Victron 200ah lithium dropped to 20% overnight with barely any load - BMS playing up?

Had almost the identical thing happen on my boat last winter — woke up to the Victron BMV-712 screaming at me showing 18% state of charge, yet I'd only had the anchor light and bilge pump float switch running overnight. Hardly what you'd call a heavy load.

Turned out it wasn't the BMS misbehaving at all. The culprit was a combination of two things:

1. Cold cell temperatures dragging the actual usable capacity down significantly — lithium really does suffer below about 5°C
2. A rogue background draw I hadn't accounted for. My chartplotter was staying in standby rather than actually powering off, pulling around 0.8A continuously

The BMS was actually doing its job correctly — it was reporting accurately, I just hadn't fully understood what I was looking at.

Worth checking a few things on yours:

• What's the cell temperature reading? If you're in a cold location right now, that alone could explain a lot
• Have you run a proper parasitic draw test with a clamp meter on the negative bus?
• Is the battery reporting actual cell voltages via the Victron app, or just a calculated SoC?

The Victron ecosystem is pretty transparent if you dig into the VictronConnect app — you can pull historical data and usually spot the anomaly pretty quickly. I'd start there before assuming the BMS itself is faulty.

What does your overnight consumption log actually show in VictronConnect? That'd be the first thing I'd want to see before drawing any conclusions.

@CarlWalker cold kills lithium capacity faster than my ex-wife killed my bank balance — what was the battery temperature when it threw a wobbler?

@CarlWalker worth checking whether your BMV-712's Peukert exponent and charge efficiency factor are calibrated correctly for lithium — the defaults are often set for AGM and will skew SOC readings significantly at low temperatures.

Also, did the BMV synchronise to 100% on a proper absorption/float cycle before that overnight period? If it didn't hit the tail current threshold, the SOC calculation starts from a false baseline and compounds the error throughout the discharge.

On the BMS side — if you're using Victron's own Smart Lithium batteries, pull the VictronConnect history and check for any cell undervoltage events. A single weak cell hitting low-voltage cutoff prematurely will look identical to a genuine capacity drop from the BMV's perspective.

What's the battery age and how many cycles showing in the BMS data?

@CarlWalker this one took me down a rabbit hole on my motorhome last February. Turned out my Victron BMV-712 had never done a proper full synchronisation cycle — the 100% reference point was completely wrong from the start. The BMV needs to see the battery hit its tail current threshold at full charge before it trusts its own calculations. If yours has never properly synchronised, every percentage reading since installation has been drifting further from reality.

Check your BMV history screen — if "time since last full charge" reads something embarrassing like 847 hours, that's your culprit. Set your charged voltage and tail current parameters correctly in VictronConnect and let it run a proper absorption cycle to reset.

@RetiredNurse49 and @RiverRunner both raise valid points, but fix the synchronisation first before chasing ghosts.

@CarlWalker this happened on my motorhome last winter and turned out to be a parasitic draw I'd completely missed — the Victron Cerbo GX and a cheap 12v USB adapter were quietly draining maybe 3-4A all night. Doesn't sound much but over 8 hours that's 24-32Ah gone before you've even noticed.

Worth checking a few things:

• Isolate everything and measure actual draw with a clamp meter at the battery terminals
• Any inverters on standby? Even in sleep mode they pull current
• Check your fuse board for any mystery circuits

Also — has the BMV-712 done a proper full charge cycle recently to recalibrate its SoC? If it hasn't hit absorption and float properly in a while the percentage figure drifts badly. Mine was reading 30% out when I finally did a proper charge from near empty.