Anyone else's Victron MPPT arguing with their Fogstar lithium like a divorced couple at Christmas?

My 100/30 keeps throwing a "battery safe" strop every time the Fogstar 200Ah drops below 20% — cuts out completely rather than just throttling back like a sensible adult.

Running 200W of Renogy panels on the roof of my Transit, 12V system, and the SOC readings between the SmartShunt and the BMS are having a completely different conversation about what percentage we're actually at.

Anyone cracked the right absorption/float/tail current settings to get these two talking nicely, or is couples therapy (i.e. a Cerbo GX) the only answer?

@ExFarmer21 oh this took me months to sort on the boat. The issue is your MPPT doesn't actually know your battery state — it's just reacting to voltage drop, which on lithium looks like a cliff edge compared to lead-acid.

Go into VictronConnect and check your absorption voltage is set correctly for the Fogstar — mine needed dropping to 14.2V rather than the default 14.4V. Also set your tail current to around 2% of battery capacity (4A in your case).

The "battery safe" cutout usually means the MPPT thinks it's seen a short or a fault because lithium voltage collapses so fast when genuinely low. The Fogstar BMS and Victron aren't talking — they're just guessing at each other. If you can run a VE.Smart network with a battery sense dongle that changes everything.

@ExFarmer21 had the exact same nonsense in my van build. The fix that sorted it for me was going into the MPPT settings and manually configuring a proper lithium charge profile rather than letting it auto-detect — the "battery safe" shutdowns were happening because the absorption voltage was set too low and it was misreading state of charge entirely.

In VictronConnect, set:

• Absorption: 14.2V
• Float: 13.5V
• Tail current: around 2-4%

Also worth checking whether your Fogstar has a BMS communication cable — if you're not running a proper VE.Smart network or at least a BMV battery monitor, the MPPT is essentially flying blind on actual SoC.

200W into a 100/30 is well within spec so it's definitely a comms/settings issue rather than hardware.

@LisaKelly66 is on the right track but didn't finish the thought, so I will.

Your 100/30 is using its own internal voltage-based SOC estimation, which bears essentially no relationship to what the Fogstar's BMS is actually seeing. Below ~24.5V under load it panics.

Two proper fixes:

1. Smart Battery Sense + VE.Smart networking — gives the MPPT actual temperature-compensated voltage from the battery terminals, not the controller's input side
2. Cerbo GX / SmartShunt via DVCC — lets the BMS communicate proper SOC to the whole system and the MPPT throttles intelligently rather than just throwing a strop

@RayPowell's manual threshold fiddling is a workaround, not a solution. You'll just rediscover the problem at a different voltage when temperatures change.

I run this exact stack on my shepherd's hut build. DVCC via Cerbo sorted it permanently.

@SolarNotSure mate did you also not finish the thought, because this thread is basically a relay race where everyone drops the baton 🏃‍♂️ — sorted mine in the garden office by enabling the VE.Direct cable so the Victron actually talks to the BMS rather than just guessing like my ex deciding what I wanted for dinner.

Haha @GlenSimon you've noticed the pattern then 😂

Right, let me actually finish a sentence for once in this thread. The issue is almost certainly your absorption/float voltage settings not matching what Fogstar recommend for their cells. Their 200Ah wants 14.6V absorption and 13.6V float — if your 100/30 is set lower it'll misread the state of charge and panic unnecessarily.

Go into VictronConnect, hit your MPPT, and manually set a custom lithium profile rather than using the preset. While you're in there check your "tail current" setting too — dropping that to around 2-4% of battery capacity stops it cutting out prematurely.

@ExFarmer21 what firmware version are you running on the MPPT? Older versions had a known strop with lithiums below 20% that a simple update sorted.

Right, @DanFisher89 don't you dare leave us hanging as well! 😄

@ExFarmer21 — what I suspect nobody's quite got to yet is that your fix is probably straightforward. In VictronConnect, go into your 100/30 settings and check your battery voltage parameters. The "battery safe" cutoff is almost certainly triggering because Victron's default low voltage disconnect doesn't match what Fogstar actually recommend for their cells.

The Fogstar 200Ah should have a BMS doing its own protection anyway, so your MPPT doesn't need to be quite so jumpy about it. Try nudging the low voltage cutoff down a touch — around 46.4V if you're 48V, or 23.2V on a 24V system — and see if that stops the unnecessary drama.

What voltage is your system running at? Makes a difference to the exact numbers.

@LesHarris is clearly about to nail it, so I'll jump in before this thread becomes a relay race with no finish line 😄

The key thing nobody's mentioned yet — check your Victron Connect app under Battery Settings and look at the "tail current" value. If it's set too high relative to your Fogstar's actual capacity, the MPPT thinks it's never properly full and starts behaving erratically at the bottom end too.

Mine did exactly this with a 100Ah Fogstar. Dropped tail current from 4% to 1% and it sorted itself completely. Took about 10 minutes.