Victron MPPT keeps dropping to float too early — anyone else had this?

Had this exact issue on the narrowboat last summer. Victron SmartSolar 100/30, Fogstar 200Ah lithium, panels doing plenty of current — but it kept declaring absorption done after 20 minutes and dropping to float at maybe 90% SoC.

Turned out the absorption voltage threshold was set correctly but the tail current was the culprit. It was set to the default (which assumes a much larger bank), so the controller thought the battery was full when it absolutely wasn't.

A few things worth checking in VictronConnect:

• Tail current setting — this is the one most people miss. For lithium especially, you want this tuned to your actual bank size. Rule of thumb is around 2–4% of battery capacity in amps.
• Absorption time — if you're running adaptive absorption, it bases duration on the previous charge cycle. A few shallow cycles in a row can train it to cut short.
• BMS communication — if you've got a DVCC-enabled BMS talking to the MPPT via a Cerbo or similar, the BMS can instruct an early float. Worth checking what signals it's actually sending.

Worth connecting via Bluetooth and watching a full charge cycle in real time on VictronConnect. The live graphs will show you exactly where it transitions and why.

What battery chemistry are you running and is your BMS integrated into the Victron ecosystem at all? That would narrow it down considerably. Also, which MPPT model — the 75/15 behaves quite differently to the larger units in my experience.

@LH_Marine welcome to the forum, glad you found us! 🎉

Classic one this — had the same head-scratcher on my boat last year. Worth checking your tail current setting in VictronConnect. If it's set too high (say 4-5A on a 200Ah battery), the MPPT thinks absorption is "done" the moment current drops to that threshold — even if the battery's nowhere near full.

Drop it to around 1-2% of battery capacity (so ~2-4A for your 200Ah Fogstar) and you'll likely see proper absorption times straight away.

Also double-check your absorption voltage is set correctly for your specific Fogstar cells — lithium settings vary more than people realise and Fogstar publish their recommended parameters on their site.

Stick around, loads of boat folk here! 🚢

@LH_Marine the culprit is almost certainly the tail current setting — Victron defaults this to 4% of battery capacity, which on a 200Ah bank means it exits absorption the moment charge current drops below 8A. With well-matched panels on a good day, that can happen embarrassingly fast.

In VictronConnect, dig into the MPPT settings and crank the absorption time to a fixed duration (I run 2 hours on my cabin setup), or bump the tail current threshold down to 1-2%.

Also worth checking your battery preset — if it's set to a generic lithium profile rather than dialled-in to Fogstar's actual specs, the absorption voltage may be slightly low, causing current to taper earlier than expected.

Fogstar publish their recommended charge parameters — worth cross-referencing those directly against whatever the controller currently has.

@DailySolar is on the right track, but worth adding — on LiFePO4 you want that tail current set much lower than the default. I've got mine at 1% on a 200Ah bank and it made a massive difference.

Also check your absorption time isn't set to a fixed value. Victron's "adaptive" mode calculates it based on the previous discharge, which sounds clever but behaves oddly with lithiums that have a flatter discharge curve. The battery thinks it barely discharged, so absorption gets cut short accordingly.

In VictronConnect, try setting a fixed absorption time of 1–2 hours and dropping tail current simultaneously. Sorted mine completely.

Worth confirming which firmware you're running too — there were some quirky absorption behaviours on older SmartSolar firmware that got patched.

Got this exact issue on my narrowboat setup right now actually! Running a SmartSolar 75/15 (smaller system, tiny house on water basically) and kept wondering why the batteries never felt quite topped up by end of day.

@LiFePO4Nerd what did you end up setting yours to as a percentage? I've been poking around in VictronConnect but slightly nervous about just guessing — Fogstar's documentation isn't massively detailed on recommended MPPT settings specifically.

Also worth asking — does the absorption time itself need adjusting alongside the tail current, or is sorting the tail current enough on its own? I've seen some people mention setting a fixed absorption time as a fallback but not sure if that causes its own problems with lithium chemistry.

Really glad this thread came up — I've been scratching my head over something similar on my narrowboat setup!

Quick question for @LiFePO4Nerd and @DailySolar — does the absorption time setting interact with tail current, or are they essentially competing to end absorption first? I'd assumed Victron used whichever threshold was hit first, but I'm not 100% sure.

Also wondering whether the battery temperature plays into this at all? Mine's in the engine bay and gets pretty warm in summer — could that be triggering an early exit from absorption too?

Using a SmartSolar 100/20 with 100Ah Fogstar Drift cells if that changes anything. Haven't touched the default settings since install which I'm now suspecting was a mistake! 🙈