1. Problem Statement: Drift in LOGO! 0BA6 Battery Voltage Display
When a Siemens LOGO! 0BA6 is powered from a 12 V lead-acid battery that also starts a backup generator and an LPG boiler, the on-screen battery voltage reading drifts by approximately 1.3 V across the 11–15 V measurement window. The drift is symmetric: a calibration performed at 15 V reads 13.0 V at 12 V, and a calibration performed at 12 V reads 13.8 V at 15 V. This non-linearity makes the displayed value unsuitable for driving generator start, battery alarm, and LPG boiler enable logic, where a 0.5 V error in the displayed terminal voltage can mis-trigger a low-voltage disconnect or leave the battery in a deep-discharge state.
The root cause is twofold:
- Hardware: The LOGO! 0BA6 base module analog inputs are fixed at 0–10 V. A voltage divider is required to bring the 0–20 V battery range into the 0–10 V input window. A 5 kΩ potentiometer used as a manually-adjusted divider introduces wiper resistance, temperature coefficient, and tolerance errors that are non-linear over the divider travel.
- Software: Scaling the analog input value (0–1000 raw counts) directly to a voltage assumes an ideal divider. Any deviation between the divider ratio and the theoretical 0.5:1 ratio produces a constant gain error, but pot wiper noise and the AI input impedance load (≥78 kΩ on 0BA6) introduce a variable error across the input range.
The corrective procedure is to (a) build a fixed-resistor divider that maps 0–20 V to 0–10 V with low-tolerance resistors, (b) configure the LOGO! to scale 0–1000 raw to 0–20 V (or 0–21.28 V to compensate for the AI input impedance loading), and (c) apply a two-point linear calibration to remove the residual non-linearity.
2. LOGO! 0BA6 Analog Input Hardware
The 0BA6 generation (LOGO! 6, 2009–2012) base module has the following analog input characteristics that are relevant to this application:
| Parameter | Specification |
|---|---|
| Analog inputs | AI1, AI2, AI3, AI4 (shared with I7, I8, I11, I12 on 12/24 V DC unit) |
| Input range (voltage mode) | 0 V DC to 10 V DC |
| Input range (current mode) | 0/4 mA to 20 mA (requires AM2/AM2 RTD module) |
| Input impedance (voltage mode) | ≥ 78 kΩ |
| Resolution | 10 bits (1024 steps, displayed as 0–1000) |
| LSB weight (voltage) | 10 mV per count |
| Absolute maximum overvoltage | ±30 V DC (sustained input above 12 V risks damage) |
| Update rate | Per program scan cycle (typical 10–50 ms) |
| Voltage mode configuration | Default; no jumper required on base module |
Voltage mode is the default for AI1–AI4 on the 12/24 V DC base module when no current-mode jumper is fitted. The raw value returned by the AI block in LOGO! Soft Comfort V6.x or V7.x is an integer 0–1000 that corresponds linearly to 0–10 V at the input pin. The 0–10 V to 0–1000 scaling is performed in firmware and is not user-configurable on the 0BA6.
3. Voltage Divider Design (0–20 V → 0–10 V)
A 2:1 divider is the simplest solution for a 12 V battery system that must read up to 20 V (the maximum expected is 14.4 V during float charge, but the divider must accommodate a worst-case 20 V for safety margin). With the divider midpoint connected to AI1 and the low end tied to LOGO! common ground:
| Component | Value | Tolerance | Rationale |
|---|---|---|---|
| R_top (battery + to AI) | 10 kΩ | 1% metal film | Sets divider ratio, limits input current |
| R_bottom (AI to GND) | 10 kΩ | 1% metal film | Matches R_top for unity 2:1 gain |
| Input current at 20 V | 1.0 mA | — | Negligible battery drain (≈9 Ah/yr) |
| Power dissipation at 20 V | 20 mW per resistor | — | 1/8 W parts are adequate |
| Filter capacitor | 100 nF X7R | 10% | Attenuates RF/AVR noise at AI pin |
The transfer function with both resistors equal is:
V_AI = V_batt × R_bottom / (R_top + R_bottom) = V_batt / 2
The Thevenin resistance seen by the LOGO! AI pin is R_top || R_bottom = 5 kΩ. The loading effect with the AI input impedance (Z_AI ≥ 78 kΩ) gives an error factor of:
k_load = Z_AI / (Z_AI + R_th) = 78 k / (78 k + 5 k) = 0.940
This means the AI pin sees 94% of the ideal divider voltage — a constant 6% gain error across the entire range. To compensate, either:
- Set the LOGO! full-scale to 21.28 V (gain 0.02128 on the analog amplifier block) to correct the 6% loss in software, or
- Reduce R_top to 9.4 kΩ (use 9.1 kΩ in series with a 300 Ω trimpot) to bring the divider gain to 0.515, which compensates the AI input loading in hardware.
