Smartsolar MPPT 100/20, battery voltages are below set values

Had the exact same head-scratcher with my Victron SmartSolar 100/20 on the garden office setup — set absorption to 14.4V for my Fogstar AGM, watched VictronConnect smugly report 13.8V instead like it was doing me a favour.

Turns out there are a few culprits worth checking:

• Wire losses — Victron measures voltage at the controller terminals, not the battery. If your cable run is longer than a couple of metres with undersized wire, you're losing voltage before it even gets there. Add a battery sense dongle or use the Bluetooth sensing feature and it'll compensate properly.
• Temperature compensation — if you've got the temp sensor attached, it'll quietly adjust voltages downward in warmer weather. Worth checking what it's actually reading.
• State of charge logic — the MPPT will sometimes drop out of absorption early if it reckons the battery is full. Not always wrong, but worth cross-referencing with a decent battery monitor like a Victron BMV-712.

The Sunwind AGMs are Scandinavian spec'd — their 14.22V recommendation is a bit conservative compared to what most UK installers would set for a similar battery, but it should still be hitting that target regardless.

Anyone else running AGMs on a SmartSolar and finding the voltages drift seasonally? Wondering if this is more common than people realise once British "summer" temperatures get involved.

@RockyMender classic one this — almost always wiring resistance to blame. The controller hits 14.4V at its terminals but by the time current travels through dodgy or undersized cable, you're losing voltage along the way. Battery sees less than it should.

Few things to check:

• Cable gauge — undersized wire = voltage drop under load
• Connection quality — corroded terminals are silent killers
• Measure at the battery terminals directly with a multimeter while charging, not just trusting VictronConnect

Also worth checking if you've accidentally left it in low temperature compensation mode — it'll drop the target voltage if it thinks it's cold.

Had this exact faff on my own garden office last winter. Turned out a loose Anderson connector was robbing me of nearly 0.4V. Tightened everything up, job done.

@QuietTrekker is right about the wiring but nobody ever wants to hear "your cable's too thin" — measure the voltage at the battery terminals while charging and prepare to be humbled by the laws of physics.

My tiny house setup was dropping nearly 0.4V across a run I'd convinced myself was "fine" — spoiler: it wasn't fine, nothing about it was fine.

Victron's own docs recommend keeping voltage drop under 0.1V, so grab a multimeter and check the actual drop under load before you go fiddling with absorption setpoints and making everything worse.

@ROW_OffGrid honestly "your cable's too thin" is just Victron's way of saying "we knew, we just didn't want to be the ones to tell you" — chuck a proper multimeter across the battery terminals while it's charging and if your voltage drop is more than 0.3V you've found your culprit, classic Ohm's Law doing its thing in the most inconvenient way possible.

@FogstarGal Ha, fair point, but Victron did account for it — that's exactly what the battery voltage sense feature is for on the SmartSolar range. Wire a separate sense pair (0.5mm² is plenty) directly to the battery terminals and the MPPT compensates for the drop automatically. Done this on my motorhome build where the controller is mounted remotely from the 200Ah Fogstar Drift — the controller was seeing ~13.9V but batteries were genuinely at 13.6V. Enabled remote sense, problem vanished overnight.

Worth noting: in VictronConnect under Battery settings there's a Sense option — make sure it's not just showing "internal" if you've wired external sense wires. Caught me out the first time.