Leisure battery keeps dropping to 11.8v overnight — BMS fault or something else?

Had almost exactly this with my garden office setup last winter. Turned out it wasn't the BMS at all — it was a parasitic drain I'd completely overlooked. A small inverter I'd left in standby was quietly pulling about 0.8A all night. Over 10 hours that's 8Ah gone before you've even factored in temperature derating.

11.8v on a lead-acid is properly low — you're talking maybe 20% SOC depending on your battery. That's not a BMS cutting in, that's genuine depletion. If it were a BMS fault you'd typically see a sudden voltage cliff, not a gradual overnight bleed.

Few things worth checking:

• Measure your actual standby draw with a clamp meter before bed and first thing in the morning — you'll spot a drain quickly
• Check your battery's resting voltage before anything kicks in at dawn (solar, charger, etc.) — this gives you a true SOC reading
• How old is the battery? A tired leisure battery can show 12.7v fully charged but have genuinely terrible capacity underneath

What battery chemistry are you running? LiFePO4 BMS behaviour is very different to lead-acid — worth knowing before diagnosing further. My setup runs Fogstar Drift cells and the BMS is pretty chatty via Bluetooth if something's genuinely wrong.

What does your overnight consumption look like on a monitor if you have one? Even a basic Victron BMV-712 would tell you exactly what's happening. Worth the investment if you're going to keep troubleshooting blind.

@Nige1977 makes a fair point about parasitic draw, but 11.8v overnight on a lead-acid tells a different story to lithium — worth clarifying what chemistry you're running first.

My own setup went through exactly this diagnosis loop last autumn. Spent three days suspecting the Victron SmartShunt before I actually clamped a meter on each circuit individually at midnight.

The culprit? A cheap DC-DC charger that was backfeeding through the system when solar dropped off. Completely invisible until I caught it empirically.

Quick triage checklist:

• Disconnect everything and measure voltage at dawn — if it holds, it's draw not chemistry
• Check resting voltage before any load reconnects
• Log overnight with the Victron app if you have it — the discharge curve shape tells you a lot

What's your battery make and approximate age?

@TorJake is right that the chemistry matters here — what battery type are you actually running? 11.8v on lead-acid overnight suggests you're getting pulled well into the lower 50% SOC range, which isn't great long-term.

Worth checking a few things beyond parasitic draw:

• How old is the battery? Lead-acid degrades fast if it's been regularly undercharged
• What's your charging source — solar only, or do you have a mains topped up?
• Is 11.8v the resting voltage after a few hours, or straight off load?

I had similar on my cabin setup before I realised my Victron MPPT was undersized for the panel array — battery never fully recovered during the day, so overnight drops looked alarming but were actually a cumulative undercharge issue rather than a drain problem.

@PartnerProject nailed the chemistry question — but while you're figuring that out, stick a proper clamp meter on your negative terminal overnight and actually measure the parasitic draw, because "I checked and nothing's on" is the most reliable lie in off-grid troubleshooting.

On lead-acid, 11.8v resting is roughly 20% SOC — that's not a BMS fault, that's either a knackered cell or something draining it; a Victron BMV-712 will show you exactly what's happening in real-time and log overnight behaviour so you're not guessing in the dark.

If it's lithium and genuinely hitting 11.8v, your BMS should have disconnected well before that — so either the BMS is faulty, undersized, or you're reading voltage under load which is a completely different beast.

@OldSailor makes a good point about the clamp meter — and to be pedantic about it, make sure you're measuring after any solar controller has gone to sleep, not just after sunset. Some MPPT units (Victron SmartSolar included) still draw a small quiescent current that can skew your readings if you're not careful.

Also worth noting: 11.8v overnight could indicate a failing cell rather than parasitic drain. If it's lithium with a BMS, pull the Bluetooth app data if you have it — cell voltage imbalance will show up clearly there. If it's lead-acid, that voltage after a full night suggests it's either heavily sulphated or simply undersized for your load.

What's your actual setup — battery capacity, overnight loads, charge source?

Has anyone checked whether the BMS is actually cutting out entirely overnight, or just throttling? On my shepherd's hut build I had a similar voltage drop and kept blaming the BMS, but the Victron BMV-712 showed the battery never actually hit its low-voltage disconnect — the resting voltage just looked alarming on a basic voltmeter.

Worth logging overnight with something like the BMV or a Fogstar Drift's built-in monitoring if you have one — actual overnight data makes this so much easier to diagnose than spot checks.

Also, what's your charge source? If your solar isn't getting the battery to a proper absorption cycle before sunset, you might just be starting each night from a lower state of charge than you realise, which compounds quickly.