To connect a charge controller to solar panels, wire the solar panels to the charge controller's PV input terminals first, then connect the battery to the charge controller's battery terminals. Always connect the battery before the solar panels, and check polarity carefully before making any connection.
Getting your charge controller wired correctly is one of the most important steps in any solar installation. A miswired controller can damage both the controller and your battery, so taking a methodical approach pays off. This guide walks through the process step by step for both PWM and MPPT controllers.
What You Need Before You Start
Gather the following before touching any wires:
- Charge controller (PWM or MPPT)
- Appropriately rated DC cable (check the controller's maximum input current)
- MC4 connectors or compatible plugs for the solar side
- Inline fuse or circuit breaker between the battery and controller
- Multimeter
- Screwdrivers and wire strippers
Always size your cables to handle at least 125% of the maximum current. Undersized cable is a fire risk.
Step-by-Step Wiring Process
Step 1: Connect the Battery First
This is the most important rule in charge controller wiring: always connect the battery before connecting the solar panels. The charge controller needs power from the battery to initialise its electronics. Connecting the solar array first can cause voltage spikes that damage the controller.
Connect the positive battery cable to the battery terminal marked BAT+ (or BATT+), and the negative cable to BAT-. Install an inline fuse as close to the battery as possible, ideally within 150mm of the positive terminal.
Step 2: Connect Any DC Loads (Optional)
If your controller has a dedicated load output for running 12V or 24V devices, connect those next. This step is optional and only applies if you are using the controller's built-in load control feature rather than drawing directly from the battery.
Step 3: Connect the Solar Panels
Before connecting the panels, use your multimeter to check the open-circuit voltage (Voc) of the solar array. For an MPPT controller, confirm this is within the controller's maximum PV input voltage rating. Exceeding the rated voltage will permanently damage the controller.
Connect the positive solar cable to the PV+ terminal and the negative cable to PV-. Once connected, the controller should display the array voltage and begin regulating charge.
Wiring Multiple Panels
Series Wiring
Wiring panels in series adds the voltages together whilst keeping the current the same. This is the preferred approach for MPPT controllers, especially over longer cable runs, because higher voltage means lower current and therefore smaller cable losses.
Parallel Wiring
Wiring panels in parallel keeps the voltage the same but adds the currents together. This suits PWM controllers and situations where shading on one panel would otherwise drag down the whole array.
Common Mistakes to Avoid
- Reversing polarity: Double-check positive and negative before connecting. Many controllers have no reverse polarity protection.
- Exceeding the PV voltage limit: Cold temperatures increase panel Voc. Calculate worst-case voltage using the temperature coefficient on your panel datasheet.
- No fusing: Always fuse the battery connection. A short circuit without a fuse can cause a fire.
- Mixing panel brands or sizes: Mismatched panels in series cause the weakest panel to limit the whole string.
Checking the Installation
Once everything is connected, the controller display or app should show:
- Battery voltage (matching your bank's current state of charge)
- PV input voltage (if the panels are in sunlight)
- Charging current flowing into the battery
If the controller shows no charging current in direct sunlight, recheck your PV connections and verify the panels are producing voltage with a multimeter.
Safety Notes
Solar panels produce electricity whenever light hits them. There is no switch to turn them off. Always cover panels with an opaque cloth or cardboard before working on the wiring, and never work alone on larger systems. If you are installing a system larger than 3.5kWp in the UK, consult a qualified MCS-accredited installer.
