RV solar not charging: Panel or controller

Both of these produce the complaint "solar has stopped charging", and neither is a fault. Ruling them out first saves you an afternoon.

One: The battery is full

Victron lists this first in its own troubleshooting order, and Go Power documents it too: once the battery is full, the controller will stop or significantly reduce its charge current, and that is normal rather than a fault. If your controller shows little or nothing in the afternoon after working all morning, that is the system doing its job. Check the battery's actual state rather than the controller display.

Two: The controller is set for the wrong battery chemistry

Zamp's own manual states plainly that its controllers ship with the default setting on AGM, and carries the warning that an incorrect battery type setting may damage your battery. If you have fitted lithium and left the controller on the lead-acid profile, the controller will run equalisation and temperature compensation that a lithium bank must not see, and the battery will shut the charge down rather than accept it. That looks exactly like a failing panel. This is a setting rather than a fault, and it is worth checking before you test anything electrical.

Related, and easy to miss: Victron warns that lithium batteries are prone to damage when charged below about 5 degrees Celsius, which is 41 Fahrenheit. The physical limit underneath that is freezing point: below 32 Fahrenheit, charging does damage the cells. So a battery that stops accepting charge on a cold morning is protecting itself rather than failing. Controllers handle it differently, and some will show an error rather than simply going quiet.

Victron publishes its own ranked list of why solar stops charging. The causes are: the battery is fully charged; reversed panel polarity; panel voltage too high; reversed battery polarity; the controller being disconnected from the battery through a cable, fuse or breaker problem; incorrect charger configuration such as a low charge voltage or current setting; the charger being externally controlled by a system such as an energy management feature; and the temperature compensation feature misbehaving or being misconfigured with a battery that is too hot.

On your own RV, the order to check is:

The battery is already full

First, because it is free, and because a full battery is the first thing Victron's list names.

The controller setting is wrong

Second, because it is also free and it will quietly undercharge your bank for months if you leave it.

A fuse or breaker in the solar circuit

Victron's fifth cause is the charger disconnected from the battery through a cable, a fuse or a breaker. Controllers are often fitted with a fuse between the controller and the battery, it is easy not to know it exists, and it is worth checking before concluding the controller is dead.

Shading

Even a small shadow on part of a panel can greatly reduce its output. Go Power's own wording: an object as small as a broomstick held across the solar module may cause the power output to be reduced. A new aerial, a roof vent, a tree that grew, or even a cable draped across a panel can do it. If output dropped suddenly without any work being done, look up before you look at the wiring.

Wiring faults come next: loose wires, loose connections and badly crimped connectors. Solar connectors that were crimped by hand rather than with the right tool are a weak point worth checking on any system that has been worked on.

Reversed polarity, panel side or battery side. Worth checking specifically if someone has recently worked on the system, because reconnecting things is how it happens.

Solar is easy to test because there are only three points, and each one has an expected reading.

Measurement one: What the controller is receiving

Start at the controller, with everything connected and the sun on the array. Measure the voltage at the controller's solar input terminals. If the array's operating voltage is there, the panel and its wiring are delivering and you can leave them alone. If it is missing, or far below the battery voltage, the fault is upstream of the controller and it is worth isolating the panel to test it. Victron's 5 volt rule is the figure on its own MPPT chargers: charging begins when the panel voltage is 5 volts above the battery voltage, and continues while it stays 1 volt above. A PWM controller needs far less than that and starts as soon as the panel is above the battery at all, which is why the two types behave differently on an overcast day. Either condition may simply not be met in poor light, and that is a reason for no charging rather than a fault.

Measurement two: What the controller is sending

Measure at the controller's battery terminals, on the terminals rather than on the lugs. That is what the controller is sending, and while the sun is out it should be tracking the battery up through bulk and absorption. A reading there at or below the resting battery voltage, with the array delivering into the controller, means the controller is not passing power along.

Measurement three: The panel, isolated

Only if Measurement one showed nothing at the controller's input is this worth doing, because it means taking the array apart. Cover the panels first, with a blanket or a sheet of cardboard, because a panel in sunlight is a live source that cannot be switched off. Then disconnect them and measure open circuit voltage against the figures on the panel's label. A reading much lower than the value on the label means the panel is the suspect rather than the wiring. Short circuit current is the other figure on the label, and it is the one to be careful with: shorting a panel through a multimeter draws an arc when the connection is broken, and most meters are fused at 10 amps, which a larger array will exceed. A DC clamp meter on a working circuit measures current without any of that.

