Akkus auf 46% und trotzdem Erhaltungsladung

Anyone else had their Victron MPPT swear blind the batteries are full when they're clearly not?

Mine did something similar last winter — Fogstar Drift 48V LiFePO4 sitting at about 51V (so roughly 45-50% by my reckoning), yet the solar controller had already flipped into float like the job was done. Smug little LED glowing green while my battery monitor was sat there going "mate, that's not right."

Took me an embarrassing amount of forum-digging to work out the culprit: absorption time set far too short, combined with the MPPT trusting voltage more than it should on a cold January afternoon. LiFePO4 cells are notoriously flat in their voltage curve — a couple of volts tells you almost nothing about actual state of charge, which is why relying purely on voltage thresholds is a mug's game.

A few things worth checking if you're in the same boat:

• Tail current settings — is your system actually hitting the absorption end-amps before it exits?
• Temperature compensation — disabled or set for lead-acid? Big no-no on lithium
• BMS comms — if you've got a BMS talking to Victron via VE.Can or VE.Direct, conflicting signals can confuse the whole lot

Also worth pulling up VictronConnect and having a look at the history graph — the charge curve will usually tell you exactly where it bailed out early.

Has anyone else found Cerbo GX logging helpful for diagnosing this sort of thing, or is it just me staring at graphs at midnight pretending I understand them?

Hey @LazyRanger, classic false absorption trigger that one. Your MPPT is likely seeing a voltage spike when the panels first kick in and mistakenly thinking "job done, switch to float."

Worth checking your absorption voltage setpoint — for LiFePO4 48V you want around 55.2V (3.45V per cell). If it's set too low, the charger thinks it's hit target when it hasn't really.

Also double-check your tail current setting in VictronConnect. If it's set too high, the charger bails out of absorption prematurely and drops to float/storage mode way before the cells are properly topped up.

The Fogstar Drift BMS might also be occasionally tripping and causing voltage weirdness that confuses the MPPT. Have you got comms between the BMS and MPPT via VE.Direct or are they running independently? That could be your culprit right there.

Really interesting thread this — I had something similar with my setup last autumn. Quick question for @DODNerd and @LazyRanger: are you both using the VE.Smart Networking feature to link your MPPT to a BMV or SmartShunt?

When I added a SmartShunt into the loop, the MPPT stopped chasing phantom voltage spikes and started trusting the actual state of charge instead. Made a huge difference to my Fogstar cells hitting proper absorption.

Also worth checking — what have you set your tail current to? If it's too high, the charger thinks absorption is "done" prematurely and flips to float way too early. A lot of default settings are quite conservative out of the box.

What firmware version is your MPPT on? There were some absorption-related fixes pushed out relatively recently that might be relevant here.

Seen this exact issue on my boat. Worth checking whether your cable runs are creating a voltage drop between the MPPT and battery terminals — the controller measures at its own terminals, so if there's any resistance in the wiring, it'll read artificially high and think absorption is done early.

On a 48V system even 0.5V drop can fool it. Victron's solution is to use the VE.Smart network with a battery monitor (BMV-712 or similar) doing the actual voltage sensing and feeding that back to the MPPT wirelessly. Made a massive difference for me.

Also double-check your absorption voltage setpoint against Fogstar's actual recommended figures — their Drift cells are sometimes happier slightly higher than generic LiFePO4 defaults in VictronConnect.

Good shout from @KevScott on voltage drop — worth quantifying it properly rather than guessing. Grab a multimeter and measure directly at the battery terminals while the MPPT is in bulk. If you're seeing, say, 52.5V at the controller but 51.2V at the battery under load, that's your culprit. Victron's absorption trigger is based solely on what the MPPT sees at its terminals, not at the battery.

On my cabin setup I ran 35mm² cable between the MPPT and the Fogstar pack specifically to keep drop negligible — anything thinner on longer runs causes exactly this phantom-full behaviour.

Also worth checking: in VictronConnect, what absorption voltage are you actually set to? For a 48V LiFePO4 system it should typically be around 55.2V (3.45V/cell × 16 cells). If someone's set it lower at some point, you'll get early absorption triggers consistently.

Had this exact problem at my cabin last winter. Turned out my Victron SmartSolar had the absorption voltage set a touch too low for LiFePO4 — it was hitting that threshold early and deciding job done, even though the cells were nowhere near balanced.

Worth checking your charge profile settings in VictronConnect. For most LiFePO4 packs the absorption voltage should be around 56.8–57.6V for a 48V system, and the tail current threshold matters a lot too — if it's set too high, the charger exits absorption prematurely.

@SolarCol makes a fair point about voltage drop as well. Both issues can compound each other. Sort the cable losses first so you've got accurate readings, then look at your charge parameters. No point tweaking settings based on dodgy numbers at the terminals.