Here is the part the brochures skim over: LiFePO4 is not a like for like replacement for lead acid, even when the box is the same shape. The cells behave differently while charging, while discharging and while being protected, and the rest of the installation has to accommodate that.
None of this is a reason to avoid lithium. It is a reason to plan the conversion as a system change rather than a battery swap. This guide sets out what genuinely changes, drawn from the work we do as a Victron and Fogstar approved installer.
The biggest practical gain is depth of discharge. Lead acid banks are conventionally sized so that only a modest portion of the rated capacity is used regularly, because taking them lower shortens their life sharply. LiFePO4 tolerates deep cycling far better, so a smaller nominal bank can deliver more usable energy.
That changes how you size the installation. It is common to end up with less weight and less space consumed while gaining endurance, which matters on a boat where both are finite.
Every charge source needs to be set to a lithium profile, and some will need replacing outright. Chargers that only offer lead acid curves will overcharge or, more often, sit in an absorption stage that lithium neither needs nor benefits from. Solar controllers need the same treatment.
A lead acid bank naturally tapers its acceptance as it fills, which quietly protects the alternator from running flat out for hours. Lithium does not taper in the same way, and a standard alternator asked to deliver maximum output continuously can overheat and fail.
The usual answers are external regulation with a temperature sensed profile, or a DC to DC charger between the engine battery and the lithium bank so the draw is capped. Which is appropriate depends on the alternator, the engine installation and how you use the boat.
Every lithium bank has a battery management system that will disconnect the cells to protect them, typically on low temperature charging, over voltage, or cell imbalance. That disconnection is the crucial design consideration, because it can leave a running alternator with nowhere to send its output, which damages the alternator and sometimes the electronics on the same circuit.
A properly designed installation handles this deliberately, with the BMS able to signal the charge sources to stop, or with a lead acid battery kept permanently in circuit to absorb the load. This is exactly where drop-in batteries and integrated systems diverge in practice.
On lead acid, resting voltage is a rough but workable state of charge gauge. LiFePO4 holds a nearly flat voltage across most of its usable range, so the panel voltmeter will read reassuringly normal right up until the bank is nearly empty.
Shunt based monitoring is therefore not optional on a lithium boat. It counts amp hours in and out and gives you a state of charge you can actually plan around, and on a Victron installation it also gives you the data to see whether the charging is behaving as configured.
LiFePO4 must not be charged below freezing, which matters for boats left on the hard or wintered afloat in the UK. Many banks handle this in the BMS, and heating or a charge inhibit arrangement may be needed depending on where the bank lives.
The cells will also deliver very high current into a fault, so fusing, cable sizing and the physical installation have to be right. Lithium conversions are a genuine fire risk when done badly, and this is not sensible DIY territory. Have the design and the installation done by a qualified marine electrician who works with these systems regularly.
Sometimes. If both support a configurable LiFePO4 profile they can usually stay. If they are older fixed profile units, they need replacing as part of the conversion.
It depends entirely on the boat. Drop-ins suit simple installations with modest charging, but where there is a substantial alternator or several charge sources, an integrated system with proper BMS control is the safer design.
Not usually, though its low voltage cut-out should be reviewed against the lithium bank's working range so the two do not disagree about when the bank is empty.
It normally stays as lead acid. Starting is a short high current job that lead acid does perfectly well, and keeping it separate simplifies the charging design.
We design and install LiFePO4 systems as complete installations, with the charging, protection and monitoring set up to match.