Problem diagnosis · Victron Energy

Victron MPPT Error Codes — PV Overvoltage, String Sizing & SmartSolar Diagnosis

Your Victron SmartSolar or BlueSolar MPPT controller is showing an error code — most likely Error 33 (PV overvoltage) or Error 38 (PV input shutdown). The solar charge controller has stopped generating and the battery is no longer charging from solar. Error 33 is almost always a string configuration issue, and it gets worse in cold weather. Error 38 is a battery over-voltage protection with a different cause.

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SmartSolar 150 & 250 series
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MPPT error stopping your solar charging?

We review VRM solar data, calculate your string Voc with cold-weather correction, and determine whether the fix is a reconfiguration or a controller replacement.

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Written by Ronald · Independent Solar Engineer Last updated 25 September 2026

Safety note: PV strings can produce dangerous voltages — up to 250V DC or higher depending on configuration. Do not disconnect MC4 connectors under load or in direct sunlight. If you need to isolate the PV string, use the DC isolator switch. Never work on PV wiring without confirming zero voltage with a multimeter.

Diagnostics

5-step MPPT error code diagnosis

MPPT errors 33 and 38 are overwhelmingly caused by PV string voltage exceeding the controller's maximum input. Cold weather makes panel voltage rise — a string that works in summer can trip the controller on a cold morning. Start with the error code, then verify the maths.

Identify the specific MPPT error code from VictronConnect or VRM

Open VictronConnect (Bluetooth) or the MPPT device page in VRM. The error code is shown on the main status screen:

Common MPPT error codes

Error 2 — Battery voltage too high: Battery overvoltage detected — check charge parameters and BMS communication

Error 17 — Controller overtemperature: MPPT too hot — check ventilation, mounting location, ambient temperature

Error 18 — Controller overcurrent: Controller output current exceeded its limit — switching large loads, sudden irradiance changes, or inverter overload

Error 33 — PV overvoltage: PV input voltage exceeded maximum Voc rating — most common error

Error 38 — PV input shutdown: PV input internally shorted to protect the battery from over-charging — check the battery voltage setting on every charger on that battery

Check the error history in VictronConnect under the History tab. If Error 33 only appears on cold mornings and clears by midday, this confirms a cold-weather Voc issue.

Calculate your PV string open-circuit voltage including cold-weather correction

The key calculation: panels in series × panel Voc × cold correction factor

Example calculation

Panel: 400W, Voc = 46.2V (from datasheet at STC / 25°C)

String: 4 panels in series → 4 × 46.2V = 184.8V at STC

Cold correction (−10°C): +12% → 184.8V × 1.12 = 207.0V

SmartSolar 150/45 max: 150V → exceeds by 57V — Error 33 guaranteed

SmartSolar 250/100 max: 250V → within limits ✓

The temperature coefficient for Voc is on the panel datasheet — typically −0.25% to −0.35% per degree Celsius. For a rough UK calculation, add 10–15% to the STC Voc to cover worst-case winter conditions (−10°C ambient, panel at sub-zero before sunrise).

Use the Victron MPPT Calculator at victronenergy.com to verify your configuration. The calculator includes location-based temperature data.

Check whether the string configuration exceeds the controller rating

Common UK configurations that trigger Error 33:

• SmartSolar 150/45: 4× high-power panels (400W+) in series — Voc typically 184V+ at STC, 206V+ cold-corrected. Must be reconfigured to 2s2p or 3s1p.

• SmartSolar 150/35: Even 3× panels with Voc above 50V can exceed 150V cold-corrected. Check panel specs carefully.

• SmartSolar 250/100: 6× panels in series with Voc above 42V can exceed 250V in cold weather. Less common but possible with newer high-efficiency panels.

• Mixed panels: If panels of different models are wired in series, use the highest individual Voc for your calculation — mismatch can cause unexpected voltage distribution.

The fix for an oversized string is to reconfigure: reduce panels in series (add parallel strings if needed), or upgrade to a higher-voltage MPPT controller.

Inspect PV wiring for damage or partial shading issues

If the string Voc calculation is within the controller rating but errors persist, investigate the wiring:

• MC4 connectors: Check for moisture ingress, corrosion, loose connections, or cracked housings. Damaged MC4s can cause voltage spikes.

• Cable insulation: Inspect for physical damage along cable runs — rodent damage in lofts, UV degradation on exposed runs, pinch points at roof penetrations.