The residual error after this single-point correction is below 0.1% over the 0–20 V range, which is within the LOGO! AI quantization noise of ±10 mV (one count). For a 12 V battery system that only needs to display 10–15 V accurately, a 2:1 divider with software compensation is sufficient.
For higher precision (better than 50 mV), use 0.1% metal-film resistors with a temperature coefficient of 25 ppm/°C or better (Vishay RN55D or similar). Alternatively, add a unity-gain op-amp buffer (e.g., Texas Instruments TLV2372, ON Semiconductor MCP6002) between the divider and the LOGO! AI input to eliminate the loading effect entirely. The buffer draws under 10 µA from the divider and presents a near-infinite impedance to the LOGO! AI pin.
4. Wiring the AI Channel
Wire the divider as follows for a 0BA6 12/24 V DC base unit (8DI/4DO/4AI):
- R_top (10 kΩ) between battery positive terminal and AI1 input pin (I7 on the 0BA6 base).
- R_bottom (10 kΩ) between AI1 and LOGO! common ground (M terminal on the power supply side).
- Add a 100 nF X7R ceramic capacitor directly at the AI1 pin to ground. This filters high-frequency noise from the LPG boiler ignition coils and the generator AVR.
- Use shielded cable if the divider is located more than 200 mm from the LOGO! module. Ground the shield at the LOGO! end only to avoid ground loops.
- If the battery negative is not the same chassis as the LOGO! ground, tie them together at one point near the divider to prevent ground-loop noise from injecting into the AI input.
Pinout (LOGO! 0BA6 12/24 V DC base):
| Pin | Function | Notes |
|---|---|---|
| I7 | AI1 (analog/digital selectable) | Use this for the battery monitor |
| I8 | AI2 | Available for a second analog channel (e.g., temperature) |
| I11, I12 | AI3, AI4 (expansion port) | Use only if AI1/AI2 are already in use |
| M | LOGO! common (return for AI inputs) | Same potential as LOGO! power supply negative |
The AI pins are physically shared with digital inputs but are sampled by the analog channel when the AI block is configured in the program. The default LOGO! Soft Comfort V6.x I/O configuration enables AI1 (I7) and AI2 (I8) as analog channels. AI3 and AI4 on the 0BA6 base are routed to dedicated expansion connector pins and require a different block reference (B-auxiliary) in older firmware.
Inline SVG topology of the divider and LOGO! AI input:
5. Software Scaling: Block Configuration in LOGO! Soft Comfort
The scaling chain in LOGO! Soft Comfort V6.x for the battery monitor consists of four blocks in series:
- AI block (B001): Reads I7 (AI1) and outputs the raw value 0–1000.
- Analog amplifier block (B002): Applies gain and offset to the raw 0–1000 value. Configure Gain = 0.02128 (maps 1000 counts → 21.28 V to compensate AI input loading) and Offset = 0.
- Arithmetic block (B003): Multiplies the analog amplifier output by the two-point k factor (e.g., 1.1765) using integer math.
- Arithmetic block (B004): Subtracts the two-point d offset (e.g., 2.235) from B003 output. The output is the corrected battery voltage in volts × 1000 (e.g., 12,000 = 12.000 V).
Block diagram of the scaling chain:
Parameter values for the Analog Amplifier block (B002):
| Parameter | Value | Notes |
|---|---|---|
| Sensor type | 0–10 V | Default for AI1 voltage mode |
| Gain | 0.02128 | 21.28 V / 1000 counts; compensates 78 kΩ loading |
| Offset | 0.000 | Adjusted during two-point calibration if preferred |
| Display range min (B005) | 0.0 | Message text block parameter |
| Display range max (B005) | 21.28 | Message text block parameter |
If only the 11–15 V range is of interest, the gain can be set to 0.020 (full-scale 20.0 V) and the display min can be left at 0.0 V. The 1.3 V drift reported in the original setup is consistent with the AI block being scaled 0–10 V (gain = 0.010) while the user was reading a 12 V battery through a manually-adjusted 5 kΩ pot — the calibration at one end pushed the display into the non-linear region of the pot travel.
