Midnight sun confuses MPPT?

Not strictly midnight sun territory here in the UK obviously, but I've been wondering about something similar after last summer on the canal.

During those long June evenings — especially up north near the Scottish border — I noticed my Victron SmartSolar 100/30 was staying "active" well past 10pm. No meaningful watts coming in, but the PV voltage from my two Renogy 175W panels was apparently still high enough that the controller wouldn't properly transition into night mode.

The practical concern for me was around the tail current settings and whether the MPPT was correctly logging absorption/float cycles. If it never properly registers "night," does that mess with the next day's charge algorithm? I genuinely wasn't sure.

A few things I've been considering:

• Street/ambient light near marina moorings could compound this — artificial light keeping PV voltage elevated overnight
• Whether adjusting the "panel voltage to start" threshold in VictronConnect would help force a proper cutoff
• If this causes any issues with lithium BMS wake/sleep cycles (running Fogstar Drift 100Ah)

Has anyone actually seen this cause measurable problems — like shortened battery life or incorrect state-of-charge readings — or is it essentially harmless and the controller just sorts itself out at dawn?

Curious whether anyone running solar on a narrowboat or a remote cabin in Scotland has had proper hands-on experience with this, especially through the summer solstice period. Worth tweaking settings or just leaving it alone?

PylontechGal | 847 posts

@Emma1996 Ooh, interesting one! I had something similar on my narrowboat last June around the solstice. Those long dusks where the panels are still trickling in 15-20W at 10pm genuinely confused my Victron SmartSolar — it kept resetting its absorption timer because the voltage never quite dropped enough to trigger the end-of-day cycle properly.

Worth checking your MPPT's "tail current" setting if it has one. On Victron kit you can set absorption to end when current drops below a percentage of battery capacity rather than relying purely on time or voltage — much more reliable in those weird lingering-light conditions.

Also, are you seeing any actual battery damage or just the controller behaving oddly? Because if it's just confused logging, honestly I wouldn't lose sleep over it! 😄

LuckyHiker | 1,243 posts

Worth adding to what @PylontechGal touched on — the issue is often the MPPT's absorption/float transition logic rather than the controller being "confused" per se.

On my motorhome with a Victron SmartSolar 100/30, those extended dusk periods in June (especially further north) mean panel voltage stays stubbornly above the bulk threshold for hours. The controller interprets this as sustained charging opportunity and keeps cycling back into absorption unnecessarily.

Key things worth checking in VictorConnect:

• Tail current threshold — lowering this helps the controller accept that the battery is actually full
• Maximum absorption time — hard cap prevents endless cycling
• Repeated absorption interval — how frequently it re-triggers

LiFePO4 cells (I run Fogstar Drift 100Ah) are particularly unforgiving here because they genuinely are full by early afternoon, and the BMS will start rejecting current regardless of what the MPPT thinks it should be doing.

SimonKelly | 2,156 posts

Good points from @LuckyHiker on absorption/float transitions — worth adding that Victron MPPTs handle this particularly well once you dig into VictronConnect and set a proper tail current rather than relying purely on time-based float triggers.

On my motorhome with 200Ah of Fogstar Drift LiFePO4, I set tail current to around 2% of capacity (4A) which tells the MPPT "battery's genuinely full, drop to float" regardless of what the sun's doing at 9pm in June.

The long-evening confusion happens because the controller sees sustained irradiance and keeps pushing absorption unnecessarily — tail current solves that elegantly.

Check your BMS logs too if you have them; you might find your cells are hitting balance limits before the MPPT even realises absorption is complete.

BordersOffGrid | 634 posts

@SimonKelly @LuckyHiker so basically my MPPT is having more of an identity crisis than I did when I bought my first narrowboat and thought "how hard can it be" 😅

Genuinely though — I've noticed on my cabin setup that the long Scottish summer evenings make the