Overview
Connecting a lithium-ion battery bank to a Siemens LOGO! logic module for a small solar monitoring application is a common engineering exercise, but the LOGO! 24RC (catalog number 6ED1052-1FB08-0BA2) cannot perform this measurement on its own. The base module provides only eight digital inputs (I1–I8) and four relay outputs (Q1–Q4); it has no analog-to-digital conversion hardware. This article walks through the exact hardware additions, voltage scaling math, and wiring practice required to monitor a 4S 18650 pack (nominal 14.8 V, full-charge 16.8 V) using a LOGO! AM2 analog input extension and a properly sized resistive voltage divider.
Prerequisites
Before beginning the wiring, confirm the following parts and tools are on hand.
- Siemens LOGO! 24RC base module (6ED1052-1FB08-0BA2) or compatible variant
- Siemens LOGO! AM2 analog input extension (6ED1055-1MA00-0BA2), 2 channels, 0–10 V or 0/4–20 mA
- LOGO! Soft Comfort V8.4 or later programming software (current release V8.4 SP1)
- 4S 18650 battery pack with BMS (battery management system) rated for ≥ 5 A continuous discharge
- Precision resistors for the voltage divider: 1% tolerance metal film, 0.25 W minimum
- Twisted-pair shielded cable for analog signal run, ≤ 10 m recommended
- Multimeter with 0.5% DC accuracy for calibration verification
- Small grid-tie inverter compatible with 14.8 V battery input (e.g., ENVERTECH EVT 248, Growatt MIN 2500 TL-XH, or similar 24 V-class micro-inverter)
Hardware Identification: Why the 24RC Cannot Read Voltage Directly
The LOGO! 24RC ships with the following I/O inventory, taken directly from the LOGO! Product Information manual:
| Parameter | Specification |
|---|---|
| Catalog number | 6ED1052-1FB08-0BA2 |
| Supply voltage | 24 V DC (20.4–28.8 V) |
| Digital inputs | 8 (I1–I8), 24 V DC sinking |
| Relay outputs | 4 (Q1–Q4), 10 A / 250 V AC, 5 A / 30 V DC |
| Analog inputs | None |
| Analog outputs | None |
| Expansion slots | Right-side bus, supports AM2, AM2 RTD, DM8, etc. |
Only base modules labeled with "...CE" (e.g., LOGO! 24RCE) and 12/24 V DC variants (e.g., LOGO! 12/24RCE) embed analog inputs directly on the base. Modules with analog inputs on-board are identified by the labels AI1, AI2 near I7/I8 and AI3, AI4 near I1/I2. The 24RC carries no such labels.
The AM2 Analog Input Module
The LOGO! AM2 (6ED1055-1MA00-0BA2) is a two-channel analog input expansion that snaps onto the right side of the LOGO! base. Its terminal layout is shown below.
| Terminal | Function | Polarity |
|---|---|---|
| U1+ | Channel 1 voltage input (0–10 V) | Positive |
| M1 | Channel 1 ground reference | Negative |
| U2+ / I2+ | Channel 2 voltage OR current input | Positive |
| M2 / I2- | Channel 2 ground reference | Negative |
Key AM2 specifications:
- Input range (voltage mode): 0–10 V DC
- Input range (current mode): 0–20 mA
- Resolution: 10 bits (0–1000 counts, 10 mV/count in voltage mode)
- Input impedance: ≥ 100 kΩ in voltage mode, 250 Ω in current mode
- Galvanic isolation: None — the M terminal is internally bonded to the LOGO! DC common/ground
- Maximum permissible input voltage on U+ pin: 12 V DC continuous
4S 18650 Battery Pack — Electrical Envelope
A 4S pack built from 18650 lithium-ion cells (Samsung INR18650-25R, LG MJ1, or similar) produces the following DC levels at the pack terminals:
| State of Charge | Pack Voltage | Cell Voltage |
|---|---|---|
| 100% (full charge, 4.20 V/cell) | 16.80 V | 4.20 V |
| 75% (storage) | 15.20 V | 3.80 V |
| 50% (nominal) | 14.80 V | 3.70 V |
| 25% | 14.00 V | 3.50 V |
| 0% (cutoff) | 12.00 V | 3.00 V |
| Over-discharge (damage threshold) | < 10.00 V | < 2.50 V |
Every one of these values exceeds the AM2's 0–10 V measurement window. A resistive voltage divider is therefore mandatory — and, because the maximum pack voltage (16.8 V) is more than 1.5× the AM2's absolute-maximum (12 V), the divider must be sized with a safety margin of at least 30%.
