5V voltage difference for MPPT needed for startup or normal operation?

Been running a similar head-scratcher on the narrowboat this season and thought it worth opening up for discussion here.

My setup is a Victron SmartSolar 150/35 paired with a 24V LiFePo4 bank (Fogstar cells, 8s configuration) and around 800Wp of panels wired 2s2p on the roof. Last summer everything tracked beautifully — controller would pick up within minutes of decent light and crack on. This summer I'm noticing the MPPT occasionally struggles to kick in properly when the battery is sitting high (say, above 27V after overnight storage) and panel voltage isn't sufficiently above that.

Which got me thinking about the 5V differential rule — is that 5V headroom strictly a startup requirement, or does the controller actually need to maintain something close to that margin during normal bulk/absorption operation as well?

My reading of the Victron documentation suggests it's primarily a startup threshold, and once tracking is established the controller can operate with a tighter margin. But real-world behaviour on the boat seems a bit inconsistent with that, especially on overcast mornings where panel Voc is suppressed.

Few things I'd be curious to hear from others on:

• Has anyone with a higher cell-count LiFePo4 bank (16s especially) found this more pronounced at high states of charge?
• Does shading or partial cloud significantly shift where that startup threshold effectively lands?
• Any difference noticed between Victron firmware versions on this behaviour?

Would be particularly interested if anyone running boats or vans with roof-mounted arrays has found panel orientation or tilt affecting how reliably the controller wakes up on marginal mornings.

@ExSquaddie49 — the 5V differential is specifically a startup requirement for the MPPT algorithm to initially "wake up" and begin tracking. Once it's running, the Victron will happily operate with much tighter margins.

Worth noting on a 24V LiFePO4 system: your fully-charged bank sits around 28.8V, so your panel Voc needs to comfortably clear ~34V minimum to reliably trigger startup, particularly on cold mornings when cell voltage sags slightly.

From my own SmartSolar 100/30 experience — if it's sluggish waking up at dawn, nine times out of ten it's the panels barely scraping over that threshold rather than a controller fault.

Check your panel string Voc in the VictronConnect app history. If you're borderline, adding a panel in series sorts it immediately.

Great thread @ExSquaddie49 — worth adding that once your SmartSolar is running, it'll happily continue operating with a much tighter voltage differential. I've seen mine keep chugging along on a 24V bank even when panel voltage drops surprisingly close to battery voltage during heavy cloud cover or late afternoon.

The practical implication for narrowboat life is that morning startup can be the tricky moment — particularly if you're moored under trees or facing east with partial shade early on. Worth checking your absorption voltage setting too, as a high absorption target gives you more headroom for that initial 5V gap to be satisfied earlier in the morning.

@LDVCamper has the right of it on the startup distinction. Just don't panic if it seems slow to kick in on dull mornings — it's usually working exactly as designed.

Really useful points from @LDVCamper and @Tommo10 there.

One thing worth adding from my own experience — on cloudy mornings the panel Voc can sit frustratingly close to battery voltage, which means your SmartSolar might not wake up at all until conditions improve. I've seen this on my setup during overcast Welsh winters.

Worth checking your absorption voltage setpoint too. If you've got a high state of charge already, the differential narrows even further, making startup trickier.

A practical workaround: tilt your panels to catch the best early-morning angle, maximising that initial Voc spike. Sometimes just a few extra volts at dawn is enough to kick the algorithm into life and then it'll track happily all day.

Also, Victron's VictronConnect app shows live panel voltage — dead handy for diagnosing exactly where the gap is sitting. 📱

My static van taught me this the hard way — caught my Victron SmartSolar having a lie-in on a perfectly sunny morning because the panels were borderline on that startup threshold. Slapped an extra panel on the string and it's been leaping out of bed ever since. The Victron Connect app is your mate here — watch the "PV voltage" vs battery voltage live and you'll see exactly when it decides to bother waking up. Worth checking your panel string voltage isn't getting dragged down by a dodgy connection too, because a loose MC4 on a 24V system can quietly steal just enough volts to keep the controller snoozing all morning like a teenager. 🔌