Optional low-pass filter: Insert a 4-sample moving average (using a shift register or an integrator block with K=4) after the analog amplifier to smooth reading jitter from the divider noise. This is particularly useful if the LOGO! is switching the generator or LPG relay and the supply rail is dipping on each switch event. A 4-sample average at a 50 ms cycle time gives an effective 200 ms time constant, which filters 50 Hz mains hum and relay transients without slowing the display noticeably.
For threshold-only applications, use the Analog Threshold Trigger block instead of the analog amplifier. The threshold trigger block has a hysteresis parameter and operates directly on the raw 0–1000 value, eliminating the need for a separate scaling block. To trigger a low-battery alarm at 11.5 V with a 2:1 divider and gain 0.02128: threshold = (11.5 / 21.28) × 1000 = 540 counts. Hysteresis: 20 counts (200 mV) prevents chattering around the threshold. This is the preferred approach for start/stop and alarm logic; the scaled message text is only used for the on-screen display.
6. Two-Point Linear Calibration
The two-point method removes any residual non-linearity from pot wiper resistance, AI input impedance, temperature drift, or resistor tolerance. It is implemented as a separate scaling block or as the gain/offset of the analog amplifier.
Procedure:
- Apply a known reference voltage V1 (for example, 12.00 V from a calibrated bench supply or a fresh, rested 12 V battery) to the battery terminal of the divider.
- Read the LOGO! displayed value V_disp1 (for example, 12.10 V).
- Apply a second known reference voltage V2 (for example, 15.00 V) to the same terminal.
- Read the LOGO! displayed value V_disp2 (for example, 14.65 V).
- Compute the linear correction coefficients:
k = (V2 − V1) / (V_disp2 − V_disp1)
d = V1 − k × V_disp1
Worked example using the values above:
k = (15.00 − 12.00) / (14.65 − 12.10) = 3.00 / 2.55 = 1.1765
d = 12.00 − 1.1765 × 12.10 = 12.00 − 14.235 = −2.235
The corrected voltage is:
V_actual = 1.1765 × V_displayed − 2.235
In the LOGO!, this is implemented as two arithmetic blocks in series:
- B003 (Arithmetic): V_displayed × 1.1765 → use 11765/10000 if float math is available, or scale via 10000 then divide by 8500 in integer math.
- B004 (Arithmetic): B003 − 2.235
The output of B004 is the corrected battery voltage. Drive the message text block from B004.
If using firmware V3.x or later, the analog amplifier block accepts a 4-decimal floating-point gain and offset. Enter the computed k and d directly into the analog amplifier block. The output then feeds the message text block with no further arithmetic needed.
Recompute the two-point calibration annually, or whenever the LOGO! is moved to a significantly different ambient temperature. The calibration is most sensitive to the gain resistor values; check the resistor values with a multimeter if the k factor drifts more than 2% between annual calibrations. The reference source for the calibration should be a calibrated bench supply (Fluke 87V or similar) with ±0.05% accuracy. A rested 12 V lead-acid battery is repeatable to ±0.05 V over a 24-hour period and is acceptable for field re-calibration.
7. Battery-Specific Compensation
A 12 V lead-acid battery has a resting voltage between 11.8 V (50% depth of discharge for a flooded cell) and 12.7 V (fully charged, rested). Under load, the terminal voltage drops by 0.2–0.5 V. While charging from the generator, the terminal voltage rises to 13.8–14.4 V (float for a 12 V lead-acid at 25 °C). The display should be configured to show the entire 0–20 V range, but the practical operating window is 10–15 V.
Recommended display alarms:
| Voltage | Condition | Action |
|---|---|---|
| < 10.5 V | Critical discharge | Stop non-essential loads, raise alarm |
| 10.5–11.8 V | Warning | Start generator, run LPG boiler priority |
| 11.8–12.7 V | Normal (rested) | Standby |
| 12.7–13.8 V | Charging | Generator running, alternator active |
| 13.8–14.4 V | Float (12 V lead-acid) | Hold float, monitor temperature |
| > 15.0 V | Overcharge / regulator fault | Disconnect alternator, raise alarm |
Temperature compensation: A lead-acid battery terminal voltage drops by approximately 3 mV/cell/°C (18 mV/battery/°C) when cold and rises when hot. If the LOGO! is installed in a temperature-controlled enclosure, the offset term d can be adjusted seasonally. For a permanent install, add an external NTC thermistor divider on AI2 and apply a software temperature correction to the d coefficient (see the Analog Devices voltage monitoring design note for a reference implementation of multi-rail temperature-compensated monitoring).
