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

Had this exact saga with my 100/30 last summer — turned out my Fogstar 100Ah LiFePO4 had a dodgy BMS that was throttling charge current early, and the Victron was just doing what it was told, the obedient little rectangle.

Check your absorption voltage threshold first — if it's set too low, the controller thinks it's done its job before the battery's actually full. On VictronConnect you want to look at the charge current when it drops to float; if it's still pulling decent amps rather than tapering off properly, something upstream is lying to it.

A few things worth ruling out:

• Battery temp sensor reading incorrectly (cold battery = lower voltage target)
• Absorption time set too short in the custom settings
• BMS communication cutting current before the Victron gets a proper absorption cycle in
• Loose MC4 connections dropping panel voltage intermittently

The Victron algorithm is actually pretty clever — frustratingly so, because when it misbehaves it's usually something else being daft and the Victron just faithfully reporting the chaos.

What battery chemistry are you running and have you checked the VRM logs if you're connected to a Cerbo? The charge cycle graphs will tell you exactly where it's bailing out, which narrows it down enormously.

Also worth checking — is this worse on cloudy days? Intermittent shading can cause it to restart the absorption timer repeatedly until it just gives up and floats. My garden office setup did this for weeks before I noticed a buddleia had grown into the panel line.

@DuctTapeDave — classic BMS miscommunication scenario, and honestly one of the trickier ones to diagnose because from the Victron's perspective it's behaving perfectly.

Worth checking your VictronConnect history graphs — if you're seeing the absorption voltage being reached suspiciously fast rather than the timer just expiring, that's your smoking gun. The BMS is almost certainly reporting a full cell early and pulling back current, so voltage spikes up, Victron goes "job done, float time!" when the battery is nowhere near actually full.

My Fogstar 200Ah had a firmware update that sorted similar behaviour — worth hammering their support chat. They're generally pretty decent about it.

Also double-check your battery's tail current setting in VictronConnect. Dropping it lower means the controller won't transition to float until current genuinely tapers off naturally, rather than being artificially starved by a grumpy BMS.

Really interesting thread this — I've got a similar setup on my boat (Victron 100/30 + Fogstar 100Ah) and I've been scratching my head about exactly this issue.

Quick question for the group: is there a reliable way to tell whether it's the BMS throttling or whether the Victron's absorption voltage is just set too low? I've been using the VictronConnect app but honestly I'm not sure I'm reading the graphs correctly.

Also — does anyone know if enabling the Battery Life algorithm makes this worse? I've seen mixed opinions and I genuinely can't work out whether that setting is helping or fighting against the BMS on a LiFePO4 setup.

@DuctTapeDave did you end up reflashing the BMS firmware or did you just swap the whole unit out?

Been through something similar with my van build — but in my case it turned out to be the battery temperature sensor placement rather than the BMS itself. I'd mounted the sensor on the outside of the battery casing near a warm spot, so the Victron thought the battery was warmer than it actually was and was derating the charge current accordingly. Dropped to float embarrassingly early every time.

Worth checking what your absorption voltage is actually set to in VictronConnect. If it's hitting that target voltage prematurely (because reduced current means less voltage drop under load), the timer kicks in faster than expected.

@DucatoLife — on a boat with higher ambient temps this could be even more of a factor for you. What's your absorption time set to, fixed or adaptive?

@DucatoLife fellow boat owner here — worth checking your absorption voltage setting too. On my narrowboat I found the Victron was hitting absorption target voltage faster than expected because of longish cable runs with undersized wire creating a voltage rise at the controller terminals (less load = less drop = inflated reading). Switched from 6mm² to 10mm² on the battery-to-controller run and absorption time extended noticeably. Easy to verify — stick a multimeter directly across the battery terminals during charging and compare to what VictronConnect reports. If there's a meaningful difference (even 0.2–0.3V), your wiring's lying to the MPPT about the true battery state.