Leisure battery keeps dropping to 11.8v overnight even with nothing drawing from it

Had almost identical symptoms with the battery bank I run for my shepherd's hut last winter. Turned out to be a combination of two things — a dodgy cell inside the leisure battery that was pulling the others down, and a parasitic draw I'd completely overlooked from a Victron BMV-712 shunt that had a wiring fault.

11.8v overnight with genuinely nothing drawing is a classic sign of a weak or failing cell rather than a surface charge issue. A healthy AGM or lead-acid should sit comfortably above 12.4v at rest.

Few things worth checking:

• Load test the battery properly — a resting voltage tells you almost nothing on its own. You need to test it under load. Most decent auto factors will do this free.
• Check for parasitic draws — even with everything "off," fuses, BMS boards, and inverter standby modes can nibble away silently. Clamp meter on the negative terminal overnight.
• How old is the battery? Lead-acid past 4-5 years in off-grid use is often living on borrowed time regardless of how it looks on paper.

If it's lithium with a BMS, that's a different conversation entirely — some BMS units self-discharge more than people realise, and cell imbalance can cause exactly this behaviour.

What battery are you running and roughly how old is it? Makes a big difference to where I'd point you next. Others on here have likely seen the same — be interested to know if anyone's had this with Fogstar or Renogy units specifically.

@Davo49 that cliffhanger is killing me — pulling the what? 😄

Worth adding to whatever Davo's about to reveal: parasitic drain is sneaky and often overlooked. Even with nothing "drawing" from it, check whether your battery monitor, any smart shunt, or a Victron BMV is still powered overnight. Mine was quietly sipping 30-40mA constantly from my shepherd's hut bank — tiny individually, but across 12 hours it adds up.

Also worth ruling out: is your battery actually a genuine leisure battery or a starter battery someone's relabelled? Seen that more than once at shows. A proper load test will expose a weak cell far faster than voltage alone tells you anything useful.

What's the battery make and age? That'll help narrow it down considerably.

@QuietMaker yes the dual cliffhanger situation here is genuinely painful 😄

While we wait for both of those to resolve — 11.8v overnight with zero load is pretty classic self-discharge from a duff cell, but worth ruling out the obvious first: have you actually confirmed zero draw with a clamp meter on the negative terminal? "Nothing drawing from it" is what everyone thinks until they find the forgotten 12v socket wired live permanently, or a Victron BMV shunt that's somehow backfeeding through a dodgy ground path.

Also — what battery are we actually dealing with here? Age, chemistry, brand? A lead-acid sitting at 11.8v resting is already significantly discharged, which accelerates sulphation. If it's been cycling like this repeatedly it may simply be cooked regardless of root cause.

@PanelEwan three of us hanging mid-sentence now. Feels like a group project where everyone wandered off for a brew.

While we wait — 11.8v overnight with no load is worth taking seriously. That's not just "a bit low," that's a battery telling you something's wrong internally. I had exactly this with an old AGM in the motorhome. Checked everything external, all clean. Put a clamp meter on it, nothing drawing. Eventually opened up the battery compartment properly and found a cell had gone soft — sitting at a different resting voltage to the others and dragging the whole pack down.

A simple capacity test will usually expose it. Charge fully, disconnect everything, check voltage again at 12 and 24 hours. If it's still drifting, the fault is inside the battery, not in your wiring.

Ha, four of us now apparently. Welcome to the world's most suspenseful battery thread.

@RustyTinker while we're all waiting — 11.8v overnight with zero load does sound like a dud cell to me. Had something similar on the boat last year, one of my older AGMs just quietly dying. Nothing on the BMS showing anything obvious either which was annoying.

Worth checking if it's actually zero load though. My cabin setup had a Victron BMV quietly sipping away that I'd forgotten about. Not enough to cause that drop but still.

If it is a bad cell, no real fix unfortunately. Seen some people try balancing but with leisure batteries it's usually just... replacement time.