Surface charge caution: A 12 V lead-acid battery that has just been removed from a float charge may show 13.2–13.5 V but drop to 12.6 V within minutes. For a meaningful state-of-charge reading, sample the voltage only after the battery has been at rest for 30 minutes. The LOGO! program can include an on-delay timer block (B007, preset 30 min) that latches the displayed value 30 minutes after the generator stops. This prevents a false "battery is full" indication during the post-charge relaxation period.
Starter dip interpretation: When the LOGO! calls the generator starter motor, the battery terminal voltage will drop to 9–10 V for 0.5–2 s. Do NOT interpret this as a low-battery condition. Use a pulse generator with a 3 s on-delay to suppress the low-voltage alarm during cranking. Wire the pulse trigger to the same output that energizes the starter solenoid.
8. Verification Procedure
After the wiring and scaling are complete, perform the following acceptance test:
- Power the LOGO! from a calibrated bench supply set to 12.00 V (±0.05 V). The display should read between 11.95 V and 12.05 V. If not, recompute d.
- Increase the supply to 13.80 V. The display should read between 13.75 V and 13.85 V. If not, recompute k.
- Repeat step 1 and step 2 iteratively until both endpoints are within ±0.05 V.
- Sweep the supply from 10.0 V to 15.0 V in 0.5 V steps. The display should track the supply within ±0.1 V at every step. Record the readings in a calibration log.
- Apply a step change from 12 V to 15 V. The display should settle within ±0.1 V of the final value in under 1 second (depending on the filter time constant).
- Disconnect the bench supply and reconnect to the actual battery. The reading should be within ±0.2 V of a handheld multimeter reading at the same battery terminal.
- Run the generator. The display should rise to 13.8–14.4 V. Confirm the relay logic for generator start, LPG boiler enable, and battery alarm are all operating from the corrected scaled value, not the raw AI value.
If step 4 shows a non-linear error greater than 0.2 V, the divider is loading the AI input or the wiper resistance is still in circuit. Replace the pot with fixed resistors and redo the two-point calibration.
If step 6 shows a discrepancy greater than 0.2 V between the LOGO! display and a multimeter, the issue is likely a ground-loop or a voltage drop in the supply wire between the battery terminal and the divider. Move the divider to within 50 mm of the LOGO! power input, or use Kelvin connections to the battery (separate sense wires from the load-carrying wires).
Sample calibration log template:
| Date | Reference V | LOGO! V | Error (V) | Technician |
|---|---|---|---|---|
| 2024-01-15 | 12.00 | 12.02 | +0.02 | — |
| 2024-01-15 | 13.80 | 13.78 | −0.02 | — |
| 2024-01-15 | 15.00 | 15.03 | +0.03 | — |
9. Common Pitfalls and Noise Sources
| Symptom | Cause | Remedy |
|---|---|---|
| Display drifts 1–2 V across the 11–15 V range | Pot wiper resistance non-linearity | Replace with 10 kΩ 1% fixed resistors |
| Display reads 6% low at all voltages | AI input impedance loading the divider | Set gain to 0.02128 or use op-amp buffer |
| Display jitters by ±0.3 V during generator running | AVR noise coupling into AI input | Add 100 nF capacitor at AI pin; add ferrite bead on supply |
| Display reads 0 V or 21.28 V at 12 V | Open or shorted divider resistor | Check R_top and R_bottom with multimeter |
| Display is stuck at 0 even with battery connected | AI block not configured in program | Add AI block B001 in LOGO! Soft Comfort and connect to I7 |
| Display shows correct value on bench but wrong in field | Voltage drop in supply wiring | Use heavier gauge wire (≥1.5 mm²), move divider closer to battery |
| Display shows −0.5 V at 12 V | Ground loop between battery negative and LOGO! ground | Tie grounds at one point near the divider |
| LOGO! resets when generator starts | Voltage dip below 10.5 V on LOGO! supply | Add 1000 µF capacitor on LOGO! power input; check wire gauge |
| Display reads correctly on day 1, drifts 0.5 V by day 30 | Thermal coefficient of divider resistors | Use 25 ppm/°C metal-film resistors or move divider away from heat sources |
| Display shows half-scale ripple at 100 Hz | Mains-coupled noise from LPG boiler controller | Add 10 µF electrolytic + 100 nF ceramic at AI pin; shielded cable |
For applications requiring higher precision (better than 50 mV), dedicated battery monitor ICs (e.g., Texas Instruments BQ34Z100, Analog Devices LTC2943, Maxim MAX17823) provide ±1 mV accuracy and Coulomb counting. The LOGO! approach is appropriate for display and threshold-only monitoring, not for state-of-charge estimation. For a 3-phase supply monitor (mains/generator transfer switching), use a dedicated Phoenix Contact 3-phase voltage monitoring relay with over/undervoltage and phase sequence detection. For academic reference on voltage monitoring theory and signal conditioning, see the Voltage Monitoring and Supply Controlling System paper from ScienceDirect.
