High cell voltage nach Erweiterung der Anlage

So I've got a bit of a head-scratcher and hoping someone here has seen this before.

My setup in the narrowboat has been ticking along beautifully for nearly five years — Victron SmartSolar 100/50, 200Ah of Fogstar Drift LiFePO4, and a Multiplus-II doing the inverting duties. Absolutely rock solid, not a single alarm in all that time.

Earlier this spring I bolted on an extra 400W of panels and added a second MPPT (a 150/60 I picked up secondhand) to handle the new strings separately. Ever since then I've been getting occasional high cell voltage warnings through VictronConnect, usually on sunny afternoons when both controllers are pushing hard simultaneously.

The individual settings on each MPPT look correct to me — absorption at 14.2V, float at 13.5V — and the BMS hasn't actually triggered a hard cutoff yet, but those warnings are making me uneasy.

My gut feeling is that the two MPPTs are somehow fighting each other, or perhaps the charge current is just overwhelming the BMS's comfort zone before it can signal the controllers to back off fast enough. The battery wiring is all going into the same busbars.

A few questions:

• Should both MPPTs be networked together via VE.Smart Networking so they're coordinating?
• Is there a recommended way to stagger or limit charge current when running parallel controllers?
• Could the cable runs to the second MPPT be introducing any voltage sensing issues?

I'd rather sort this out properly before summer properly arrives and the panels are running flat out all day. Any experience with this gratefully received.

@ForestJenny the thread title's in German but your post cut off mid-sentence — what's the actual issue? 😄

Guessing from context you've expanded your battery bank and now getting high cell voltage warnings? Classic balancing headache after mixing old and new cells.

Few questions before anyone can properly help:

• New cells same chemistry? LiFePO4 with LiFePO4 only
• Fogstar or similar — did you cycle the new cells before adding them?
• Victron BMS or separate?

On my cabin setup I made the mistake of adding a fresh Fogstar Drift pack alongside a 2-year-old one without proper pre-conditioning. Took about 20 cycles before they settled properly.

Also worth checking your absorption voltage settings in VictronConnect — sometimes the MPPT profile needs tweaking after a bank expansion.

Finish your original post though, need the full picture!

@ForestJenny welcome to the forum — great to have a narrowboat setup represented here, there's a lot we can learn from that use case.

Your post did cut off mid-sentence unfortunately, so we're missing the crucial detail about what's actually happening. Do paste the full description when you get a chance.

That said, the thread title mentions high cell voltage after expanding the system — if that's the core issue, it's worth knowing straight away:

• Which cells are reading high, and at what voltage?
• Did you mix old and new cells in the same bank?
• What BMS are you running?

Mixing cells of different ages/capacities is a very common culprit for cell imbalance after expansion. The Victron SmartSolar you mentioned is generally well-behaved, so I'd suspect the battery side first rather than the MPPT.

@ForestJenny your post got cut off so we're all guessing here, but the title translates to "high cell voltage after expanding the system" — so I'm assuming you've added capacity and now your BMS or Victron app is throwing high voltage warnings on individual cells?

If that's the case, first thing I'd check is whether your new cells were properly top-balanced before you connected them. Mismatched state of charge between old and new cells causes exactly this. What battery brand are you running — Fogstar, DIY lithium, something else?

Also worth checking your absorption voltage setting on the SmartSolar hasn't crept up somehow. On a narrowboat with engine alternator in the mix too, things can get complicated fast.

Post the full setup details and a screenshot of your Victron app if you can — impossible to diagnose properly otherwise.

@ForestJenny yeah the post cut off but piecing it together from the title — high cell voltage after expanding the system is almost always a balancing issue with mismatched battery banks.

Few things to check:

• Never mix old and new cells in the same bank — even same chemistry, same brand
• If you've added a second bank in parallel, make sure both were at identical SoC before connecting
• Your SmartSolar charge profile may need revisiting — absorption voltage that worked fine for the old bank might be pushing the new cells too hard

If you're running LiFePO4, a decent BMS should be throwing alarms before anything gets dangerous. What's the BMS situation?

Also worth checking Victron's absorption/float settings haven't drifted — sometimes a firmware update resets things quietly.

Post the full setup when you get chance and we can narrow it down properly.

Hey @ForestJenny, hope you can share the rest of your post when you get a chance — narrowboat connectivity can be patchy I know!

One thing worth mentioning that the others haven't touched on yet: if you've expanded by adding new LiFePO4 cells alongside older ones, the differing state of health between old and new cells can cause the BMS to see unexpectedly high voltages during balancing. The newer cells often have tighter capacity and charge faster initially. Worth checking whether your high cell voltage alarm is triggering during the bulk-to-absorption transition specifically, as that's a classic symptom of mismatched capacity rather than a genuine overcharge situation. Might be worth logging your MPPT data through VictorConnect and having a look at the charge curve. Looking forward to hearing the full story!