Victron 200Ah lithium showing full charge but dropping off quick under load — BMS fault?

Had something similar on the boat last summer. Victron showed 100% then voltage tanked almost immediately once I fired up the inverter. Turned out one of the cells was well out of balance — the BMS was cutting out to protect it but from the outside it just looked like the battery had fallen off a cliff.

Few things worth checking:

• Cell voltage spread — if you've got a Victron BMS or a Cerbo, you can often see individual cell voltages. Anything more than ~0.1V spread under load is a red flag
• BMS logs — some units record fault events, worth digging into if yours supports it
• Charge settings — are you actually hitting absorption properly or just hitting a voltage threshold and calling it done?

My fix ended up being a proper top-balance of the cells using a bench power supply on each one individually. Tedious but sorted it.

Also worth double-checking your cable connections — a dodgy terminal can cause voltage drop that looks like a battery problem but isn't. Had that fool me once too.

What BMS is actually in the battery? If it's a Fogstar or Redodo-style pack with an integrated BMS they can be a bit opaque about what's actually going on inside. Victron's own smart batteries are more transparent but even they can have cell issues over time.

Anyone else had cell drift on a Victron lithium? Curious how common this is getting as these packs age a bit.

@LDVWanderer classic "wolf in sheep's clothing" cell that one — looks fine until you actually ask it to do something useful, bit like my brother-in-law tbh.

Worth checking if the BMS is actually balancing or just claiming to balance. Victron's own BMS can be a bit optimistic about its achievements — like a

@LDVWanderer had exactly this in my garden office setup — Fogstar 200Ah sitting pretty at 100% then flopping like a wet lettuce the moment the kettle kicked in, turned out one cell was basically a chocolate teapot.

Quick checks worth doing:

• Cell voltage spread under load (anything over ~50mV difference is sus)
• Whether your BMS is actually balancing or just sitting there looking decorative
• Victron's battery monitor history — the minimum voltage dip tells the real story

Run a proper discharge cycle and watch the Victron BMV like a hawk — you'll spot the dodgy cell almost immediately when it drags the pack down early.

@LDVWanderer this takes me back to a painful winter in a layby outside Inverness. My Fogstar 206Ah was doing the exact same pantomime — proudly sitting at 100% on the Victron dashboard, then the moment I switched the kettle on, voltage nosedived like a stone off a bridge.

Grabbed a cell-level Bluetooth monitor and found one cell peaking at 3.65V whilst the others were still climbing. BMS was calling "full" based on that rogue cell alone.

The fix that worked for me: long slow absorption charge at a lower current — gave the weaker cells time to actually catch up. Took three cycles before things levelled out properly.

Worth checking your cell voltages individually rather than trusting the pack voltage alone. The BMS is only ever as honest as its weakest cell.

What @LDVWanderer describes rings very familiar from my static caravan setup. I had a Victron 200Ah that looked textbook healthy on the MPPT dashboard — 100%, float, the works — but the moment the garden office load kicked in across the cable run, voltage would crater within minutes.

The thing nobody mentions is that State of Charge readouts are essentially educated guesses until the BMS has seen a proper full cycle under realistic load. Mine had never been properly capacity-tested after installation.

Ran a deliberate discharge cycle using a known resistive load, logged it through VictronConnect, and found I was actually sitting on roughly 140Ah of usable capacity. One cell was clearly the weak link dragging the pack down.

Worth grabbing a proper cell-level voltage readout via Bluetooth if your BMS supports it — the headline percentage tells you almost nothing in this scenario.