Victron 200Ah lithium showing full but dropping off fast under load — BMS issue?

Had this exact same thing on my van build last summer. Victron MPPT was reporting 100% SOC but the moment I fired up the induction hob it'd tank to 60% in seconds then stabilise. Drove me mad for weeks.

Turned out my BMS wasn't properly calibrated after I'd done a few partial cycles without ever doing a proper full charge/discharge cycle. The cells weren't balanced and the BMS was lying about the SOC based on voltage rather than actual capacity.

Few things worth checking:

• Cell voltage spread — if one cell is racing ahead of the others under load, that's your culprit
• BMS comms — are you running it into the Victron via VE.Can or just relying on voltage sensing? Big difference in accuracy
• Resting voltage — let it sit for 2 hours off charge and check what it actually reads

I'd also double-check your cable sizing and connection torque. A loose terminal or undersized cable creates resistance that can trick the system into thinking cells are drooping when really it's just a voltage drop across a dodgy connection.

What brand is the battery? Some of the cheaper Fogstar or Enjoybot units have BMS settings that are a bit conservative out of the box — might just need a reconfigure if you've got Bluetooth access to it.

Anyone else dealt with SOC drift on lithium after a lot of partial cycling? Curious whether a full capacity test sorted it for others or if recalibration was needed.

@BurnBen classic voltage sag issue mate. The MPPT sees resting voltage and calls it 100%, but under a big load spike like an induction hob you're seeing the true internal resistance of the cells.

Few things worth checking:

• Cell balance — if one cell group is weaker it'll drag the whole pack down fast
• BMS current limits — some units throttle hard under sudden high draw
• Cable sizing — undersized cables will cause massive voltage drop before it even hits the BMS

Had similar on my Fogstar 200Ah setup. Turned out one of my busbar connections was just slightly loose. Tightened everything up and the sag halved immediately.

Worth grabbing a clamp meter and checking actual current draw vs what Victron reckons it's pulling. Numbers rarely lie once you dig into them.

BreezyCaptain replied:

@BurnBen @OllieThompson73 makes a good point about voltage sag, but worth checking your cell-level balancing too. If one or two cells are lagging behind, the BMS will cut off protection early even when overall voltage looks healthy. Connect via Bluetooth and watch the individual cell voltages under load — if you're seeing more than about 50mV spread between cells you've likely got a balancing issue rather than a pure voltage reading problem. Also worth double-checking your busbar and cable connections are properly torqued; a loose terminal on a 200Ah pack can cause some surprisingly dramatic apparent SOC drops under induction hob loads. What firmware version is your BMS running?

@BurnBen had almost identical grief with my van setup two winters back. Turned out my battery cables were undersized — 10mm² where they really needed to be 25mm². Under a 2kW load spike the resistance in those cables was enough to fool the BMS into thinking cells had sagged past threshold.

Worth checking your connection points too. A loose terminal will absolutely murder your voltage readings under load. Got a proper torque wrench on mine and the problem nearly halved before I even touched the BMS settings.

The Victron SmartShunt is decent but it's only as accurate as what it's measuring. Garbage in, garbage out.

What gauge cabling are you running between the battery and your inverter?

@GafferTapeKing is onto something — undersized cables are the silent killer nobody talks about, but I'd also bet your BMS is throttling output before the MPPT even registers the sag properly.

Classic symptom: SOC reading is essentially lying because it's calculated from voltage, and lithium's flat discharge curve means a tiny voltage dip looks catastrophic to dumb SOC algorithms.

Proper fix: current-based SOC tracking via a shunt — Victron SmartShunt is about £80 and transforms the whole picture. My motorhome build was guesswork until I fitted one.

Also check your BMS discharge current rating — a 200Ah cell pack can still have a BMS rated only 100A continuous, and an induction hob will spike well past that instantaneously before it backs off.

@BurnBen this is a classic tale I know well from my motorhome build. What nobody's mentioned yet is the cell spread at high load — if one cell group is slightly weaker, the BMS will see it hit its low-voltage cutoff threshold and throttle output fast, which looks exactly like a dramatic SOC drop but is actually protection kicking in.

Connect a Victron SmartShunt and watch the individual cell voltages under load via the VictronConnect app. You're looking for any cell diverging more than ~50mV from the others during that induction hob spike.

Worth noting: the MPPT SOC figure is notoriously unreliable without a proper shunt — it's essentially guessing from voltage alone. Once I fitted the SmartShunt on my motorhome the "phantom drops" made complete sense immediately.