Solar being restricted

Been scratching my head over this one for a few days now and thought I'd ask here before I start pulling cables.

Running a 48V system on my shepherd's hut build — 200Ah of Fogstar Drift LiFePO4 cells, with 1,200W of panels (3 × 400W Renogys) feeding into a Victron SmartSolar 100/50. The whole point of this setup was to run a decent induction hob and a small portable AC unit during summer lets without the guests losing their minds.

Problem is, the MPPT seems to be throttling back well before the batteries are anywhere near full. I'm seeing maybe 600–700W actual input on a clear day when by rights I should be pulling close to the full 1,200W. The Victron app shows absorption kicking in far too early — we're talking 80% SoC and it's already backing off.

My suspicion is the absorption voltage is set slightly low for this particular battery chemistry, or the tail current threshold is too aggressive, so the controller thinks the pack is fuller than it is. I've cross-referenced the Fogstar spec sheet but the recommended charge parameters feel quite conservative.

A few specific questions:

• Has anyone dialled in Victron absorption/float voltages specifically for Fogstar Drift cells?
• Is there a case for tweaking the tail current setting upward to let it stay in bulk longer?
• Could the BMS itself be signalling the MPPT to curtail via the VE.Direct comms, or does that only apply when using a full Cerbo setup?

Situation isn't desperate — batteries are healthy and the hut is letting well — but I'd like to get the most out of those panels heading into the back end of summer.

@SprinterProject classic symptom of a few things — what's your charge controller setup?

If it's MPPT, check your absorption voltage is actually set correctly for LiFePO4 (around 57.6V for 48V system). Lots of controllers default to lead-acid profiles.

Also worth checking:

• BMS current limits — Fogstar Drift has its own charge current ceiling, easy to hit with decent panels
• High temp cutoff — had this bite me on my narrowboat last summer
• Cable sizing — undersized wire between panels and controller will throttle output silently

What does your controller display actually show when it's "restricting"? Watts, voltage, any fault codes? That'll narrow it down massively before you start pulling anything apart.

@PanelGraham beat me to the absorption point, so I'll go a different direction — check your battery temp sensor isn't lying to your MPPT, mine on the narrowboat was convinced it was -40°C and throttled everything back like it was protecting penguins.

Got a 48V Fogstar setup myself on my shepherd's hut — had something almost identical last summer.

Worth checking your BMS communication if you've got it wired into the Victron (assuming you're using a Victron MPPT?). If the BMS is sending a charge current limit via DVCC and it's set conservatively, the controller will obey it and cap your input. Looked like solar restriction to me but it was actually the BMS being cautious because one cell group was slightly out of balance.

Pull up VictronConnect and check the DVCC screen — it'll show you if a current limit is being imposed externally. Also worth looking at the charge history tiles to see if it's hitting the limit repeatedly at a specific time of day.

Good points from everyone already on the absorption and temp sensor fronts — one thing nobody's mentioned yet: have you checked whether your MPPT is actually configured for LiFePO4 chemistry, or is it still defaulted to a lead-acid profile?

A lot of controllers ship with AGM/gel presets and if you've not manually set the charge voltage ceiling correctly for LiFePO4 (typically 54.4–55.2V on a 48V bank depending on your BMS limits), the controller may be pulling back prematurely thinking it's reached a correct absorption threshold when it hasn't.

What MPPT controller are you running? If it's a Victron SmartSolar you can check the exact profile in VictronConnect — worth screenshotting your charge settings and posting them here so we can actually see what's happening rather than guessing.

What's your cable run length from panels to MPPT? Had this exact issue on my motorhome last year — turns out my 4mm² cable was way too thin for the run I had, voltage drop was brutal and the Victron was seeing phantom high voltage and throttling back.

Chuck your figures into a voltage drop calculator. If you're losing more than 1-2% you'll get weird restricting behaviour that looks like a settings problem but isn't.

Also — daft question but worth asking — have you got partial shading on any panels? Even one shaded cell can hammer output massively if they're wired series. Might explain why it's intermittent rather than constant.

What MPPT are you running and what's the array config?