Resolving S7-300 CPU 315 BATF LED: Battery Fault Diagnostics

1. Problem Summary

The BATF (battery fault) LED on a Siemens SIMATIC S7-300 CPU 315 remains illuminated after the backup battery has been replaced. The PLC continues to execute the user program normally, but every time the supply voltage is removed the work memory is lost and the program must be reloaded from the SIMATIC Memory Card (MMC) or a PG/PC. The fault is not cleared by a power cycle, a STOP/RUN transition, or a memory reset (MRES). This article documents the diagnostic path, the underlying hardware conditions that drive the BATF indicator, and the migration option to a maintenance-free MMC-only CPU variant.

2. BATF LED Behaviour and Definitions

The BATF LED is part of the CPU status display alongside SF, BF, DC5V, FRCE, RUN, and STOP. According to the SIMATIC S7-300 CPU 312 IFM through CPU 318-2 DP manual (chapter 1.3.2), the BATF LED is lit when:

• The backup battery is missing.
• The backup battery is faulty (open-circuit voltage below the internal monitoring threshold).
• The backup battery is not yet charged (relevant for the optional rechargeable buffer on certain older 6ES7-315 variants).
• A rechargeable battery is installed but the user program is not backed up by the rechargeable cell — i.e. the BATF line is also driven when the CPU detects a rechargeable battery where a primary lithium cell is expected for that firmware.

The BATF LED has no effect on the cyclic program execution. It is purely a diagnostic indication. However, a sustained BATF condition means the retentive areas (M, T, C, DB instance data marked as retentive) are not protected against power failure. After every STOP-to-RUN or power-off-to-power-on transition the CPU will start in a cold-restart state.

3. CPU 315-2 DP Hardware Identification (MLFB)

Always verify the exact MLFB (Maschinenlesbare Fabrikatebezeichnung, the Siemens order number) before ordering parts. The 6ES7-315 family has multiple hardware revisions with different battery requirements and MMC support:

Only CPUs from firmware V2.0 onwards (6ES7 315-2AG10 and later) support operation without a backup battery when an MMC is inserted. On older hardware, removing the battery with no MMC installed will drive BATF and clear retentive data on every power-down.

4. Battery Types, Voltage, and Shelf Life Specifications

The Siemens-approved backup cell for the S7-300 family is a 3.6 V lithium-thionyl-chloride (Li-SOCl₂) primary cell. The reference part is 6ES7 971-1AA00-0AA0 (single cell, 2.3 Ah) with a self-discharge rate of approximately 1 % per year at 25 °C. The 6ES7 971-1BA00 and 6ES7 971-1CA00 designations are equivalent in function with different harness lengths.

5. Diagnostic Procedure: Step-by-Step

1. Read the MLFB. Open the cabinet, photograph the front of the CPU, and record the 6ES7 order number including the -0AB0 suffix. Without this, you cannot select the correct battery or firmware.
2. Pull the existing battery and measure. With the supply OFF, eject the battery from its holder. Set a digital multimeter to DC V and measure the open-circuit voltage. Reading < 2.7 V: cell is dead. Reading 2.7–3.3 V: marginal, will not sustain BATF OFF. Reading 3.4–3.65 V: nominal.
3. Inspect the holder. Look for green-white corrosion on the spring contacts, particularly the negative return. Even a fine oxide layer breaks the sense loop and BATF will stay on.
4. Inspect the cable harness. The 6ES7 971-1AA00 lead is a 200 mm two-wire harness with a 2-pin Molex-type connector. Wiggle-test with the meter connected — an intermittent open-circuit reading (OL on the meter) indicates a broken conductor at the strain relief.
5. Insert a known-good battery. Use a cell with a verified date code and a measured open-circuit voltage > 3.5 V. Match the chemistry exactly: do not substitute an alkaline AA cell or a NiMH rechargeable cell unless the CPU is rated for the rechargeable option (some very early 6ES7-315 revisions shipped with a NiMH back-up that is recharged from the 5 V rail).
6. Power-cycle. Apply 24 V DC, observe BATF during power-up. The LED should extinguish within 3–5 seconds of RUN.
7. Read the diagnostic buffer. Connect STEP 7 V5.x or TIA Portal and navigate to Online > Diagnostic Buffer (CPU > Diagnostic Buffer). Look for entries with Event ID 13B1 hex (battery voltage low) or 13B2 hex (battery exhausted). See Section 6.
8. Force a battery test. From STEP 7, execute PLC > Diagnostics > Battery Test. The CPU draws a brief load through the cell and reports PASS / FAIL.

6. Diagnostic Buffer Event ID Interpretation

The diagnostic buffer is the authoritative record of the battery state. The CPU logs the following events when the battery monitoring hardware transitions state:

If the diagnostic buffer shows repeated 13B1 / 13B2 entries even with a freshly measured-good battery, the cause is almost always the sense-loop contact resistance inside the holder, not the cell itself.

