The short answer: if your bank is healthy and nothing is switched on, it should lose very little overnight. A bank that arrives at morning visibly down has either a standing load nobody accounted for, or cells that no longer hold what the meter says they hold. Those two causes look identical from the panel and are separated by measurement, not guesswork.
Work through them in order. Ruling out the standing load first is quicker, cheaper and far more often the answer than replacing perfectly serviceable batteries.
Charge the bank fully, then isolate it completely at the main switches and leave it disconnected overnight. Read resting voltage before and after. If the isolated bank holds its voltage but the connected bank does not, something on board is drawing current. If the isolated bank still sags, the cells are the problem and no amount of tracing circuits will help.
Resting voltage has to be read after the surface charge has settled, so allow a couple of hours after the charger comes off. A reading taken straight off charge flatters every battery ever made.
Very few boats are genuinely at zero draw when the panel switches are off. Equipment wired ahead of the switch, memory circuits and anything with a standby mode all keep ticking. Individually they are small. Together, over a fortnight on a mooring, they empty a bank.
With everything switched off, a DC clamp meter on the main negative cable gives you the total standing draw in amps. Multiply by the hours you leave the boat and compare it with what actually disappeared. If the numbers agree, you have found your answer and only need to identify which circuit it belongs to.
From there it is a process of pulling one fuse at a time and watching the clamp reading fall. The circuit that changes the number is the circuit responsible. Anything wired ahead of the distribution board will not respond to this test, which is itself a useful clue about where to look next.
Lead acid banks lose usable capacity long before they fail outright, and a bank that once ran the boat for two nights will quietly become a bank that runs it for four hours. Sulphation, a single weak cell dragging a series string down, and years of sitting partially charged over winter all produce the same symptom.
A load test tells you far more than a voltmeter. So does a shunt based monitor left in place for a week, because it records what went in and what came out rather than what the panel guessed. On lithium banks the pattern is different again: LiFePO4 holds voltage almost flat until it is nearly empty, so voltage alone will never warn you, and a BMS that keeps waking to balance can itself be the standing load.
A bank can also appear to drain overnight simply because it never reached full. Short engine runs, an undersized charger, a solar controller set to the wrong battery profile, or a split charge arrangement favouring the engine start battery will all leave the domestic bank permanently part charged. It then behaves exactly like a bank with a mystery drain.
This is worth ruling out before you buy anything. Leave the boat on shore power for a full absorption and float cycle and see whether the behaviour changes. If it does, the fix is in the charging setup rather than in the batteries.
Testing voltage and pulling fuses is reasonable owner territory. Anything that involves cutting into main battery cabling, altering how the bank is fused or isolated, or working around a lithium installation is not. Battery banks store enough energy to melt a spanner and start a fire in seconds, and a short across main cabling gives no warning at all.
If the trace points at heavy cabling, at the charging system, or at the bank itself, hand it to a qualified marine electrician. We do this as fault finding work, and it is usually resolved in one visit because the diagnostic order is already established.
A properly isolated healthy bank should lose very little indeed over one night. The number that matters is the standing draw measured in amps on your own boat, because every installation has a different set of always-live circuits.
It protects against standing loads, but it also disables bilge pumps and alarms, which most owners will not accept. It is better to identify the draw and keep the safety circuits fed by a charger or solar.
Frequently. A panel can carry a standing load through daylight hours so nobody notices until autumn, when the same drain finally shows up as a flat bank each morning.
No. A parasitic load draws the same current whatever chemistry it is connected to. Lithium gives you more usable capacity to absorb it, which hides the symptom rather than fixing the cause.
We trace standing loads with a clamp meter and a shunt rather than by replacing parts, and we work across the South Coast from Brighton to Lymington.