Solar not charging
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- Where does the solar stop? Meter in order, midday, panels in full sun: panel voltage at the controller input, controller output at the battery, and the controller's display. Expect: Panel open-circuit voltage 18 to 22 V for each 12V nominal panel (more for series or high-voltage panels), controller output 13.5 to 14.6 V, amps flowing in sun
- Open-circuit voltage at each panel's connector? On the roof, unplug each panel's MC4 pair and meter it in sun. Expect: 18 to 22 V per 12V nominal panel
- Voltage drop from the controller output to the battery posts, with amps flowing? Red lead on the controller's battery positive, black on the battery positive post. Then the same on the negative side. Expect: Under 0.3 V each side
Causes and repairs
Failed panel or junction box
A cracked panel, a burned diode or connection in its junction box, or a pulled MC4. In series strings one dead panel kills the whole string.
Tools: multimeter
Check the junction box on the back. Replace the panel. Reseat MC4 connectors until they click.
Open between the roof and the controller
The roof combiner fuse, a solar disconnect or breaker, a broken wire at the roof entry gland, or a loose terminal at the controller.
Tools: multimeter
Meter at each break point down the run. Replace the fuse or repair the wire. Reseal the roof gland.
Controller has no battery connection
The controller powers from the battery side. A fuse between the controller and the battery, a disconnect switch, or a lithium BMS that has opened.
Tools: multimeter
Fuse and connections on the battery side. If the battery reads 0 V at its posts, see the lithium guide.
Connection or wire size
A loose lug, a corroded fuse holder, or wire too small for the run. The controller sees full voltage and backs off before the battery is full.
Tools: hand tools
Clean and torque the connections. Upsize the wire on long runs.
Battery type setting
A controller set to AGM or flooded stops early on lithium, or floats lead-acid too low. Some controllers also stop charging in heat.
Tools: manual
Set the battery type to match. Check absorption and float voltages against the battery maker's numbers.
Expectations and shade
A 200 W array makes about 50 to 70 Ah on a good summer day and less than half in winter. Shade on one cell cuts a whole panel. A PWM controller on a high-voltage panel wastes most of it.
Tools: clamp meter
Clean the panels, remove shade, match the controller type to the panels. Add panels if the loads need it.
Controller error
Over-voltage (wrong battery type, or a lithium BMS disconnected so the controller sees panel voltage), over-temperature, reverse polarity, or a shorted load output.
Tools: multimeter
Read the code in the manual. Confirm the battery is connected and the type is right. Reset.
Keep going on 12V and batteries
Learn it
- Volts, amps and ohms, the simple way
- Voltage drop: the test that finds bad grounds
- Practice: find the bad connection
- Batteries: lead-acid vs lithium
- Fuses, breakers and grounds
Prove it
- Back lights are dim
- Battery dies even on shore power
- Lithium won't charge on cold mornings
- Coach is dead, battery is fine
- Batteries dead after storage