Voltage Divider Calculation
Design a divider with the following targets:
- Full-scale input (Vin_max) = 17.5 V (5% above pack full charge to allow for transient overshoot)
- Full-scale output (Vout_max) = 9.5 V (5% below the 10 V AM2 ceiling)
- Divider ratio: k = Vout_max / Vin_max = 9.5 / 17.5 = 0.5429
For a standard two-resistor divider Vout = Vin × R2 / (R1 + R2), solve for resistor values:
R2 / (R1 + R2) = 0.5429
R1 / R2 = (1 / 0.5429) - 1 = 0.8421
R1 = 0.8421 × R2
Choose R2 = 10.0 kΩ (1% metal film). Then R1 = 8.42 kΩ. The nearest E96 1% standard value is 8.45 kΩ.
Recompute the exact ratio with R1 = 8.45 kΩ, R2 = 10.0 kΩ:
k_actual = 10.0 / (8.45 + 10.0) = 0.5420
With this divider:
- 16.80 V in → 9.106 V out (91% of AM2 full scale)
- 14.80 V in → 8.022 V out
- 12.00 V in → 6.504 V out
- 10.00 V in → 5.420 V out (BMS cutoff floor)
Power dissipation in the divider at full charge (worst case, no inverter load):
I_divider = V_in_max / (R1 + R2) = 16.8 / 18450 = 0.911 mA
P_total = V_in × I_divider = 16.8 × 0.000911 = 15.3 mW
Both resistors dissipate under 8 mW — well within the 0.25 W rating of 0805 SMD or 1/4 W through-hole parts.
Step-by-Step Wiring Procedure
- Power down the LOGO! base module. Remove the 24 V DC supply at the L+ / M terminals.
- Snap the AM2 module onto the right-hand bus of the LOGO! 24RC until the interlock tab clicks. The AM2 receives power and address assignment automatically from the base.
- Connect the divider input to the battery pack positive terminal via the BMS output positive rail. Insert a 100 mA inline fuse (Littelfuse 0451004.MRL or equivalent) for fault protection.
- Connect the divider low side (R2 bottom) to the battery pack negative terminal. This is the same node that bonds to the AM2 M1 terminal in step 6.
- Add a 100 nF X7R ceramic capacitor across R2 (between the divider tap and ground) to filter switching noise from the inverter.
- Route the divider tap (junction of R1 and R2) to the AM2 U1+ terminal using a shielded twisted pair. Connect the shield drain wire to the AM2 M1 terminal at one end only (LOGO! side), to prevent ground loops.
- Bond the AM2 M1 terminal to the LOGO! base M terminal with a short 2.5 mm² jumper. This establishes the common reference the AM2 requires.
- Reapply 24 V DC supply to the LOGO! base and verify the AM2 power LED illuminates.
Wiring Diagram (ASCII)
4S 18650 Pack (12.0–16.8 V DC)
+ ────[ 100 mA Fuse ]────┬──── R1 (8.45 kΩ) ────┬── U1+ (AM2 Ch1)
│ │
│ 100 nF
│ │
- ─────────────────────────┴──── R2 (10.0 kΩ) ───┴── M1 (AM2 Ch1)
│
│
[ LOGO! M terminal ]
Configuring the AM2 in LOGO! Soft Comfort V8.4
- Launch LOGO! Soft Comfort and open your project.
- In the Tools → Module Selection dialog, add "LOGO! AM2" to slot 1 (right of the base module).
- Open the Analog Input block (U1) and set:
- Sensor type: 0–10 V
- Scaling gain: 1.000
- Offset: 0.000
- Insert an Analog Math block (or use a math constant block) to convert the 0–9.106 V reading back to pack voltage:
V_pack = V_AM2 / 0.5420 - Add threshold comparators for alarm points:
- Low-voltage cutoff: V_pack < 12.0 V → trigger Q1 (load disconnect)
- High-voltage alert: V_pack > 16.8 V → trigger Q2 (charge disable)
Solar Charger and Grid-Tie Inverter Interfacing
The LOGO! 24RC relay outputs can supervise a small DC-coupled solar system as follows:
| Relay | Function | Contact Rating |
|---|---|---|
| Q1 | Load disconnect (battery cutoff relay) | 10 A / 30 V DC |
| Q2 | Solar charger enable / MPPT relay | 10 A / 30 V DC |
| Q3 | Grid-tie inverter enable | 10 A / 250 V AC |
| Q4 | Alarm / status indicator | 10 A / 30 V DC |
Two of the user's four relays have failed, leaving two healthy outputs. Assign Q3 and Q4 to the critical paths (inverter enable and alarm) and abandon the load-disconnect and charger functions, or use an external interposing relay driven from a single Q output. If a failed relay is welded closed, de-energize the affected circuit by removing the 24 V supply before re-commissioning.
