Float voltage changing from multiplus

Been noticing something similar on my narrowboat setup recently and curious if others have seen this.

Running a Fogstar Drift 200Ah LiFePO4 paired with a Multiplus 12/1200 and a couple of Victron MPPT controllers. On warmer days — and I know we don't get many here! — the float voltage in VictronConnect seems to drift slightly compared to what I've set. Not massively, but enough to notice.

My understanding is the Multiplus can adjust charge parameters based on temperature compensation, but I'm not entirely sure if that's what's happening or whether something else is going on. The BMS comms side of things always feels a bit murky to me in terms of what's actually overriding what.

Few things I'm wondering:

• Is the BMS sending charge voltage adjustments via the VE.Bus that effectively change what you see as "float"?
• Does anyone use a Battery Protect or external temp sensor to manage this more deliberately?
• Is it worth setting up a ESS assistant or sticking with the simpler charge profile approach?

On a boat you do get more temperature variation than a static setup — engine bay can get warm in summer, and obviously cold moored up in winter — so I suspect thermal management matters more than I'd initially thought.

Anyone with a similar Victron stack had to dig into this? Particularly interested if you've got BMS comms properly integrated rather than just running standalone charge profiles.

@BirchTrevor interesting one — are you running the Multiplus in Li mode with the BMS in charge control, or have you manually set the absorption/float voltages yourself?

On my shepherd's hut build I initially had the Multiplus fighting with the BMS because I hadn't disabled the float stage properly for LiFePO4. Once I set float to 13.5V and absorption to 14.2V it settled right down.

Worth checking whether your Fogstar Drift's BMS is sending any DVCC signals if you've got a Cerbo or Venus device in the loop — that could explain why the float seems to be shifting around. Is the voltage creeping up or down?

@BirchTrevor had almost identical behaviour on my boat last winter — Fogstar Drift 200Ah with a Multiplus 12/3000.

Worth checking whether your BMS is sending a DVCC signal via VE.Bus or if it's just relying on the charge disconnect relay. If the BMS is cutting charge current rather than communicating properly, the Multiplus can get confused about state of charge and start hunting between absorption and float.

Key things I'd verify:

• DVCC enabled in VenusOS (if you've got a Cerbo or similar)
• Float set no higher than 13.5V for LiFePO4
• Absorption timeout kept short — mine's at 20 mins

The Fogstar Drift's built-in BMS doesn't natively communicate over CAN, so you're relying on voltage-based control. That's fine but leaves less margin for the Multiplus to behave predictably.

@BirchTrevor the Multiplus has a known behaviour where it'll nudge float voltage based on temperature compensation — even when you think you've locked it down. Worth checking if you've got a VE.Bus Smart Dongle attached, because that'll feed ambient temp data back and the charger quietly adjusts.

Also — and this catches people out constantly — if your Fogstar Drift's BMS is asserting charge enable/disable via a VE.Bus BMS or similar, the Multiplus can interpret reconnection events as a new charge cycle and briefly renegotiate absorption before settling into float. Looks alarming in VictronConnect but is often completely benign.

Pull the 30-day history graph from VictronConnect and check whether the float deviations correlate with temperature drops overnight. That'll tell you immediately whether it's compensation drift or something genuinely misbehaving. 🔋

Hey @BirchTrevor, worth checking whether your MPPT chargers and the Multiplus are actually talking to each other via VE.Bus/VE.Smart networking. If they're not properly networked, each device calculates its own float independently and you can end up with them essentially competing — one device finishes absorb and drops to float, then the other still pushing a slightly different voltage causes the Multiplus to bump back up again. Saw this exact thing on a mate's widebeam last summer. Connecting everything through VE.Smart sorted it almost immediately as they sync to a shared charge profile. Also double-check your battery temperature sensor is actually fitted and reading correctly — a dodgy or missing sensor can cause some odd compensated voltage behaviour even on a static setup. What firmware version are you running on the Multiplus?