Siemens S5-95U Battery Replacement Without Losing Program

Siemens S5-95U Battery Replacement Without Losing Program

The SIMATIC S5-95U compact PLC (order number 6ES5 095-8MA03) holds its STEP 5 program, data blocks, flags, timers, and counters in volatile RAM that is sustained by an internal lithium backup cell. The single rule that decides whether the program survives a battery swap is documented in paragraph 4.4.2 of the S5-90U/S5-95U system manual: "Only insert or replace the battery while the PLC is switched on. Otherwise, an OVERALL RESET is required after POWER ON." This article translates that rule into a concrete field procedure, with verification, recovery, and diagnostics for engineers who must service the cell on a running machine.

S5-95U Hardware and Battery Backup Architecture

The S5-95U integrates CPU, 16 digital inputs (24 V DC sink/source selectable per channel), 16 digital outputs (relay or transistor variant by order suffix), and a configurable analog I/O footprint into a single housing. Two power sources keep RAM valid: the onboard 24 V supply when the PLC is running, and the 3.6 V lithium backup cell when mains is removed.

Without the backup cell, any power outage or battery removal while mains is OFF forces the CPU into an OVERALL RESET ("Gesamtlöschung") on the next POWER ON. OVERALL RESET clears:

• All RAM-resident OB, FB, FC, PB, SB, and DB blocks
• Bit memory, timers, and counters
• Process image
• Retentive flag area as defined in the program
• NET configuration and MPI interface parameter block
• Analog I/O scaling constants held in RAM

Only EPROM, EEPROM, or plug-in Flash card blocks survive OVERALL RESET. Programs loaded into RAM through the programmer (PG) are wiped; programs written into EPROM with a programmer's EPROM writer survive. Hot-swap with the cell energized is therefore the supported path for routine battery replacement.

Backup Battery Specifications and Part Numbers

Service life of a new cell under three profiles:

• Continuous 24 V mains applied, CPU always powered: 1 year minimum before BATT LED asserts (often 2–3 years in practice)
• Cyclic duty (8 h on / 16 h off): 3+ years typical
• Cold-storage profile (10 °C): 5+ years typical

Memory Map and What the Battery Actually Holds

If the S5-95U is configured with a user-program EPROM (e.g., a 6ES5 376-... EPROM card) and that EPROM is the working copy, OVERALL RESET will not destroy the program. If the working copy was transferred from the PG into RAM and never EPROM-burned, OVERALL RESET wipes it.

Prerequisites Before You Touch the Battery

1. A fresh backup cell meeting the spec in Battery Specifications; DMM-measured open-circuit voltage ≥ 3.50 V.
2. A small flat-blade screwdriver for the S5-95U terminal cover; no metallic tools near live power; ESD-safe preferred.
3. ESD wrist strap or grounded ESD mat.
4. A PG backup of the program on disk, with a timestamped filename. Archive a printed protocol if your site's policy requires it.
5. Operator sign-off that the machine is in a safe state. The upstream ESTOP or hold circuit is engaged. No one will trip the breaker during the swap.
6. Clean dry work surface. Cell contacts must not be touched with bare fingers.
7. Documentation of the S5-95U's MPI / PROFIBUS / RS485 net address and any partner definitions.
8. A free DMM (true-RMS preferred, 600 V minimum rating, ≥ 1000 counts).
9. If the S5-95U is networked: confirm the partner PLCs tolerate one slave disappearing from the bus for a brief moment.
10. A printed step-by-step copy of the procedure (this article, or the manual excerpt §4.4.2).

Step-by-Step Replacement Procedure

The complete swap, with cell voltage pre-check and post-install verification, takes about 5 minutes for an experienced engineer.

Step 1 — Pre-check the new cell

1. Unpack the replacement cell on the ESD mat.
2. Measure the open-circuit voltage across the cell pigtail. Reading must be ≥ 3.50 V DC. Normal range is 3.50 V–3.66 V at delivery.
3. Confirm the polarity of the mating connector. The plug key sits on the positive side; do not force the lead into the socket.

