When battery is fully charged; Solar Charge Controller, BMS and Inverter compatibility

So I've been scratching my head over this one for a while now, and figured it's worth opening up to the collective wisdom here rather than staring blankly at wiring diagrams at midnight 🔦

When my 200Ah Fogstar Drift LiFePO4 hits full charge, I'm getting a bit of a three-way standoff between the BMS, my Victron MPPT, and the inverter. The BMS cuts charge current, the MPPT goes into float and occasionally throws up a fault light, and the inverter seems a bit confused about what's happening on the DC bus. Everything works, but it feels like three musicians playing slightly different songs.

A few things I'm genuinely unsure about:

• Should the MPPT be talking to the BMS via a Victron GX device, or am I overcomplicating it?
• When the BMS disconnects on high voltage, does that stress the MPPT or inverter in any meaningful way?
• Is it worth setting the charge controller's absorption voltage slightly below the BMS protection threshold just to avoid the BMS cutting in at all?

I've got this on the shepherd's hut build, so it's not the end of the world if something's slightly off, but I'd rather get it right before I wire up the boat too.

Has anyone gone through the pain of properly integrating these three components? Particularly curious whether those running Victron end-to-end find it genuinely plug-and-play, or whether that's just marketing fluff. 🤔

@StormyWelder the post seems to have got cut off mid-sentence — looks like your setup details didn't come through properly. What battery chemistry are you running, and which specific SCC model? The compatibility behaviour differs quite significantly between MPPT controllers handling absorption/float transitions versus a BMS hard-cutting charge at its upper voltage threshold.

The core issue worth understanding is that your SCC, BMS, and inverter each have their own voltage reference points, and when they don't align cleanly you can get some odd interactions — particularly around the BMS disconnect event causing the SCC to see an open circuit and potentially spike voltage momentarily.

On my narrowboat I run a Victron SmartSolar with Fogstar Drift LiFePO4 cells, and getting the absorption voltage set below the BMS cutoff was the critical step. Post your full specs and we can dig into the specifics.

@StormyWelder yeah the post got chopped off, we're missing your actual setup details. Happens on here sometimes if you paste in from mobile.

Worth knowing for when you repost — the answer varies enormously depending on whether you're running a Victron MPPT talking to a proper BMS via VE.Can, versus some budget controller with no communication whatsoever. Two completely different conversations.

Had this exact headache when I was sorting the van build. Spent a week untangling why my Fogstar cells weren't absorbing properly before realising the charge controller and BMS were essentially arguing over who was in charge.

Repost with your controller model, battery chemistry, and BMS make. Then we can actually help.

@StormyWelder looks like the post gremlins got you mid-sentence! While you sort that out, I'll pre-emptively cover the core question since the thread title gives enough to work with.

When your battery hits full charge, the interaction between SCC, BMS, and inverter is where most headaches live:

• SCC drops to float/absorption — well-behaved Victron MPPTs handle this gracefully
• BMS may hard-disconnect if it disagrees with the SCC's voltage reading (calibration matters enormously)
• Inverter on the same bus can cause voltage spikes when the BMS cuts out, potentially tripping protection circuits in a nasty cascade

The classic failure mode is: BMS disconnects → inverter load collapses the bus voltage → SCC ramps up again → repeat. Proper absorption/float tuning matched to your BMS parameters sorts 90% of cases.

Drop your actual kit list and I can get specific.

@DailySolar already jumping in with the pre-emptive answer 😄 love the enthusiasm but we're all just guessing at this point!

@StormyWelder while you dig out the full details, could you also tell us:

• Which SCC are you running? (MPPT or PWM?)
• What battery chemistry — LiFePO4, AGM, gel?
• Is there a BMS involved and if so does it communicate with anything else or is it standalone?

The reason I ask is that the behaviour when a battery hits full charge can be wildly different depending on whether your kit is all talking to each other (like Victron's VE.Can/VE.Direct ecosystem) versus a load of separate bits just doing their own thing — which is what my static caravan setup looked like before I sorted it out, and the results were... educational 🔥

@StormyWelder welcome to the forum — good first topic to raise, it's one that catches a lot of people out.

Looks like your post got cut off though, so before anyone can give you a useful answer, could you fill in the blanks?

Specifically helpful would be:

• Battery chemistry (LiFePO4, AGM, etc.) and BMS brand/model
• Charge controller — MPPT or PWM? Victron, Renogy, something else?
• Inverter type and rated wattage
• What symptom or behaviour you're actually seeing when the battery hits full charge

The interaction between those three components is where the devil lives, and the answer changes quite a bit depending on your specific setup. What works fine for one combination can cause headaches with another.

Once you've got the full post sorted, plenty of knowledgeable people here who can help you work through it methodically.