MPPT Charge Controller Over-voltage

Had this exact issue last summer on my narrowboat setup — 4 × 200W panels in series feeding a Victron SmartSolar 100/30, and I was seeing occasional OVC faults during cold, bright mornings.

The thing people often miss is that panel Voc climbs significantly in cold temperatures. Most datasheets quote Voc at 25°C (STC), but on a crisp January morning at, say, 5°C, you can easily add 10–15% on top of that figure. If you've strung panels in series without accounting for this temperature coefficient, you can clip the controller's input ceiling even though your "paper" calculations looked fine.

Worth checking a few things:

• Recalculate your array Voc at lowest expected ambient temp — the formula is roughly Voc_cold = Voc_stc × [1 + (Tcoeff × ΔT)] where Tcoeff is typically around -0.3%/°C for monocrystalline
• Check your controller's absolute max input voltage, not just the rated operating voltage — on most Victron units there's a margin, but it's not infinite
• Consider reconfiguring series/parallel if you're regularly brushing against the ceiling

I ended up dropping one panel out of series and running a 3S1P arrangement, which solved it entirely. Slight reduction in harvest on overcast days, but the peace of mind is worth it.

What controller are you running, and what's your current string configuration? Knowing your panel specs and local climate would help diagnose whether this is a genuine over-voltage risk or something else triggering the fault code.

Been there with a similar setup, @FenlandSolar! The cold morning issue is classic — panels produce noticeably higher Voc in low temperatures, sometimes 10-15% above their rated figures. Worth running the numbers through Victron's MPPT calculator with your actual minimum expected temperature rather than the standard 25°C STC rating.

Four 200W panels in series could easily push you dangerously close to (or over) that 100V input limit on a frosty morning. Have you considered reconfiguring to 2S2P instead? Halves your string voltage while keeping the same wattage. Bit more wiring faff but it'd likely solve your OVC faults permanently without replacing the controller.

Yeah @DustyHiker's right about the cold Voc spike — caught me out too with my garden office setup.

Worth actually running the numbers before assuming you're safe. Panels are rated at 25°C, but on a crisp winter morning you could easily see -5°C or lower. Most panels derate roughly -0.3% per °C, so that's a 9% Voc bump over the rated spec.

4S on 200W panels can push you uncomfortably close to that 100V limit on the SmartSolar 100/30 even if your datasheet looks fine at STC.

Victron's MPPT calculator is genuinely useful for this — plug your panel Voc in and check the cold-corrected figure. If you're marginal, dropping to 3S and reconfiguring might save the controller long term.

@FenlandSolar been exactly here with my shepherd's hut setup — the fix that saved my Victron was simply reconfiguring to 3S rather than 4S, dropping well below that 100V input ceiling even on a crisp January morning when panels are practically begging to overvolt everything in sight.

Quick maths worth doing:

• Check your panel's Voc at STC, then apply the temperature coefficient (usually around -0.3%/°C)
• At 0°C you're realistically adding 10-15% on top of rated Voc
• 4 × open-circuit Voc at cold temps on a 100V controller is just asking for trouble

Fogstar and Victron's own sizing calculators handle this properly if you can't face the arithmetic before your morning brew.

Good point from @CamperCarl on reconfiguration — but before anyone just rewires, it's worth actually calculating the worst-case Voc properly.

Take your panel's datasheet Voc at STC (typically measured at 25°C), then apply the temperature coefficient. Most crystalline panels derate roughly -0.3% per °C. On a sharp winter morning at say -5°C, that's a +9% uplift on your Voc figure.

Four panels in series at 200W are likely sitting around 45–48V Voc each — push that to -5°C and you're potentially hitting 210V+, which absolutely exceeds the SmartSolar 100/30's 100V absolute maximum input.

The Victron MPPT calculator tool handles this automatically if you feed it your panel specs and local minimum temperature. I always use -10°C as my design floor for anything in eastern England — fenland winters catch people out.

Reconfiguring to 2S2P is the sensible outcome here, but understand why first.