To build a reliable Arduino solar monitoring system using CTs (Current Transformers) for an off-grid storage setup, you need an Arduino microcontroller, a split-core AC CT (like the YHDC SCT-013), and a burden/bias resistor network to step the induced AC current down to a readable 0-5V analog signal. While the Arduino handles the data logging, the true success of the system relies on correctly sizing the upstream power architecture. This guide maps the CT monitoring circuit to a complete 48V LiFePO4 solar storage system, detailing battery sizing math, inverter selection, and precise CT calibration techniques used on the bench.
System Architecture & Battery Bank Sizing
A robust off-grid monitoring system must measure power at the point of consumption. Our system block flows from source to load as follows: Solar PV Array (2400W) → MPPT Charge Controller (60A) → 48V LiFePO4 Battery Bank → Pure Sine Wave Inverter → Main AC Load Panel. The Arduino monitors the inverter's AC output by clamping SCT-013 CTs directly onto the L1 and L2 hot legs feeding the AC panel.
Let's size the battery bank for a realistic scenario: a 1500W continuous AC load running for 5 hours daily (7.5 kWh total). Sizing math for lithium requires adjusting for inverter efficiency, typically 88% for high-frequency pure sine units. The DC energy required from the battery is 7.5 kWh / 0.88 = 8.52 kWh.
Unlike lead-acid batteries, which suffer severe capacity loss at high discharge rates due to Peukert's Law (exponent ~1.3), LiFePO4 chemistry exhibits a Peukert exponent of roughly 1.05. Therefore, high-draw penalties are negligible. We derate the bank strictly for round-trip inverter efficiency and Depth of Discharge (DoD) limits to preserve cycle life.
| Configuration | Nominal Voltage | Total Capacity (Ah) | Max Continuous Discharge (C-Rate) | Usable Capacity (80% DoD) |
|---|---|---|---|---|
| 1P 4S (Standard 12V block x4) | 51.2V (48V nominal) | 100Ah | 1C (100A / 5120W) | 4.09 kWh |
| 2P 4S (Parallel 12V blocks) | 51.2V | 200Ah | 1C (200A / 10240W) | 8.19 kWh |
| 3P 4S | 51.2V | 300Ah | 0.5C per cell limit* | 12.28 kWh |
| 16S (Raw cells in series) | 51.2V | 100Ah | 1C (100A / 5120W) | 4.09 kWh |
*Note: When paralleling raw cells, the effective C-rate limit of the individual cell dictates the max draw. A 100A BMS per string is recommended.
Series vs. Parallel Consequences: Wiring cells in series increases system voltage while maintaining the Ah rating of a single cell; this is critical for reducing DC current and minimizing I²R heating in busbars. Wiring in parallel increases Ah capacity at the same voltage, but requires precise cell matching. For our 8.52 kWh requirement, a 2P 4S configuration (51.2V, 200Ah) yields 10.24 kWh nominal, providing 8.19 kWh usable at 80% DoD—perfectly covering the daily load.
Inverter Sizing & Charge/Discharge Limits
Selecting the right inverter requires looking beyond continuous wattage. Our stated load is 1500W continuous, but inductive loads (refrigerator compressors, well pumps, power tool motors) draw 3x to 5x their running wattage for a few milliseconds during startup. Applying a standard 25% overhead for continuous thermal headroom, plus surge tolerance, a 3000W 48V Pure Sine Wave Inverter is the correct sizing choice. This ensures the inverter operates at 50% capacity during continuous loads, keeping internal MOSFET temperatures low and extending lifespan.
Charge and Discharge Limits: The 2P 4S LiFePO4 bank (200Ah total) is governed by strict C-rate limits to prevent lithium plating and separator degradation.
- Max Discharge Rate: 1C continuous (200A). At 51.2V, this equals 10,240W, well above our 3000W inverter's maximum DC draw (~65A accounting for efficiency).
- Max Charge Rate: 0.5C continuous (100A). This dictates your MPPT sizing. A 60A MPPT controller (max 3000W PV input) is ideal, as it charges the bank at 0.3C, optimizing cell longevity.