Alternative hardware paths:
- Current-mode output: Use a 4–20 mA current loop from a battery monitor IC to the LOGO! AI input. The 0BA6 current-mode AI range is 0/4–20 mA, but you must set the rear-panel jumpers on the AM2/AM2 RTD module. This is more noise-immune than 0–10 V for long cable runs.
- Op-amp buffer: Insert a TLV2372 or MCP6002 rail-to-rail op-amp as a voltage follower between the divider and the LOGO! AI input. The buffer presents a 10 MΩ input impedance (no loading effect on the divider) and a near-zero output impedance (no noise pickup on the cable). Add a 10 kΩ resistor in series with the op-amp output to protect against capacitive loads.
- Modbus RTU module: Use a CMK2000 or similar Modbus slave battery monitor and read it via a LOGO! CM EIB/KNX expansion. Requires LOGO! 8 (0BA8) or later for native Modbus support via the LOGO! CM 8AI/4AI modules (part number 6ED1055-1MA00-0BA8).
10. Safety, Fusing, and Field Commissioning
The voltage divider is connected directly to the battery terminal. A short circuit of R_top or R_bottom during installation can dump the full battery current through the LOGO! AI input and destroy the base module. Always include the following protection:
- Series fuse: Install a 100 mA fast-bloom fuse (Littelfuse 0451004.MRL or equivalent) in series with R_top. This fuse limits the worst-case fault current to a level the wire and PCB traces can survive.
- TVS clamp: Add a 15 V bidirectional transient voltage suppressor (Littelfuse SMAJ15CA or equivalent) across R_bottom. This clamps load-dump transients from the generator AVR and alternator field collapse, which can reach 30 V for 100 ms.
- Series resistor at AI input: Add a 100 Ω resistor in series with the AI1 pin, immediately at the LOGO! terminal. This limits the inrush current if the TVS clamps during a transient and prevents the AI pin protection diode from conducting.
- Reverse polarity protection: Add a 1N5408 diode (3 A) in series with the battery + wire. This prevents reverse-polarity wiring damage during commissioning.
Field commissioning checklist:
- Verify battery polarity with a multimeter before connecting the LOGO!.
- Power the LOGO! from a current-limited bench supply (set to 500 mA limit) during initial programming.
- Connect the divider with the LOGO! powered off. Use a separate multimeter to confirm the divider output is 0 V at the AI pin before powering the LOGO! on.
- Power on the LOGO!. Read the AI value in online mode (LOGO! Soft Comfort → Tools → Online → AI1). Confirm the raw value is between 460 (10 V battery) and 690 (15 V battery).
- Transfer the program to the LOGO! and verify the message text block shows a reasonable voltage.
- Run the two-point calibration procedure from Section 6.
- Connect the generator and LPG boiler outputs. Verify the start/stop logic operates correctly at the threshold voltages from Section 7.
- Disconnect the bench supply. Connect the actual battery. Verify the display matches a handheld multimeter reading within ±0.2 V.
- Document the final k and d coefficients in the program header comment for future re-calibration reference.
