Boat solar panels not charging (or barely)

A solar panel hardly wears out: when it produces little, it is most often because it is in the shade, badly angled, dirty, or because the current is lost between it and the batteries. Before concluding that a panel is dead, you need to know what you can reasonably expect from it — a panel only gives its rated power in full sun, facing the sun, while cool — and then check the path of the current, in order. The manufacturer’s manual for your panel and your controller always has the last word.

1. What you notice

• The controller shows a very low current in full sun.
• The charge collapses as soon as a shadow falls on a small part of the panel.
• The batteries no longer come back up at anchor despite fine days.
• The controller shows “float” or “absorption” and the current is low.
• A panel produces nothing at all any more, or the controller shows an error.

2. Safety first

A panel in the light produces current as soon as it sees daylight, even when disconnected from everything else: its connectors are live. Several panels wired in series can reach a high DC voltage, and a DC electric arc does not go out by itself. Before touching the wiring, cover the panels with an opaque sheet or work at nightfall, and follow the connection order given by the controller manufacturer (most ask you to connect the battery before the panels, and to disconnect in the reverse order). On the battery side, beware of short circuits: no metal tool across two terminals. Finally, panels are often on an arch or a bimini: do not climb up there at sea, and not without being secured.

3. Likely causes, from the most common to the rarest

• Shade, even partial — the cells of a panel are linked in a chain: the shadow of a boom, a halyard or a radar on a few cells can make the output of the whole panel drop, far more than the shaded area would suggest.
• Angle and time of day — a flat panel produces less early in the morning, late in the evening, in winter and further north; at anchor, the boat swings and the panel never faces the sun for long.
• The battery is full — this is normal: the controller deliberately reduces the current as the battery approaches full charge (the “absorption” then “float” stages). The panel has nothing to do with it.
• Dirt and salt — salt deposits, dust, bird droppings: a single spot can act like a shadow.
• A corroded or badly seated connector — panel plugs live outdoors, in salty air; a plug that heats up or discolours loses current.
• An open fuse or breaker — between the panel and the controller, or between the controller and the batteries.
• A wrongly configured controller — wrong battery type selected, charging voltages too low, no lithium profile: the controller stops too early. A PWM controller (the simplest kind) also loses part of the power when the panel voltage is much higher than the battery voltage; an MPPT controller recovers it.
• A cable that is too thin or too long — the voltage drops in the cable; the controller receives less than it needs, especially with a PWM controller.
• A faulty panel — cracked cell, flexible panel delaminated or browned, hot spot, damp junction box: rarer, but the surface often shows it.

4. What you can check yourself, safely

• The right conditions for measuring — in the middle of the day, clear sky, panel free of shade and facing the sun as much as possible. A measurement taken under thin cloud tells you nothing.
• Shade — watch the panel for an hour: boom, shrouds, wind generator, antenna, dinghy. Note whether output rises again when the shadow moves away.
• Cleanliness — rinse with fresh water and a soft sponge, without any abrasive product or tool that scratches.
• The controller display — panel voltage, charging current, charging stage, battery type set. Compare the type set with your actual batteries. If the stage is “float” with full batteries, everything is normal.
• The panel’s open-circuit voltage — with the panel covered, disconnect it from the controller following the manufacturer’s order, uncover it, then measure with a multimeter between its two wires. Compare with the “open-circuit voltage” printed on its label: in full sun, you should be close to it. Well below, the panel or its cable is at fault. Cover it again before reconnecting.
• The current — on the controller display, or with a DC clamp meter around the cable: compare with the “short-circuit current” or the “current at maximum power” on the label, bearing in mind that an almost full battery limits the current.
• The voltage at both ends of the cable — if the voltage read at the controller is clearly lower than at the panel output while charging, the cable or a connection is losing current.
• Connectors and fuses — by eye, with the panels covered: discoloured plug, melted sheath, blackened fuse, open breaker.
• Sizing — the solar simulator helps you compare what your panels can deliver, depending on the season and the area, with what the boat consumes. Many “breakdowns” are in fact panels too small for the use.

5. When to stop and call a professional

Call in a marine electrician if the open-circuit voltage is far below the label in full sun, if a connector or cable has overheated, if a fuse blows again after replacement, if the controller shows an error that its manual does not let you clear, if your panels are wired in series at a high voltage, or if the installation needs reworking (cable size, adding an MPPT controller, a new panel, switching to lithium). A professional measures line losses, checks the configuration and sizes the whole system together with the batteries — see also batteries that run down.

6. Stopping it from happening again

Rinse the panels regularly, especially after sailing upwind. Check the connectors and the cable glands every season. Plan where the panels go according to the shadows of the rigging, and keep the controller’s manual and settings on board. If the batteries change technology, the controller must be reconfigured the same day. Before a long voyage, the guide preparing a transatlantic crossing helps you draw up a complete energy budget.

Record in the Hullteam log the fitting of the panels, their power, the controller settings and every job, with the date and the invoice: it is this history that will make the boat’s value at resale — see which proofs count.