Step 2 — Verify the S5-95U is RUN and stable

1. Read the front-panel LED state. Expected:

• PWR solid green, 24 V rail nominal.
• RUN solid green.
• BATT may be solid amber; that indicates the existing cell is under voltage and is expected during the swap.

2. Confirm with the operator that the controlled process is paused and the ESTOP/hold chain is active.
3. If BATT is NOT amber before the swap, the existing cell is still healthy — note this on the work order.

Step 3 — Open the battery cover

1. Open the orange-coded terminal cover on the S5-95U housing. The cell compartment sits behind the same panel that houses the PG port.
2. Locate the black 2-pin keyed connector latched into the on-board socket. Gentle pressure releases the connector; do not pull on the wires.
3. Confirm wire orientation: red = +, black = −. The connector only fits one way but document orientation.

Step 4 — Hot-swap the cell (PLC remains powered)

This is the only step that decides whether the program survives. The PLC must remain powered throughout.

1. Press the small connector latch and pull the old cell plug straight out.
2. The CPU enters a brief internal battery-fail window. The BATT LED may blink or stay solid. Logic program execution is uninterrupted because the on-board bulk capacitor bank retains the RAM image for approximately 30 seconds without a battery.
3. Within 10–15 seconds, plug the new cell into the keyed connector until the latch clicks.
4. Observe the BATT LED: it must extinguish within 3 seconds. If it stays lit, the cell is low or the plug is not seated; reseat and recheck.

The 10–15-second window between unplug and plug is well within the on-board bulk-capacitor hold-up. Do not exceed 20 seconds. If the plug cannot be inserted within that window, cease the swap and reschedule for a planned maintenance window with 24 V retained.

Step 5 — Reassemble and document

1. Tuck the cell lead into the compartment. Do not pinch the lead against the housing edge.
2. Close the terminal cover.
3. Mark the cell or holder strap with install date (YYYY-MM-DD) and the voltage measured at install.
4. Update the CMMS / maintenance log with cell part number, install date, and engineer signature.

Post-Replacement Verification

5.1 LED check

5.2 Program integrity check from PG

1. PG online connection established to the S5-95U.
2. PG → Online → Block List. Confirm every block that was originally present is still listed.
3. Read the system status byte (PG "Status" readout). Confirm the BATT bit is 0.
4. Toggle a representative M flag or counter, observe it retain state through a momentary STOP / RUN cycle (this exercises the RAM-only retention path).

5.3 Optional cold-start test

1. Mode switch to STOP.
2. Cycle 24 V supply off for 10 s, back on.
3. The CPU must come up directly to RUN without OVERALL RESET. Block list identical before and after.

If the cold-start test trips SF or wipes the program, the cell was discharged in transit. Treat the freshly installed cell as suspect, replace with a verified cell, and raise an NCR with the cell supplier.

OVERALL RESET Recovery Procedure

If 24 V was lost during the swap window or for any reason the CPU performs OVERALL RESET, recovery is as follows.

1. Front-panel mode switch to STOP.
2. Bring up the PG (or connect a fresh PG cable).
3. Read STATUS: all blocks will appear as "deleted"; program memory empty.
4. Reload the program from the PG backup taken in the prerequisites. Transfer every OB, FB, FC, PB, SB, and DB present in the source file.
5. If the program lives in an EPROM card fitted in the S5-95U, that EPROM is untouched. Power off, reseat the EPROM card, restart; the CPU will load the EPROM-resident copy directly.
6. After block transfer, perform a manual STOP → RUN transition. The CPU performs a cold restart; OB 21 or OB 22 executes per the restart-up mode configured.
7. Verify the block list against the source; verify RUN is clean with SF off.
8. Re-check system status for BATT bit and PIQ fault register.