• Junction boxes: Open and inspect panel junction boxes for water ingress or burnt diodes. A failed bypass diode can change the string's voltage behaviour.

• Ground fault: An insulation breakdown between the DC circuit and earth can cause voltage anomalies. Measure string voltage to earth with a multimeter.

Measure the actual string Voc at the MPPT input terminals using a multimeter. Compare with your calculated value — a significant discrepancy indicates a wiring fault.

Assess whether the MPPT controller needs replacement or reconfiguration

Two outcomes are possible at this point:

String exceeds rating → Reconfigure

Reduce panels in series, add parallel strings, or upgrade to a higher-voltage controller. This is the most common resolution and does not require replacing the MPPT itself.

String within spec → Controller may be damaged

If the string is correctly sized but overvoltage errors persist, the controller's input protection (varistors, TVS diodes) may have degraded from a previous overvoltage event or lightning surge. The MPPT likely needs replacing.

A remote diagnostic can confirm the fault pattern from VRM historical data — we check daily peak PV voltage, error frequency, and whether the issue correlates with temperature before recommending a replacement.

If replacing, ensure the new controller's Voc rating exceeds your cold-corrected string voltage by at least 10% margin. See our VictronConnect setup guide for configuring a new controller.

Why cold weather is the hidden cause of most MPPT errors

Solar panels are rated at Standard Test Conditions (STC) — 25°C cell temperature, 1000W/m² irradiance. In the UK, panel temperatures regularly drop to −5°C or below in winter, especially in Scotland, Wales, and northern England. At these temperatures, the open-circuit voltage of a solar panel rises significantly — typically 10–15% above the STC value. A string that is safely within the MPPT controller's limit at 25°C can exceed it at −10°C. This is why Error 33 is seasonal — it appears in November or December and disappears in spring, leading homeowners to think it has "fixed itself." It hasn't. The underlying misconfiguration is still there, and it will return every winter.

The SmartSolar 150/45 is the most commonly affected controller in the UK because it hits its 150V limit quickly with modern high-efficiency panels. Three 400W+ panels in series produce around 138V at STC — apparently fine — but 155V cold-corrected, which exceeds the limit. Installers who size strings at STC without applying the temperature correction are the root cause of most Error 33 cases we diagnose. The SmartSolar 250/100 gives much more headroom and is now recommended as the standard choice for UK off-grid and hybrid installations.

MPPT error codes — common questions

What does Victron MPPT Error 33 mean?

Error 33 means PV input voltage has exceeded the controller's maximum rating. This is an overvoltage protection trip — the controller shuts down the PV input to protect itself. The most common cause is too many panels in series for the controller's voltage rating, especially in cold weather when panel Voc rises 10–15% above the datasheet STC value.

Why does my Victron MPPT trip in cold weather but not in summer?

Solar panel Voc increases as temperature drops. At −10°C, Voc can be 10–15% higher than at the 25°C STC rating. A string that's within limits in summer can exceed the controller's maximum on a cold winter morning. This is why MPPT manufacturers require cold-weather Voc correction when sizing strings — and why Error 33 is seasonal in the UK.

How do I fix Victron MPPT Error 38?

Error 38 is a PV input shutdown — the controller has internally shorted the PV input to protect the battery from over-charging. Victron attributes it to an incorrect battery voltage setting, or to another device on the same battery set to a higher voltage, such as a MultiPlus equalising at 17V. Check the battery voltage setting on every charger on that battery. Victron's recovery is to disconnect the panels, disconnect the battery, wait three minutes, then reconnect the battery first and the panels second.

Can I add more panels to my Victron SmartSolar without upgrading?

You can add panels in parallel without increasing string voltage — the controller clips excess current above its rated output. But you cannot add panels in series if doing so pushes the cold-corrected Voc above the controller maximum. If you need more series voltage for long cable runs, upgrade to a higher-voltage model (e.g. 150-series to 250-series SmartSolar).

How much does it cost to diagnose an MPPT controller fault?

Our free remote diagnostic and covers VRM performance data, MPPT error history, PV string voltage calculation with cold-weather correction, and controller health assessment. If the MPPT needs replacing, we confirm the correct model and verify the new configuration before ordering.

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MPPT error killing your solar generation?

Tell us the MPPT model (e.g. SmartSolar 150/45), the error code, your panel specs, and how many panels are in each string. We'll calculate whether a reconfiguration or controller replacement is needed.

No fix, no fee
Cold-weather Voc calculations
String reconfiguration guidance

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