Putting it back together is where controllers get damaged. Renogy, Go Power and Zamp's current install guides all state the same order: the battery goes onto the controller first and the panel second, which means the panel comes off first when the system is taken apart. A panel connected to a controller with no battery on it exposes the controller to the panel's full open-circuit voltage instead of a battery-clamped input, which is the cause Renogy names for the damage, and on a unit that senses the system voltage automatically it can also leave the controller set for the wrong one. The battery is what powers the controller up, gives it the reference it regulates against, and tells it which system voltage it is working on, which is why the order is not a formality.

What the combination tells you

Nothing at the controller's input means the fault is upstream: the panel, its wiring, or a connection. Input present but nothing coming out of the controller means the controller is the fault. Output present at the controller but a lower voltage at the bank means the wire between them, and the fix is often a thicker or shorter cable rather than a new part.

Victron's wiring guidance puts a number on it: an excessive voltage drop, more than 2.5 percent, is unacceptable anywhere in the system. That is a power-systems figure rather than a solar-specific one, but it applies directly. On long solar runs voltage drop is what to measure, and it shows up as a system that works but never performs to its rating.

There are two types of charge controller and they behave differently enough that it affects what you should expect to see.

MPPT actively hunts for the array's maximum power point, which means it can harvest meaningfully more energy, particularly when the panel voltage is well above the battery voltage or in cooler conditions. It effectively decouples the array voltage from the battery voltage, which is why it keeps harvesting where a PWM controller cannot.

PWM is simpler and cheaper and effectively connects the panel closer to the battery, so some of the array's capability is simply not used. That makes it a good low cost choice for a small system and a poor one for a large array, which is where the two types stop being interchangeable.

How this matters when something is wrong

If you have upgraded your panels but kept a PWM controller, you may have a system that is working correctly and still underperforming, which looks like a fault. Equally, if you have added panels in series to raise voltage without checking the controller's maximum input, you can exceed its rating. Panel voltage too high appears in Victron's own list of reasons charging stops, so it is worth checking the rating against what your array actually produces.

Two chemistry points that matter here

Victron states that sealed batteries and lithium batteries do not require equalisation and should not undergo the process, and that temperature compensation applies to lead-acid, so it should be disabled for lithium. If those two are set wrongly on a lithium bank, you are running a charge profile the battery was never meant to see.

A lot of "solar has stopped working" is really a controller behaving correctly, so it helps to know the stages. A solar controller runs the same three stage pattern as a converter charger.

Bulk runs from empty up to 80 percent state of charge in Victron's own table, pushing maximum available current. Absorption covers the last 20 percent at a held voltage until the current tapers off, and this is the slow part. Float then holds a lower maintenance voltage at full charge.

The numbers differ by chemistry, which is why the setting matters. Go Power's GP-PWM-10 manual gives bulk and absorption at 14.4 volts and float at 13.7 for flooded and AGM batteries, and it has no lithium setting at all. That absence is the setting problem in one line: 14.4 volts is a sane absorption figure for a lithium battery, but a lead-acid profile also applies equalisation and temperature compensation, and a lithium bank protects itself against both by refusing the charge. More panel capacity does not fix that.

So the test of "is my solar working" is not whether the controller is showing charge current at any given moment. It is whether the battery reaches full charge over a sunny day, and whether the controller goes through bulk, absorption and finally float. A system that sits in float all afternoon on a charged battery is working perfectly.

Parts and labour

A 100 watt panel kit with a 20 amp MPPT controller is a modest purchase, and a standalone MPPT controller such as a Victron 100/30 is cheaper than the kit it sits in. Individual panels cost less still. That matters because it means replacing a controller is often a better use of an afternoon than chasing a marginal fault in one.

Professional diagnosis is charged as its own fee, with labour by the hour. On a system whose main components cost about what a couple of hours of labour does, doing the three measurements yourself first is worth the afternoon.

Counterfeit controllers

Solar controllers are widely copied, so buy from a seller you can go back to rather than the cheapest listing.

Solar sits alongside the converter rather than replacing it, so if your batteries are not charging on shore power that is converter not charging. For the whole 12-volt system and the voltage-drop method, start at the 12-volt diagnostic guide.