Victron Multiplus keeps switching to float too early — anyone else had this?

Had this exact issue last winter with my Multiplus II. Kept dropping to float around 80% SOC which was doing my head in.

Turned out my battery capacity settings were way off in VictronConnect. I'd entered the Ah rating from the spec sheet but hadn't accounted for the actual usable capacity of my Fogstar Drift cells. Once I corrected that and tweaked the charge current limits, it started behaving properly.

Few things worth checking:

• Battery capacity (Ah) — is it set correctly for your actual bank?
• Absorption voltage — if it's set too low, the Multiplus hits float sooner than it should
• Tail current setting — this is the one that catches people out. If tail current is too high, it thinks the battery's full when it isn't
• Battery monitor source — are you using a BMV or SmartShunt? If not, it's just estimating

The tail current one was the big fix for me. Default settings seem tuned for AGM and just don't translate well to lithium. Dropped mine right down and it sorted itself out almost immediately.

Worth connecting via VRM if you haven't already — the charge cycle graphs make it much easier to see exactly where it's jumping out of absorption.

Anyone else running lithium with a Multiplus had similar? Curious whether it's a config thing across the board or whether some batches of units just ship with daft defaults.

@GlenWard — had almost the same saga with my shepherd's hut setup last autumn. The capacity setting was one piece of it, but what actually sorted mine was correcting the Peukert exponent in VictronConnect. Had it sitting at the default 1.25 when my Fogstar Drift lithiums wanted something much closer to 1.05.

Once that was dialled in, the SOC readings stopped lying to the Multiplus and it stopped bailing out early. Worth double-checking your battery manufacturer's recommended value — Fogstar actually publish it in their spec sheet which I appreciated.

Also had a dodgy battery temperature sensor reading cold, which was causing the charger to pull back prematurely. Simple thing to overlook in winter.

@GlenWard yeah had this with my garden office setup. Capacity was part of it for me too but the bigger culprit was the peukert exponent being set wrong — was making the SOC calculation go haywire.

Worth checking your absorption voltage as well. Mine was set too low so it was hitting the float threshold before actually being properly charged. Bumped it up slightly and it sorted itself.

Also — are you on the latest firmware? Had a weird SOC drift bug on an older version that caused similar behaviour. VRM portal showed completely different readings to what VictronConnect was reporting locally which was a nightmare to diagnose.

Good thread this — worth checking your absorption voltage threshold as well as the capacity settings. If it's set too low, the Multiplus can convince itself the battery is full before it actually is. In VEConfigure, have a look at the "absorption voltage" and cross-reference it with your battery manufacturer's spec sheet. Also, if you're running LiFePO4, make sure you've got the correct battery type selected — I've seen it left on AGM defaults which throws everything off. The tail current setting is another one to look at; if it's cutting to float based on current drop rather than time, a low figure there will trigger float prematurely on a large bank. Victron's documentation is decent on this but it's easy to miss how these parameters interact with each other.

@PanelDai makes a fair point about absorption voltage, but I'd also flag the charge current setting — people overlook this one constantly.

If your max charge current is too low relative to your battery bank, the Multiplus can hit the absorption voltage early and mistakenly "think" it's done. It's not actually measuring SOC directly — it's working off voltage and tail current.

On my boat setup (280Ah LiFePO4, Fogstar cells) I had to dial in the tail current properly in VEConfigure. Default was way too high. Once I dropped it to around 2% of capacity, it stopped bailing out early.

Worth grabbing VEConfigure rather than just using VictronConnect for this — there are parameters in there you simply can't reach otherwise.