MPPT 75/15 is sending 16v to system plus port on smart bms 12-200

Bit of a weird one this — noticed my Victron MPPT 75/15 pushing what looked like elevated voltage through to the system plus port on the Smart BMS 12-200 last week, and nearly choked on my cuppa.

Turns out it's not actually sending 16V to your loads — the BMS system plus port shows the charge voltage, not the distribution voltage, which caught me out completely when I first wired mine up on the van build. Classic "everything's on fire" panic when it's actually fine.

That said, worth double-checking a few things:

• Is your battery protect downstream of the BMS system plus? If not, your loads could theoretically see absorption voltage spikes
• Check your absorption voltage setting in the Victron Connect app — mine was set to 14.6V for LiFePO4 (Fogstar cells) but I've seen people accidentally leave it on the AGM preset closer to 14.8V, which creeps up further with temp compensation enabled
• Temperature compensation — if you've got a Smart battery sense or temp sensor fitted, that can nudge absorption voltage upward in cold conditions, especially now we're heading into autumn

Anyone else using the 12-200 BMS with a small MPPT on a marine install? Curious whether the Bluetooth comms between the two are actually doing anything useful or just looking pretty in the app. Mine claims they're networked via VE.Smart but I'm sceptical it's actually changing behaviour at all.

@DaleLover that sounds like the MPPT is seeing the battery as lower than it actually is and compensating — classic symptom when the VE.Direct comms between the MPPT and BMS aren't properly established.

Few things to check:

• Is the MPPT set to Li-ion/LiFePO4 profile rather than a sealed/AGM preset? Wrong chemistry profile will push absorption voltage way too high
• Check your VE.Direct cable is properly seated — if the MPPT isn't getting battery data from the BMS network it'll just do its own thing
• In VictronConnect what's the absorption voltage actually set to? Should be around 14.2V for LiFePO4

Had something similar on the narrowboat when I first wired mine up — turned out I'd accidentally left it on the AGM preset. Took me an embarrassingly long time to spot it 🤦

@MrBodge65 has the compensation angle covered but worth checking your VE.Direct connection too — if the MPPT can't see the battery voltage properly via the network it'll just go off its own terminal reading which can be off by a fair bit.

Had something similar with my 75/15 last year. Turned out my battery temp sensor was reading low so it was applying cold compensation on a mild day. Check in the VictronConnect app under the MPPT's settings — look at what absorption voltage it's actually targeting vs what's set.

Also — is the Smart BMS actively load disconnecting at all? If it's cutting then reconnecting rapidly the MPPT can spike a bit trying to recover. Might be worth logging a few hours of data before assuming it's purely a controller issue.

Really interesting one @DaleLover — just to add to what @MrBodge65 and @DownsExplorer have mentioned, it's worth double-checking your battery temperature sensor if you're running one. In colder weather the MPPT can push higher charge voltages as part of temperature compensation, and 16V isn't massively alarming depending on your charge profile settings.

Also have a look in VictronConnect at your absorption and float voltage settings — sometimes these get misconfigured, particularly if someone's applied a preset that doesn't match your actual battery chemistry. If you're running LiFePO4 behind that Smart BMS the voltage limits should be quite specific.

What battery are you actually using? That'd help narrow it down considerably. The Smart BMS should be protecting you from overvoltage regardless, but better to sort the root cause than rely on that as your safety net.

Good shout from @JasonEdwards but nobody's mentioned the absorption voltage setting in the MPPT itself yet. If someone's manually bumped that above the BMS's overvoltage threshold — even by 0.2v — you'll see exactly this behaviour, especially on a warm day when the bank charges faster than expected.

Had this exact situation in my motorhome last summer. Victron Connect was showing 14.6v absorption but my Fogstar Drift 200Ah BMS was tripping at 14.4v. System plus port was reading elevated because the MPPT was still trying to push past what the BMS would accept.

Worth pulling up your MPPT settings in Victron Connect and cross-referencing against the Smart BMS 12-200's spec sheet — the allowed charge voltage is in there clearly. Get those two singing from the same hymn sheet first before chasing anything else.