11. Parts List and Documentation
| Part | Value | Manufacturer / Part Number | Notes |
|---|---|---|---|
| LOGO! 0BA6 base | 12/24 V DC, 8DI/4DO/4AI | Siemens 6ED1052-1MD00-0BA6 (or equivalent) | Requires LOGO! Soft Comfort V6.x or V7.x |
| R_top, R_bottom | 10 kΩ, 1%, 1/8 W, 25 ppm/°C | Vishay RN55D1002F (or Yageyo MFR-25FBF52-10K) | Metal-film, low TCR |
| Filter capacitor | 100 nF, 10%, X7R, 50 V | Murata GRM188R71H104K | 0805 SMD or through-hole |
| TVS diode | 15 V, 400 W, bidirectional | Littelfuse SMAJ15CA | Across R_bottom |
| Series fuse | 100 mA, fast-bloom, 250 V | Littelfuse 0451004.MRL | Inline with R_top |
| Series resistor (AI pin) | 100 Ω, 1/4 W | Vishay RN60D1000F | At LOGO! AI1 terminal |
| Reverse-polarity diode | 3 A, 1000 V | ON Semiconductor 1N5408 | Inline with battery + |
Reference documentation:
- LOGO! 0BA6 System Manual (Siemens AG) — I/O specifications, wiring, and absolute maximum ratings.
- LOGO! 0BA6 Product Information (Siemens AG) — module variants, firmware compatibility, ordering data.
- Analog Devices Voltage Monitoring in Complex Systems — supervisory solutions for multiple rails.
- Phoenix Contact Voltage Monitoring Relays — 3-phase over/undervoltage and phase sequence monitoring.
- Voltage Monitoring and Supply Controlling System (ScienceDirect) — network voltage amplitude, RMS, and frequency measurement reference.
12. Frequently Asked Questions
Why does my LOGO! 0BA6 show 1.3 V drift when I measure a 12 V battery with a 5 kΩ pot?
The pot's wiper resistance is non-linear over its travel, and the LOGO! AI input impedance (≥78 kΩ) loads the 5 kΩ Thevenin resistance of the divider by about 6%. Replace the pot with two fixed 10 kΩ 1% metal-film resistors and configure the analog amplifier block for a 0–20 V (or 0–21.28 V) full scale. Then apply the two-point calibration in Section 6 to remove any residual non-linearity.
Can the LOGO! 0BA6 directly read a 12 V battery on an analog input?
No. The 0BA6 analog inputs are fixed at 0–10 V in voltage mode. Voltages above 10 V risk damage to the input pin. An external voltage divider (2:1 ratio for 0–20 V operation) is required.
What scaling gain do I enter in the analog amplifier block for a 0–20 V range?
Use a gain of 0.02000 (with a 10 kΩ + 10 kΩ divider and no load compensation) or 0.02128 (full compensation for the 78 kΩ AI input impedance). The offset is set to 0.0 initially and is trimmed during the two-point calibration. The display range max should match the gain × 1000 product.
How do I display the battery voltage on the LOGO! built-in screen?
Wire the output of the analog amplifier or arithmetic chain to a Message Text block. In LOGO! Soft Comfort V6.x, drag a Message Text block onto the worksheet, connect it to the scaled value, and enable the bar graph option. On the LOGO! device, navigate to the message text with the cursor keys. On a LOGO! TDE, the message text appears as a scrolling line.
Will a firmware update on the 0BA6 improve the analog input accuracy?
No. The 10-bit ADC resolution and the 0–10 V input range are hardware-fixed. Firmware updates add new function blocks and bug fixes but do not change the AI resolution. For better than 10 mV accuracy, use a dedicated battery monitor IC and feed the result to the LOGO! via a digital input or via a Modbus RTU expansion module.
What is the difference between analog amplifier gain and arithmetic block scaling in LOGO! Soft Comfort?
The analog amplifier block applies a fixed gain and offset to a single raw 0–1000 input, ideal for sensor scaling. The arithmetic block is a general-purpose calculator that adds, subtracts, multiplies, and divides; it can chain the analog amplifier output with a custom two-point correction. For a battery monitor, use the analog amplifier for the primary scale (0–1000 → 0–20 V) and arithmetic blocks for the two-point linear correction.
Can I use AI3 or AI4 on the 0BA6 base unit for the battery monitor?
AI3 and AI4 on the 0BA6 base are routed to dedicated expansion connector pins and require a different block reference (B-auxiliary) in older firmware. Use AI1 (I7) or AI2 (I8) for the battery monitor to avoid I/O mapping confusion. If both AI1 and AI2 are already in use, add a LOGO! AM2 module (6ED1055-1MM00-0BA6) and configure the second pair of analog inputs there.