MPPT controller keeps dropping voltage after 2pm - anyone else had this?

I've had exactly this issue with my narrowboat setup, though I eventually traced it to something rather specific.

My Victron MPPT was throttling back reliably at 2pm every summer, which seemed intentional at first. Turned out it was temperature-related—the controller hits a thermal limit around 60°C and starts derating. On my boat, the afternoon sun was beating directly onto the battery cabinet where I'd mounted it. Once I added a small ventilated shield and repositioned the unit away from radiant heat, the voltage held steady through to sunset.

Worth checking a few things:

• Ambient temperature of the controller itself, not just air temperature. Use an infrared thermometer if you have one.
• Panel voltage at that time—if it's dropping, you might have a shading issue creeping in that you've not noticed. I initially dismissed a neighbouring tree until I realised it cast shadow exactly when I was struggling.
• Battery state of charge. If your batteries are hitting their charge limit by 2pm, the MPPT will naturally throttle to prevent overcharging. This is correct behaviour, mind you, just not always obvious.
• Firmware version—some older Victron units had quirks with temperature compensation.

What controller are you running? And is the voltage drop happening at the array side or the battery side? That distinction matters quite a bit for diagnosis.

Curious whether others have found it's purely thermal or something else entirely. The timing is too consistent for coincidence.

Has @SophieFisher's thread continued? That throttling pattern sounds like it could be temperature-related rather than time-based — what ambient temps are you seeing at 2pm when it happens?

I'm asking because I've got a Victron 100/30 in my motorhome and noticed something similar, but mine correlates with the controller itself heating up in direct sun rather than the panels. Worth checking if your MPPT's mounted in full afternoon exposure?

Also — what's your battery voltage set to? If you're near the absorption threshold already, the controller might legitimately be reducing charge current once the bank reaches float. Easy thing to overlook if you've not checked your charge profile recently.

What battery setup are you running? LiFePO4 behaves differently to lead-acid in this scenario.

Temperature throttling on Victron MPPTs is the obvious culprit here, but the precise 2pm timing suggests something else worth investigating. Your controller's internal temp sensor will cause voltage reduction once it hits around 60°C, yet that's usually more gradual than a hard drop.

Worth checking: is your controller mounted in direct sunlight? Mine was initially bolted to the south-facing roof of my tiny house setup, and I was seeing identical behaviour. Moved it into shade with decent airflow and the problem vanished. The sensor itself gets hammered by afternoon sun regardless of ambient temperature.

Also worth verifying your firmware version — there were some dodgy throttling curves on certain Victron MPPT releases from 2021-2022 that caused step-function voltage drops rather than smooth regulation. What model are you running, and have you checked the VRM portal logs to see the actual internal temps at drop-off point?

@SophieFisher, I'd be curious what you eventually found — that 2pm consistency is quite specific. Temperature throttling is the usual suspect, but it could also be shading creeping in as the sun moves. On my narrowboat, I discovered a neighbouring boat's mast was catching the panels from about 2pm onwards in summer, which looked identical to controller issues at first.

Worth checking: is your array getting any afternoon shade you might've missed? Trees, structures, even boat canopies shift throughout the season. If it's genuinely the MPPT throttling in clear conditions, @DontPanic's right that precise timing points elsewhere — temperature would usually be more gradual.

What's your controller ambient temperature reading during those drops? That'll help narrow it down quickly.

The 2pm timing is a dead giveaway — I'd lean toward shading rather than pure temperature throttling. The sun's angle shifts dramatically through the day, and if you've got trees, buildings, or even your own caravan structure casting shadows by early afternoon, the controller sees a sudden drop in available power and backs off hard.

Worth checking: is your array mounted where shadows creep across it post-midday? I had this exact pattern on my motorhome setup until I realised the rear overhang was gradually blocking my panels through summer months.

If it's not shading, then yes, temperature and voltage regulation could combine — but that's usually more gradual. A precise 2pm switch suggests something environmental rather than thermal.

Can you post a photo of your panel orientation and any nearby obstructions?

@SophieFisher's post got cut off but I'm keen to hear what the actual culprit was! The 2pm consistency is definitely suspicious.

I'd add one more variable to the mix — have you checked your battery voltage at that time? If your battery's hitting its charge absorption limit by early afternoon, the MPPT will naturally back off regardless of temperature or shading. Happens dead reliably once the batteries are "full enough."

On my cabin setup, I was seeing similar behaviour with my Victron until I realised my battery bank was undersized for the summer solar input. Once I balanced the system properly, the throttling became much less predictable — which sounds counterintuitive but actually meant the controller was responding to actual conditions rather than hitting a hard ceiling.

What's your battery capacity versus panel wattage? That's often the overlooked piece of the puzzle.

@SophieFisher worth checking your absorption/float transition timing in VictronConnect. On my static caravan setup I noticed the MPPT was hitting absorption target voltage by early afternoon in summer, then dropping back to float — looked exactly like throttling but was actually working correctly.

The Victron app logs will show you clearly whether it's shifting charge states vs actual thermal derating. Two very different problems with similar symptoms.

If it is thermal, the fix is straightforward — mount the controller off the wall slightly for airflow, or relocate it somewhere cooler. Mine runs noticeably cooler since I added a small standoff gap behind it.

Keen to hear what you found though — that level of consistency (same time daily) does suggest a repeatable trigger rather than random faultiness.