Battery Status Bits and Front-Panel Diagnostics

The S5-95U exposes battery health on two channels: a hardware LED and a status flag available to the STEP 5 program.

6.1 Hardware LED (BATT)

The BATT LED on the S5-95U front panel lights amber when the cell voltage at the sense pin falls below the brownout threshold (nominally 2.85 V). The LED is a soft warning: logic execution continues; only backup during 24 V loss is no longer guaranteed.

A BATT LED that is solid before replacement simply confirms the existing cell is past its useful end-of-life. Replacement is justified.

6.2 Software status flag (S-flag, firmware-dependent)

The exact bit depends on firmware suffix. Verify in your CPU's STEP 5 System Documentation:

Example STEP 5 logic to capture the cell fault and route it to a panel flag:

A    S 35.0
JC   BATT_LOW
BEI

// BATT_LOW can be wired to an output, a flag, or a data word that the
// HMI / SCADA layer monitors.

6.3 PIQ parity on cold start

If the S5-95U reports a process-image parity error (PI / PIR fault) on cold start, the cell was empty at the moment of power-down. The CPU latches in STOP with SF on. Clearing the fault requires:

• Successful cell replacement (the new cell now holds the image).
• Manual STOP / RUN cycle via the mode switch.

Repeated PI parity after a fresh cell and STOP / RUN cycle indicates the on-board 3.3 V bulk capacitor is also failing — a main-board repair or full unit refurbishment is appropriate.

Maintenance Schedule and Battery Life Cycle

• Calendar change: every 2 years regardless of BATT LED state, on critical-service units.
• BATT-LED-driven change: immediately on BATT LED lighting, regardless of installed date.
• Post-outage audit: after any prolonged mains outage (>48 h), check the BATT LED at the next cabinet visit.
• Record-keeping: mark each new cell with install date and DMM-measured voltage at install. Update CMMS.
• Early-warning: add an alarm one month before the calendar due date.

Environmental acceleration factors:

Safety, Disposal, and Lithium Handling

8.1 Storage and handling

• Store unused cells at −20 °C to +40 °C, dry.
• Do not short-circuit. Do not solder directly to the cell body.
• Do not stack heavy objects on cells.
• Do not attempt to recharge. The cells are primary, non-rechargeable. Attempting to charge creates internal pressure and may vent.

8.2 Disposal

Used cells qualify as industrial batteries under EU Battery Directive 2006/66/EC and analogous US RCRA rules. Return cells to a certified battery recycler. Do not dispose in general waste and never incinerate. Used cells retain charge and can ignite if punctured.

Related S5 Family Notes

For modern migration, the S5-95U is typically replaced by a S7-1200 (with optional battery card for real-time-clock backup) or a S7-1500. The S7-1200 does not use a primary lithium backup cell in the same role — NVRAM with a battery card replaces it. The S5-95U hot-swap discipline does not transfer 1-to-1; consult the S7-1200 System Manual buffer-section before commissioning.

Troubleshooting Matrix

Commissioning Sheet After a Battery-Only Swap

1. Verify 24 V at the S5-95U input terminals: nominal 24 V ± 10 %.
2. Mode switch STOP → RUN. Confirm SF, BATT, BF LEDs off.
3. Watch OB 1 scan; confirm outputs return to expected state and OB 21 / OB 22 (or OB 1) initialisation completes.
4. Trigger a fault simulation (push an ESTOP chain input) and confirm the program responds.
5. Reset ESTOP; verify RUN returns and outputs re-permit as per program logic.
6. Update maintenance log: install date, cell batch, voltage at install, engineer signature.

Siemens Official Documentation Pointers

• S5-90U / S5-95U System Manual — battery replacement procedure in §4.4.2 — retrievable via the Siemens Industry Online Support portal. Search for order number 6ES5 095-8MA03 or "S5-95U System Manual".
• S5 Spare Parts catalogue (via the support portal's spare-parts list) confirming cell part numbers and dimensional specs.

FAQ