Victron MPPT keeping float voltage too high on my AGM bank — anyone else had this?

Had almost the exact same headache last summer on the boat. Running a 200Ah AGM bank with a Victron SmartSolar 100/30, and I noticed the batteries were sitting at 14.1V in float for ages — well above what the battery manufacturer spec'd.

Turned out I'd let VictronConnect auto-populate the AGM preset without actually checking what it was doing. The default float on that preset was higher than my Varta leisure batteries wanted. Classic "set and forget" trap.

Few things worth checking:

• Which AGM preset are you using? Victron has a couple — AGM spiral cell vs standard AGM behave quite differently
• Is your temperature sensor connected? Without it, the MPPT has no idea it's a warm day and won't compensate downward
• Are you on the latest firmware? There was a point where some presets had slightly aggressive values

Once I manually dialled the float down to 13.5V and got the SmartSense dongle wired in properly, the bank settled down beautifully. Temperatures in the battery compartment dropped noticeably too — always a good sign you're not cooking things anymore.

The shepherd's hut build I'm working on will be getting Fogstar lithium, so I'll sidestep this AGM fussiness entirely — but for the boat the tweaked settings have been solid ever since.

What size bank are you running, and is this a static installation or a vehicle/marine setup? Makes a difference to how much thermal variation you're dealing with.

@BoatIan yeah the default Victron AGM preset is a bit aggressive for float imo. Had similar on my garden office setup — ended up dropping float to 13.5V in the VictronConnect app and the bank has been much happier since.

Worth checking which AGM preset you're on too — there's a difference between "AGM spiral cell" and standard AGM. Easy to accidentally have the wrong one selected.

Also double-check your battery temp sensor is actually connected properly if you're using one. Mine was reading ambient instead of battery temp and throwing the voltage compensation off completely.

@BoatIan worth checking whether your controller has actually reached absorption properly before dropping to float — if it's timing out on absorption rather than hitting the tail current threshold, it'll carry residual charge into float and the voltage will read high for a while as the battery settles.

On my SmartSolar 75/15 I switched from time-based to tail current termination (roughly 2% of C20 capacity) in the VictronConnect app — made a noticeable difference. The AGM preset is fine as a starting point but genuinely treat it as a starting point, not gospel.

Also double-check your temperature compensation settings if you've got a Smart Battery Sense on there — an incorrect NTC reading can shove the voltage up without you realising it. Caught me out badly last February during a cold snap! 🙈

@Titch nails it — if absorption hasn't hit the tail current threshold, the MPPT just keeps clocking absorption time rather than genuinely finishing the cycle, so your "float" voltage is actually still absorption in disguise.

Check your absorption time setting — Victron defaults to a fixed duration rather than tail-current termination unless you dig into the MPPT settings > Adaptive absorption options in VictronConnect.

For a 200Ah AGM bank, float should sit around 13.5–13.8V max; anything above 13.8V long-term is cooking your electrolyte quietly.

My own setup runs a Fogstar AGM bank and I dropped float to 13.6V — battery temps dropped noticeably.

Also worth cross-referencing your battery manufacturer's spec sheet rather than trusting Victron's generic AGM preset — they vary more than you'd think.

@BoatIan one thing worth checking that nobody's mentioned — are you using the VE.Direct or Bluetooth connection to log the actual absorption tail current? On my static caravan setup I found the measured tail current was way off what I expected because of a loose negative connection adding phantom resistance. The MPPT thought the battery was full; it wasn't.

Also worth confirming which AGM preset you're on — Victron differentiates between standard AGM and AGM Spiral Cell. If your batteries are standard AGM but the controller's set to Spiral Cell, float voltage will be higher than the battery manufacturer specifies.

What does your battery datasheet actually say for recommended float? I'd start there and manually set the float voltage rather than trusting any preset.

@BoatIan had this exact issue with my SmartSolar 75/15 a while back — turned out my absorption voltage was set correctly but my tail current threshold was way too low, so it was timing out rather than actually finishing absorption properly.

Check your tail current setting in VictronConnect. For AGMs I'd suggest somewhere around 2-4% of battery capacity — so roughly 4-8A on a 200Ah bank. Mine was set at 1A which was basically useless.

Also worth double-checking your battery temperature situation. AGMs need lower charge voltages when warm — if you haven't got a temp sensor fitted, the controller just assumes 25°C and can get it wrong either way depending on conditions.

The Victron preset profiles are a starting point, not gospel.