Calibration and Verification
- Apply a known 14.0 V bench supply to the divider input.
- Measure the AM2 U1+ pin with a calibrated multimeter. Expected reading: 14.0 × 0.5420 = 7.588 V ± 1%.
- In LOGO! Soft Comfort online mode, verify the AI1 raw value reads 758 ± 8 counts (10 mV/count).
- If the reading is high by more than 2%, apply a software offset on the AI block. If low, reduce the offset.
- Repeat at 16.0 V to confirm linearity across the operating range.
- Connect the live 4S pack, enable the BMS, and confirm the LOGO! value tracks the multimeter to within ± 50 mV.
Troubleshooting Matrix
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| AM2 AI1 reads 0 V constantly | Open R1, or fuse blown | Check fuse continuity; measure R1 with multimeter; replace if > ± 2% of nominal |
| AM2 AI1 reads 10.0 V always (saturated) | Open R2 (tap floating to rail), or AM2 over-voltage clamp | Check R2; verify V at U1+ does not exceed 10 V |
| Reading 5% low across full scale | R1 value too low | Recalculate divider with measured R1; add 0.1% gain correction in software |
| Reading noisy, ± 50 mV jitter | Missing 100 nF cap, or shield not bonded | Install 100 nF X7R across R2; bond shield drain to M1 only at AM2 end |
| LOGO! base shows "I/O error" after AM2 install | AM2 not fully seated, or firmware < V1.08.01 on base | Reseat AM2 with power off; update base firmware via LOGO! Soft Comfort |
| Battery negative terminal sparks during connection | Divider drawing current through AM2 M before fuse engages | Add pre-charge resistor (100 Ω) across contactor; sequence fuse connection last |
| Q1 contact welding after extended use | Capacitive DC load on contact (inverter input cap) | Add snubber (RC network 100 Ω + 100 nF X2-rated) across Q1 contacts |
Notes on the LOGO! 24RC Failure Mode
A LOGO! base module with two failed relay contacts is not a candidate for in-circuit repair — the relays (typically Finder 55.34 or Omron G5LE) are wave-soldered onto the PCB and not field-replaceable. Siemens does not authorize user-level component replacement, and the warranty is voided by any such attempt. Practical options are:
- Retain the module for digital-only tasks (the surviving 4 relays and 8 digital inputs are unaffected).
- Return for repair to a Siemens-authorized service center; flat-rate repair typically costs 30–50% of a new unit.
- Replace with a new LOGO! 12/24RCE (6ED1052-1MD08-0BA2), which embeds four 0–10 V analog inputs and accepts the same divider network without the AM2 module.
FAQ
Does the LOGO! 24RC have analog inputs at all?
No. The 24RC is a digital-only logic relay. Only the 12/24V variants and the "CE" / "E" suffix modules embed analog inputs (AI1–AI4). For voltage measurement on a 24RC you must add a LOGO! AM2 (6ED1055-1MA00-0BA2) extension module.
Where exactly does the negative battery wire connect?
The battery negative connects to the M1 (or M2) terminal of the AM2 module. The same node is the LOGO! 24 V supply "M" terminal. The AM2 inputs are single-ended and galvanically tied to this common — a true differential connection is not possible without an external isolation amplifier.
What is the absolute maximum voltage the AM2 U+ input can accept?
Siemens specifies a 0–10 V nominal range and a 12 V DC continuous absolute maximum. Apply a divider that limits the tap to ≤ 10 V at the highest expected pack voltage (16.8 V for a 4S Li-ion pack), with a 30% safety margin recommended.
Can I measure current with the same AM2 module?
Yes. Channel 2 of the AM2 supports 0/4–20 mA. Insert a 250 Ω precision shunt and wire the mA loop from the battery negative through the shunt to I2+, with I2- returning to M2. The 250 Ω burden converts 20 mA to 5 V — well within the 10 V AM2 ceiling.
Is a fuse really required between the battery and the divider?
Yes. A 100 mA fast-acting fuse (0451004.MRL or equivalent) protects the divider, the AM2 input, and the LOGO! common rail from a pack short. The divider itself only draws < 1 mA; the fuse is sized to clear before the AM2 input protection diode (typically 50 mA) trips.