MPPT 450/100 flat lining

Interesting one this — been seeing something oddly similar on my own setup and it's been nagging at me.

Running a Victron SmartSolar 450/100 feeding a 280Ah Fogstar Drift LiFePO4 bank, and I've noticed that on decent solar days the MPPT seems to hit a ceiling and just... sits there. Flat line on the VRM portal. In my case it's hovering around 1.4–1.6kWh before it plateaus, regardless of whether the panels still have headroom to give more.

A few things I've been ruling out:

• BMS throttling — my Daly BMS does pull charge current down as cells approach absorption voltage, which can look like MPPT flatlining when it's actually the battery saying "steady on"
• Thermal limiting — the 100A unit does derate in high ambient temps, worth checking the controller's own temp readout in VRM
• String voltage headroom — if your panel Voc is pushing close to the 450V ceiling on a cold bright morning, the MPPT may be backing off to protect itself

What does your VRM data show for charge state at the point it flatlines? Is the battery already sitting at a high SOC, or does it happen mid-charge? That distinction matters enormously for diagnosing this.

Also curious whether anyone's seen this behaviour change after a Victron firmware update — I updated to v3.14 a few months back and my absorption behaviour shifted noticeably.

Would be good to hear from others running higher-voltage arrays into the 450/100 specifically. Feels like there's a pattern worth unpicking here.

@GoldenTrekker sounds like your OP got cut off mid-sentence, but I can probably guess where you're headed — the 450/100 entering a flat absorption plateau that just sits there rather than tapering off into float?

Seen exactly this on my motorhome build with a similar Fogstar Drift bank. Nine times out of ten it's the tail current setting in VictronConnect. Default is often 1A or 2%, which on a 280Ah LiFePO4 is far too low — the battery genuinely wants to accept more than that threshold before the MPPT considers absorption "done."

Try bumping your tail current up to 4–5% of capacity (roughly 12–14A on a 280Ah bank) and check your absorption voltage isn't set above 3.45V/cell aggregate.

Also worth confirming your BMS isn't intermittently dropping the charge enable signal — that'll cause identical symptoms and is harder to diagnose.

@GoldenTrekker — yeah, classic truncated OP, we've all been there. Curious what you're seeing though, because I had something almost identical last autumn with my garden office setup.

Fogstar Drift 280Ah, Victron SmartSolar 100/30 in my case, and the MPPT was just sitting there doing absolutely nothing mid-afternoon despite decent sun. Turned out the BMS had tripped into high-voltage protection after a dodgy absorption phase — the charger thought it was happily pushing amps, but the BMS had silently disconnected the battery side.

The tell-tale was checking the VictronConnect history graph — you could see the voltage spike cleanly right at the moment it flatlined.

Worth pulling your BMS logs if it's got Bluetooth. Fogstar's app actually surfaces this pretty clearly. What's your absorption voltage set to? Anything above 3.45V per cell on that pack and you're asking for trouble.

@GoldenTrekker — gutted the OP got clipped, but based on the hardware you've listed I'd wager you're seeing the MPPT plateau well below the expected absorption voltage whilst the BMS sits there reporting healthy SOC. Classic symptom on Fogstar Drift banks is the internal cell balancing kicking in and momentarily suppressing current draw, which the 450/100 interprets as "battery full" and throttles accordingly.

Worth checking a few things:

• VE.Direct logs — pull the raw data and look for the absorption entry timestamp vs actual charged capacity delivered
• BMS Bluetooth app — are individual cell voltages diverging more than ~20mV during that plateau?
• Tail current setting — if you've left it at default 2A on a 280Ah bank, that's almost certainly triggering float prematurely

I run a similar Victron/Fogstar combination and had this resolved entirely by adjusting tail current to roughly 1% of capacity (2.8A) and tightening the absorption voltage to 3.55V per cell.

@GoldenTrekker finish that OP when you get a chance — the suspense is killing me.

That said, I'll throw something in while we wait. I had my SmartSolar 150/35 doing something similar last autumn when my Fogstar Drift bank was nearly full. The 450/100 will throttle hard in absorption once the BMS starts pushing back, and if your cell balancing is slightly off, you can get this weird plateau where it looks like a flatline but it's actually the MPPT doing exactly what it should.

Worth pulling the VictronConnect history and checking whether the absorption voltage is actually being held steady or drifting. Also — what's your absorption setpoint? I run mine at 3.45V per cell (27.6V for 8S) and it made a noticeable difference to how the charge curve behaves toward the top end.

@GoldenTrekker my 450/100 did exactly this until I realised the BMS absorption voltage and the Victron charge profile were having a little disagreement — sorted it in VictronConnect in about four minutes, which is four minutes longer than it took me to cause the problem in the first place.