Instellingen Victron mppt 100/30

Been having a similar head-scratcher with my Victron MPPT 100/30 and thought I'd throw it out here.

Running a Fogstar Drift 100Ah LiFePO4 and everything works perfectly when the battery is at a decent state of charge — solar charges clean, absorption kicks in properly, job done.

The issue crops up when the battery is heavily discharged (I'm talking sub-20%). The MPPT seems to behave oddly — it'll start charging fine but then the current drops off far earlier than it should, almost like it's confused about which charge stage it's in. I've checked the battery profile settings and I'm running the lithium preset, but I wonder if I need to tweak the absorption voltage or the tail current threshold manually.

Anyone else seen this with lithium setups? A few questions I'm wrestling with:

• Is the tail current setting the likely culprit when charging from a low SoC?
• Should I be adjusting absorption duration — or locking it to a fixed time rather than adaptive?
• Does it matter that my panels are slightly oversized for the controller (about 380W on a 30A unit)?

I've had a poke around in VictronConnect but there are so many variables it's hard to know what to change without potentially making things worse. I'm also wondering whether the BMS on the Fogstar is communicating any current limits that might be throttling things — though I don't have a comms cable between them, so it should just be voltage-based control.

Would be good to hear from anyone running a similar combo — what settings are you using?

@NickHughes what happens at the tail end of charge is often a Victron absorption timeout issue rather than anything wrong with the battery itself. The 100/30 defaults are really set with lead-acid in mind.

For Fogstar Drift specifically, I'd check you've got:

• Absorption voltage: 14.2V
• Float: 13.5V
• Absorption time: set to a fixed short period (or use the adaptive setting carefully)

On my boat setup I had the exact same weirdness — looked like the controller was hunting around rather than settling. Turned out absorption time was running far too long and the BMS was getting twitchy about it.

Worth connecting via the VictronConnect app and watching the live data through a full cycle. You can see exactly where it starts misbehaving.

Hey @NickHughes, worth checking your tail current setting in VictronConnect if you haven't already. On LiFePO4 the Fogstar Drift is pretty fussy about that end-of-charge behaviour. I'd set tail current to around 2-4% of battery capacity (so roughly 2-4A for your 100Ah) which tells the MPPT to exit absorption once current drops that low, rather than just timing out.

Also double-check your absorption voltage is set to 14.2V and float at 13.5V — Fogstar actually publish recommended Victron settings on their website which is handy. Make sure you're not running the default lead-acid profile, as that'll cause all sorts of odd behaviour at the top end.

What firmware version are you on? Some earlier versions had quirks with the tail current detection. 🔧

@NickHughes my Fogstar Drift did the exact same thing until I realised the MPPT was essentially arguing with the BMS like two blokes in a Tesco car park over the last parking space — bump your absorption voltage down to 14.2V and let the BMS actually do its job rather than the Victron trying to be clever about it.

Good shout from @AnneButler there. One thing nobody's mentioned yet — check your rebulk voltage setting. If it's set too high, the MPPT will keep triggering new charge cycles unnecessarily as the battery rests and voltage drops slightly post-absorption. On LiFePO4 you want rebulk sitting around 26.0V (for 24V) or 13.0V (12V system).

Also worth enabling the BatteryLife algorithm off if it's somehow active — that's designed for lead-acid behaviour and can cause odd cut-off patterns with lithium.

What voltage is your absorption set to? Fogstar recommend 14.2V for the Drift if I recall correctly. Sometimes people inadvertently run it a touch high and the BMS steps in before the MPPT finishes its cycle, which creates exactly the kind of confusion you're describing.

Pop your VictronConnect screenshots up if you can — easier to diagnose from there.

@12VNerd makes a solid point on rebulk voltage — worth expanding on that.

On my 100/30 with a Fogstar Drift I've got rebulk set to 26.0V (2V below absorption at 28.0V). If yours is set too close to float, the controller keeps re-triggering bulk unnecessarily and you get this weird cycling behaviour, especially with a partial load pulling the battery down slightly overnight.

Quick checklist specific to LiFePO4 on the 100/30:

• Absorption voltage: 28.4V
• Float: 27.0V (or lower — some run 26.8V)
• Rebulk: 26.0V
• Tail current: 1-2% of capacity (so ~1-2A for a 100Ah)
• Absorption time: set to adaptive or cap it short — LiFePO4 doesn't need long absorption

What firmware version is your MPPT running? Some older versions had quirks with tail current detection worth updating past.