPU never loses backup. A typical hot-swap sequence is:

1. Verify that line power to the PS module is healthy. If the PS module itself is unpowered, the cells are the only thing holding the CPU in RAM; stop and restore PS input power before proceeding.
2. Slide the selector switch on the front of the PS module to OFF. This prevents the transient low-voltage condition of a half-disconnected cell from triggering a false BAF alarm and a diagnostic-buffer entry that has to be acknowledged and explained later.
3. Open the battery compartment cover.
4. Remove the first cell. Note the polarity marking on the holder; the new cell is installed in the same orientation.
5. Insert the new cell, observing polarity. Hold the cell in place until the retainer clip engages; a partially seated cell can produce an intermittent contact and a phantom BAF fault.
6. Repeat for the second cell if both are being replaced.
7. Close the battery compartment cover.
8. Slide the selector switch back to 2 BATT (or to 1 BATT if only one cell is fitted).
9. Verify that the BAF LED is off and that the CPU diagnostic buffer is free of any new battery entry since the switch was moved.
10. Record the replacement date, the cell lot or supplier, and the next scheduled replacement on the maintenance label of the PS module and in the plant CMMS.

Commissioning and Maintenance Best Practices

For a new S7-400 installation, or for a PS module being put back into service after a long outage, follow this sequence:

1. With line power off, install the cells. The cells begin supplying the CPU as soon as they are inserted, even before the PS module is powered from line.
2. Leave the switch in OFF while the CPU is first powered up. This avoids false BAF events during the first few minutes of operation as internal rails stabilise.
3. Apply line power and wait for the CPU to complete its start-up sequence (STOP or RUN, depending on the project).
4. Move the switch to 2 BATT.
5. Confirm the BAF LED is off and that the diagnostic buffer contains no new battery entry since the switch was moved.
6. Document the cell lot, installation date, and expected replacement date in the plant maintenance system.

For periodic maintenance, a simple checklist is sufficient:

Cell self-discharge is strongly temperature dependent. Cells installed in a cabinet that runs above 40 °C age significantly faster than cells in a 25 °C cabinet, and the maintenance interval must be shortened accordingly. Cells stored at room temperature for longer than the manufacturer's published shelf life should be discarded rather than installed; the shelf life is typically several years for fresh lithium thionyl chloride cells but the exact value depends on the cell supplier.

Troubleshooting Matrix

Field-Proven Caveats and Environmental Notes

• The selector switch is a small slide switch and is easy to bump into the OFF position during maintenance. After any work near the PS module, verify the switch position visually and check the BAF LED.
• The BAF LED is a single, shared indicator. It does not tell you which of the two cells is low. If you need to know which cell to replace, you must lift the cover and measure each cell's open-circuit voltage with a calibrated meter, or temporarily lift one cell at a time and watch the BAF LED.
• Cell self-discharge is strongly temperature dependent. A cabinet at 50 °C ages the cells roughly twice as fast as a cabinet at 25 °C. Adjust your maintenance interval accordingly.
• If the PLC has been stored without line power for longer than the cell shelf life, replace both cells before applying line power. The cells may have self-discharged below the alarm threshold and the switch will report a fault the moment monitoring is enabled.
• Some PS 405/PS 407 variants ship without a battery compartment. In those variants, retentive data is held only while line power is present; the CPU will not survive a power outage. Confirm the MLFB before commissioning.
• Do not use the OFF position to "save the cells" during a long shutdown. The cells are not measurably discharged by the monitoring circuit; the OFF position only silences the alarm.
• When the PLC is decommissioned, remove the cells before the module is scrapped. Lithium cells are hazardous waste and must be disposed of per the cell supplier's instructions and local regulations.
• The S7-400 backup architecture differs from the S7-300 (single battery on the CPU) and from the S7-1500 (no serviceable backup battery; data is held in non-volatile memory or in the SIMATIC memory card). Engineers migrating a working program from one platform to another should not assume the new platform supports the same battery-based retention model.

FAQ