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

Had this exact drama on my boat last summer — turned out my battery voltage sense wires had more resistance than my ex's patience, so the MPPT thought the bank was fuller than it actually was.

Few things worth checking:

• Absorption voltage set correctly? Victron defaults can be a bit optimistic depending on your chemistry — worth cross-referencing with your battery manufacturer (Fogstar publish decent specs if you're using their lithium)
• Tail current threshold — if you've got it set too high, it'll boot you into float before the bank's properly topped. Drop it down and see if absorption holds longer
• Temperature compensation — on a warm day the controller might be dialling back voltage thinking it's protecting the cells, but actually cutting the charge short

If you're running a Cerbo GX, pull up VRM and look at the absorption time history — you'll see immediately if it's consistently bailing out early. Mine was clocking eight minutes of absorption on a 400Ah bank which is, let's say, ambitious.

Also worth ruling out a dodgy cell dragging the resting voltage up artificially — had a duff Renogy battery once that sat at 13.1v doing absolutely nothing useful.

Anyone running the SmartSolar with BMS comms via VE.Direct found it makes this worse? Curious whether the BMS is telling it to float prematurely on some setups.

BazKnight57 | Posts: 847 | Location: Norfolk Broads

@Cerbo_Master ha, brilliant analogy mate 😄

Worth checking your absorption time setting as well — if you've got it on the adaptive algorithm and your bank hasn't been deeply discharged recently, Victron's logic can cut absorption criminally short. Had exactly this on my narrowboat setup last spring.

Also double-check your battery temperature compensation if you're running an external sensor. Mine was mounted too close to the engine bay and was reading artificially high temps, causing the MPPT to drop charge voltage prematurely and bail into float before the bank was properly topped.

What battery chemistry are you running? If it's lithium with a BMS, sometimes the BMS itself is signalling the MPPT via VE.Smart networking to back off earlier than you'd expect. Worth cross-referencing in VictronConnect to see what's actually triggering the transition.

SteveBurns99 | Posts: 312 | Location: Array

Had almost identical grief with my shepherd's hut build last autumn. Spent three weekends convinced my Victron SmartSolar was faulty before realising I'd set the absorption time too short — was on the default "adaptive" mode which was misreading my Fogstar lithium bank entirely.

Switched to fixed absorption time and bumped it to a proper duration through the VictronConnect app, and suddenly the whole system started behaving itself.

Worth checking your battery profile settings if you're running lithium — the adaptive algorithm is really designed around lead-acid behaviour and can make a total mess of things otherwise. @Cerbo_Master your voltage sense wire issue sounds like it caused exactly the same symptom from the opposite direction, which just goes to show there's half a dozen roads to the same frustrating destination with these things.

MarineGeoff | Posts: 1,204 | Location: Array

Classic voltage sense drama — if you're running Victron, plug the VE.Direct into VictronConnect and check whether the MPPT's measured battery voltage matches what your BMV-712 (or multimeter) actually reads at the terminals; if there's more than ~0.1V difference, your sense wiring is lying to the controller like a dodgy used-car salesman.

Also worth checking:

• Absorption time setting — if it's on adaptive, a partially discharged bank might genuinely be hitting float faster than expected
• Temperature compensation — a cold battery accepts charge differently, and without a temp sensor the MPPT is essentially guessing
• BMS comms — if you've got a smart lithium bank, a chatty BMS can tell the MPPT to back off early

Fogstar Drift cells + Cerbo GX with proper DVCC enabled sorts most of this nonsense in one go.

WattTony | Posts: 2,156 | Location: Shropshire

Something nobody's mentioned yet — check your absorption time settings in VictronConnect. If you've got adaptive absorption enabled and the battery was sitting reasonably charged before your solar session started, the algorithm will calculate a shorter absorption period and punt you into float earlier than you'd expect. Perfectly normal behaviour, just catches people out.

Also worth having a look at your tail current setting. If it's set too high, the MPPT interprets the bank as full before it actually is and drops out of absorption prematurely. I'd start at around 2-4% of your battery capacity as a baseline and tweak from there.

@SteveBurns99 curious whether yours was lithium or lead-acid — the symptoms can look identical but the fixes are quite different. Makes a difference knowing which way to point people! 🔋