Victron MPPT 100/30 showing absorption but batteries barely reaching 80% - am I missing something?

Had exactly this scenario last winter with my garden office setup. The MPPT was happily declaring absorption while my Fogstar 100Ah LiFePO4 was sitting at 78% according to the BMS app.

The culprit in my case was incorrect battery voltage parameters. The 100/30 was programmed with settings more appropriate for AGM — absorption voltage set to 14.4V, which the LiFePO4 technically "reaches" fairly quickly under light load, so the controller assumes the battery is fuller than it actually is. The MPPT sees target voltage achieved, transitions to absorption, counts down the timer, then drops to float. Battery never gets a proper charge.

A few things worth checking:

• What battery chemistry are you running? LiFePO4 wants absorption around 14.2–14.6V and a very short absorption time (or tail-current termination if supported)
• Cable sizing and connections — voltage drop means the controller thinks it's at absorption voltage while the battery terminals are actually lower
• Battery temperature — cold batteries accept charge slower and some BMSs will throttle current
• BMS communication — if you're running a Victron-compatible BMS via VE.Can or similar, check whether it's actually sending charge parameters or sitting silent

Connect via VictronConnect and pull up the history tab. Look at the absorption time and whether it's cutting short. Also worth checking the actual battery terminal voltage with a multimeter during what the MPPT thinks is absorption — that'll quickly tell you if there's a wiring issue.

What battery type are you running and what are your current charge profile settings? That'll narrow it down considerably.

@CarlBaker had the same head-scratcher with my shepherd's hut setup last year. Worth checking whether your absorption voltage threshold is actually calibrated to your specific Fogstar cells — the 100/30 ships with defaults that don't always play nicely with LiFePO4.

Also, are you using the battery voltage measured at the controller or trusting the BMS? Long cable runs with undersized wire will cause enough voltage drop that the MPPT thinks it's hitting absorption whilst the battery is genuinely lagging behind.

What's your cable gauge and run length? On my hut I had to bump up to 16mm² and add a proper sense wire before the readings started making sense. The Victron Connect app lets you configure a voltage sense input — absolute game changer if you haven't already enabled it.

Really common gotcha this one! @CarlBaker what absorption voltage have you actually set in the MPPT? With LiFePO4 the sweet spot is typically around 14.2-14.4V for a 12V system, but if your charge parameters are set for AGM (which is often the default from factory), the controller thinks it's done its job while the BMS is telling a completely different story.

Also worth double-checking your battery temperature - LiFePO4 slows right down accepting charge once it gets cold, and the MPPT has no way of knowing the cells are basically dragging their feet. A DVCC setup via a Cerbo GX or even just a BMV-712 giving the MPPT proper feedback can transform this situation. Once I sorted my charge profile it was like night and day. What firmware version are you running on the controller?

@CarlBaker classic case of the MPPT thinking it's done its job while the battery's still in the pub — check your charge current cutoff threshold in VictronConnect, because if it's set too high the controller declares "absorption complete" long before the cells are actually full.

@JaneGrant replied:

@CarlBaker one thing nobody's mentioned yet — have you checked your cable sizing and connections between the MPPT and battery? Voltage drop along undersized cable can fool the controller into seeing the absorption voltage at its terminals while the battery itself is still lagging behind. I had almost identical symptoms with my off-grid workshop and the culprit turned out to be a dodgy Anderson connector causing enough resistance to skew the readings. Worth putting a multimeter directly across the battery terminals during absorption and comparing that to what the Victron app is reporting. If there's more than 0.1-0.2V difference, your wiring's worth investigating before you go changing any charge parameters. VictronConnect's advanced data logging is handy for spotting this pattern over time too.

@CarlBaker worth checking your temperature compensation settings too. If you've got a SmartSolar without the temp sensor attached, it'll default to 25°C assumptions — on a cold winter day that can throw the voltage targets off enough to cut absorption short.

Also, what's your absorption time set to? Victron's adaptive algorithm can terminate early if the tail current drops, which on LiFePO4 happens fast. Min absorption time of 1hr is a reasonable floor.

Check these in VictronConnect:

• Absorption voltage (14.2V typical for 12V LiFePO4)
• Absorption time (min/max)
• Temperature compensation (set to 0mV/°C for LiFePO4)

My shepherd's hut setup had almost identical symptoms until I zeroed the temp comp — made a noticeable difference.