Marine Battery Glossary

For LiFePO4 this is typically 14.2 to 14.6 V on a 12 V system, held briefly rather than for the hours a lead-acid bank needs. A charger set to a lead-acid absorption voltage will either undercharge lithium or hold it too high for too long.

AGM batteries suspend the electrolyte in a glass mat, so they do not spill and vent far less than flooded cells. Usable depth of discharge is about 50%, and charge acceptance falls away sharply above 80% state of charge, which is what makes them slow to fill from an engine.

Marine alternators are rated for intermittent peak output. A lithium bank asks for full output continuously, which is why a DC-DC charger or an external regulator with temperature sensing is required rather than optional.

A 100 Ah battery can in theory supply 5 A for 20 hours. In practice you get the usable fraction of that: about 80% on LiFePO4 and about 50% on AGM. Always ask whether a quoted figure is nameplate or usable.

It addresses matters including overcurrent protection, conductor sizing, isolation, restraint and labelling. It is not lithium-specific. How it is applied, and whether licensed or certified work is required, depends on vessel category, jurisdiction, voltage and the work performed.

Cells in a pack drift apart over time. The BMS balances them, usually near the top of charge, by bleeding a little energy from the highest cells. A pack that never reaches its balancing voltage will drift further out of balance each cycle.

The BMS monitors cell voltages, temperature and current, balances cells, and disconnects the pack when any limit is exceeded. Protection means disconnection: the bank does not fade, it goes away. Specify one you can interrogate.

Every load and charge source connects to the bus. Two battery chemistries must never share a bus: the lithium will supply everything and the lead-acid will sit chronically undercharged.

0.5C on a 400 Ah bank is 200 A. LiFePO4 will typically accept 0.5C, which is far more than any marine alternator can deliver. This is why the charging system, not the battery, limits how fast you refill.

LiFePO4 can deliver short-circuit currents in the thousands of amps. Class-T fuses are rated to interrupt them safely, which common marine fuse types may not be. One per pack, at the positive terminal.

Keep it lead-acid and keep it separate from the lithium house bank unless your engine maker has approved lithium starting in writing. It is a warranty question before it is an electrical one.

Usually quoted as cycles to 80% of original capacity at a stated depth of discharge and temperature. Quoted figures at the budget end are frequently optimistic; we label unverified claims as claims.

It fixes the current your alternator is asked for, applies a proper lithium charge profile, and isolates the house BMS from the cranking circuit. The single most important component in an engine-charged lithium install.

LiFePO4 tolerates 80% routinely. Lead-acid and AGM want 50% or less. This one difference is why a 400 Ah lithium bank replaces a 600 Ah AGM bank.

It reverses sulphation on lead plates and is meaningless on lithium. Worse, a BMS often reads the pulses as an over-voltage fault and disconnects. A charger with desulphation that cannot be disabled is not suitable.

The alternative to assembling prismatic cells yourself. Drop-ins cost more per amp-hour and cannot usually be repaired, but they come with a manufacturer, a warranty and documentation your insurer will accept.

Essential for lead-acid, unwanted for lithium. LiFePO4 held at float sits permanently at full charge, which is the state it likes least for longevity. A lithium profile stops, or drops to a lower storage voltage.

An isolation transformer or galvanic isolator stops your anodes disappearing into the marina's earth circuit. Unrelated to battery chemistry, but the moment to fit one is when you are replacing the shore charger anyway.

Fridge, lights, electronics, pumps, inverter. It is the bank this site sizes and the one that lithium transforms. Keep it separate from the cranking battery.

The single largest intermittent load on most boats. A 2 kW inverter running a kettle draws about 170 A at 12 V. Undersized banks are almost always an inverter problem rather than a fridge problem.

The most thermally stable common lithium chemistry, with a long cycle life and a flat voltage curve. It is not the chemistry behind e-bike fires, which are typically lithium cobalt or NMC.

Charging LiFePO4 below 0 °C plates lithium onto the anode permanently and invisibly. Discharge is fine. Essential on any boat that goes south or winters unheated with a solar panel connected.

An MPPT controller converts panel voltage down to battery voltage while extracting the most available power, typically 20 to 30% more than a cheaper PWM controller. Set it to a lithium profile with no float stage.

A 12 V LiFePO4 bank is nominally 12.8 V; a 24 V bank is 25.6 V. Energy calculations use nominal voltage, which is why the same amp-hour figure means twice the energy at 24 V.

Four 100 Ah packs in parallel make a 400 Ah bank at the same voltage. Each pack needs its own fuse: a fault inside one pack is otherwise fed by the other three.

Two 12 V packs in series make 24 V at the same amp-hour rating. Only do this with packs the manufacturer approves for series use; many drop-in BMS designs do not permit it.

Fitted on the negative between the bank and everything else, it is the basis of any real state-of-charge measurement. Lithium's flat voltage curve makes a voltmeter useless for this purpose.

Heat, dirt, shading, angle and controller losses take roughly a quarter off nameplate output in Australian conditions. Our calculator derates by 25% and then credits only half the result when sizing, because a bank sized on a perfect solar day fails on the third grey one.

On lead-acid you can infer it from resting voltage. On lithium you cannot: the pack sits near 13.2 V from 90% down to 20%. Amp-hour counting through a shunt is the only reliable method.

Most of Australia averages between four and five peak sun-hours a day across the year. We use 4.25 as a national working figure, then derate for real-world losses.

80% of nameplate on LiFePO4, 50% on AGM. This is the number that should drive every sizing decision, and it is the number that is never on the label.

Proportional to current and cable length, inversely proportional to cross-section. Lithium invites higher currents than the old bank could deliver, so cable runs sized twenty years ago are frequently inadequate now.

A large intermittent load, often 80 to 150 A while raising. It rarely affects daily consumption but it does dictate cable sizing and it is a good argument for keeping the bank's voltage stable, which lithium does well.