7. Socket and Connector Inspection

The CPU senses the battery by sourcing a small test current (typically 10 µA) through the positive lead and measuring the drop across an internal resistor. Anything above ~2.7 V at the CPU end clears BATF; anything below keeps it lit. The internal resistor is in series with the cell, so any additional resistance in the holder raises the trip threshold.

Field-proven checks:

• Clean the holder with isopropyl alcohol and a non-abrasive swab. Do not use contact cleaners that leave residue.
• Measure the closed-loop resistance of the holder with the battery removed. With a fresh cell inserted, the reading across the two holder pads should be < 0.5 Ω. A reading of 1–5 Ω is enough to hold BATF on.
• Apply a thin film of NO-OX-ID or a similar contact lubricant to the spring pads. Do not use petroleum jelly; it traps chloride ions and accelerates corrosion.
• Bend the spring contacts slightly upward so they exert positive pressure on the cell button. The springs lose tension over years of vibration.

8. Battery Replacement Procedure

1. Confirm the cabinet is in a safe state. The CPU may be in STOP, but the outputs retain their last value only if a battery is present.
2. Open the hinged battery cover on the front of the CPU.
3. Disconnect the 2-pin plug from the CPU. Do not pull on the wires.
4. Remove the old cell from the holder.
5. Insert the new 6ES7 971-1AA00 cell, observing polarity (red = +, black = −). The connector is keyed.
6. Reconnect the plug. Listen for the positive click.
7. Close the cover.
8. Restore 24 V DC supply. The BATF LED should extinguish within 5 seconds of RUN.
9. Clear the diagnostic buffer from STEP 7 to remove the legacy 13B1 / 13B2 entries.

9. MMC Migration Path (Firmware ≥ V2.0)

If the CPU is a hardware revision that supports MMC-only operation, the BATF LED can be permanently cleared by inserting a Siemens MMC (6ES7 953-8LFxx-0AA0 or larger) and re-flashing the project. The CPU stores the user program and retentive data on the MMC, eliminating the need for a battery.

Migration steps:

1. Confirm the CPU MLFB supports MMC. See the table in Section 3. The 6ES7 315-2AF03 does not support MMC-only operation and will continue to require a battery.
2. Insert the MMC into the slot under the cover. The slot is spring-loaded.
3. From STEP 7, select PLC > Download User Program to Memory Card.
4. Perform a memory reset (MRES) to wipe work RAM.
5. Power-cycle. The CPU reads from MMC and BATF remains OFF because no battery sense current is drawn.
6. Verify retentive data retention by removing 24 V DC for 60 seconds, then restoring. The M, T, C, and DB retentive areas should restore to their pre-power-down state.

For older 6ES7 315-2AFxx variants that cannot run without a battery, the only permanent remedy is to replace the CPU with a current MLFB such as 6ES7 315-2AH14-0AB0. This is the long-term Siemens-recommended path for installations where the BATF LED has become a recurring maintenance burden.

10. Verification and Test Sequence

After replacement or migration, run the following verification sequence before returning the system to production:

1. Power-up with MMC inserted. Confirm BATF OFF within 5 seconds.
2. Read the diagnostic buffer. Confirm no 13B1 or 13B2 events in the last 24 hours.
3. Force a battery test from STEP 7: PASS required.
4. Power-down test: remove 24 V DC for 60 seconds, restore, confirm retentive values intact.
5. Extended power-down test (for critical plants): 8-hour power-down with field measurement of the loaded battery voltage. The voltage at the CPU end should remain > 3.0 V for the entire duration.
6. Document the date code and open-circuit voltage of the new battery in the maintenance log. The replacement interval is 5 years preventive.

11. Troubleshooting Matrix

12. Long-Term Recommendations

For plants with a fleet of S7-300 CPUs, build a battery programme around three rules:

• Date-code on receipt. Reject any backup cell whose manufacture date is more than 12 months before receipt. Open-circuit voltage on receipt must be > 3.5 V.
• 5-year preventive replacement. Replace every backup battery at year 5 regardless of diagnostic buffer state. The marginal cost of a lithium cell is far less than the production loss from a cold-restart during an unplanned outage.
• Migrate at end-of-life. Whenever a CPU 315-2AF is replaced, substitute a current 6ES7 315-2AH14-0AB0 with MMC to eliminate the battery maintenance item entirely.

For the underlying Siemens documentation, refer to the SIMATIC S7-300 CPU 312 IFM to CPU 318-2 DP manual (entry ID 21990218 on Siemens Industry Online Support), the durability and storage-life FAQ (67466247), and the S7-300/400 backup battery replacement FAQ (67466477). The Siemens Function Manual "S7-300 CPU 31xC and CPU 31x: Installation" contains the official replacement procedure and disposal guidance for Li-SOCl₂ cells.

FAQ