Adding a portable solar panel to a Birchover CBE panel

The Problem

You want to add a portable solar panel alongside the existing fixed roof panel and connect it correctly to the leisure battery via the factory CBE electrical system. You need to understand whether parallel panel wiring is compatible, whether mismatched panels cause a problem, and where fuses are required.

Solution

1. Locate the engine-run relay in the rear electrical cupboard — this relay cuts power to the habitation lights and water pump when the engine is running, while the fridge and rear 12V socket stay live regardless.

   • This is standard practice on NCC-approved conversions to prevent RF interference with vehicle electronics.

2. Connect the positive output of the solar controller to the leisure battery by piggybacking onto the lower terminal lug of the factory CBE fuse panel, using an existing rear-to-front cable that runs behind the kitchen furniture.

   • This connection is not switched by the engine-run relay, so it stays live when the engine is running.

3. Fit a 20A inline fuse in the positive cable before attaching the spade connector to the CBE terminal.

4. Wire a parallel pair of wires onto the existing solar panel input wires at the controller and terminate them in a connector such as MC4, so a portable panel can be added or removed easily alongside the fixed roof panel.

5. Confirm your controller type before wiring panels in parallel. The PV Logic 10A and 20A units fitted to these conversions are PWM (pulse-width modulation) regulators, not MPPT — they do not convert voltage, they simply switch the panel string onto the battery, so the whole parallel array is held close to battery voltage rather than at each panel's optimum. Check that the combined Voc of both panels does not exceed the controller's maximum input voltage — on the PV Logic unit this is 36V, so do not connect panels in series if their combined Voc exceeds this limit.

6. Be aware that because this is a PWM controller, mismatched Voc values genuinely reduce efficiency in a parallel setup — there is no MPPT stage to harvest the difference. The fixed 100W PV Logic panel (Voc ~21–22V) and the Anker 100W portable panel (Voc 28.5V) will result in the higher-voltage panel being dragged down to the level of the lower one, losing an estimated 25–30W at peak output. With an MPPT controller this mismatch would be far less significant, but with the PWM unit fitted the loss is real.

   • A 10A PV Logic controller caps usable output at around 120W; a 20A version supports up to around 170W.

7. Because the panels have different Voc and this is a PWM controller, fit a blocking diode (a Schottky type to minimise forward voltage drop) in the positive lead of each panel before the parallel join. Without this, when one panel is shaded or producing less, the higher-voltage panel can push reverse current back through the lower-voltage panel rather than into the battery. Many MC4-terminated panels already include an internal blocking diode in the junction box — confirm this with the panel manufacturer before adding external diodes, as you do not want two in series dropping voltage unnecessarily. If neither panel has an internal blocking diode, add external ones.

8. Add a 20A inline fuse on the positive output wire of the solar controller itself, even if the downstream socket has its own fuse, as the controller is a live power source and a short in its output wiring would not be protected by the leisure battery fuse alone.

9. Secure all wiring with p-clips at regular intervals to prevent chafing or mechanical stress.

10. As an alternative input route, you can feed the controller from the cab rather than running cable to the rear. The CBE cab outlet is a standard switched 12V accessory socket (cigar/DIN type), not a dedicated solar input port, so treat it as a 12V supply, not a regulated charge input. Wire the panel into the controller's panel input as normal, then take the controller's battery-output lead to a 12V plug for the cab socket. Fit a 20A inline fuse in the positive lead at the plug. Note two limits: the cab socket is typically fused at 15–20A at the cab fusebox (confirm the rating against your CBE/vehicle fuse chart before relying on it), and on most installs it is switched off with the ignition — so it will only pass charge with the ignition in the accessory position, making it a top-up route rather than a permanent connection. For unattended charging, use the rear CBE connection in steps 2–3 instead.