- Depth of Discharge (DoD): While LiFePO4 can technically discharge to 100%, the BMS low-voltage disconnect (LVD) usually trips at 2.5V per cell. To guarantee 6000+ cycles, set your inverter's low-battery cutoff to 48.0V (3.0V per cell), effectively capping DoD at 80-90%.
Wiring the Arduino CT Monitoring Circuit
The YHDC SCT-013-000 is a 100A split-core CT with a 2000:1 turns ratio. It outputs a secondary AC current, not a voltage. To interface this with the Arduino's 0-5V ADC (Analog-to-Digital Converter), you must build a burden and bias circuit.
The Burden Resistor Math: At 100A primary current, the secondary current is 100A / 2000 = 50mA (0.05A). We want an RMS voltage of roughly 1.65V to allow for peak swing without clipping the 5V Arduino rails. Using Ohm's Law (R = V / I): 1.65V / 0.05A = 33 ohms. A standard 33Ω 1/2W resistor is the perfect burden.
The Bias Circuit: The Arduino ADC cannot read negative voltages. We must shift the AC waveform up by 2.5V (half of VCC). This is done using two 470kΩ resistors as a voltage divider across 5V and GND, with a 10µF decoupling capacitor tied to the midpoint to stabilize the reference voltage against power supply noise.
| Microcontroller | ADC VCC | Target Bias Voltage | Burden Resistor (100A CT) | Voltage Divider Resistors |
|---|---|---|---|---|
| Arduino Uno / Nano (5V) | 5.0V | 2.5V | 33Ω (1/2W) | 2x 470kΩ |
| Arduino Nano 33 IoT (3.3V) | 3.3V | 1.65V | 22Ω (1/2W) | 2x 470kΩ |
| ESP32 DevKit V1 (3.3V) | 3.3V | 1.65V | 22Ω (1/2W) | 2x 100kΩ (lower impedance for ESP32 ADC) |
Bench Note: Never leave the secondary wires of a traditional CT open-circuited while primary current is flowing, as this can induce lethal high voltages. The SCT-013 is low-power and split-core, but it is best practice to wire your burden resistor directly at the CT jack before clamping it onto a live inverter output.
Calibration, EMI, and Code Implementation
Reading AC current with an Arduino requires sampling the waveform rapidly to calculate the Root Mean Square (RMS). The industry-standard OpenEnergyMonitor EmonLib handles the heavy lifting for sampling and RMS calculation.
However, monitoring an inverter output introduces a massive real-world variable: Electromagnetic Interference (EMI). High-frequency inverters switch DC to AC using PWM at roughly 20kHz. This switching noise easily bleeds into the high-impedance CT bias circuit, causing the Arduino ADC to read phantom current even when the load is off.
Mitigation Strategies:
- Twisted Pair Wiring: Use twisted pair cable for the leads running from the SCT-013 to the Arduino breadboard. This cancels out common-mode magnetic noise from the inverter's internal transformers.
- Physical Separation: Keep the Arduino and the CT bias circuit at least 12 inches away from the inverter's DC busbars and AC output terminals.
- Software Filtering: Implement a moving average filter in your Arduino sketch. Discard any RMS reading below 0.2A as noise floor.
Phase Shift and Real Power: If you also measure AC voltage (using an AC-AC adapter transformer) to calculate true Watts (Real Power), you must account for phase shift. The SCT-013 introduces a slight phase lead, and the AC-AC adapter introduces a phase lag. In your EmonLib initialization, you must tune the PHASECAL constant. Start with a value of 1.7 and adjust it while monitoring a known purely resistive load (like a space heater) until the calculated Power Factor reads exactly 1.00.
By correctly sizing your 48V LiFePO4 bank for the continuous load, respecting the C-rate limits, and properly biasing your CTs to reject inverter EMI, your Arduino solar monitoring system will yield lab-grade accuracy for tracking off-grid energy consumption.
