Given the age of your boat, it likely has gone through several owners. Before going down the lithium road, it would be advised to complete an electrical audit and build a diagram of your DC and AC electrical system.
A lithium house bank is usually combined with an AGM or flooded start battery. Given the incompatibility of these two chemistries, each bank must be charged seperately. Additionally, you'll need to ensure you don't over burden your alternator.
Expect to spend $1000-$3000 to update your electrical and charging system before installing a lithium battery. Wire sizes, fuse types and possible separating house load distribution from charging source distributions are all important considerations. There is no such thing as drop-in lithium.
As far as the alternator, you basically have two choirces:
a) charge the start battery. Use then use one or many DC to DC converters to charge the lithium house bank. This converter should be sized to not be beyond 70% of the max current rating of the alternator
b) charge the house battery. You then use a smaller DC to DC converter to charge the start battery. This will require that your external alternator regulator (do you have one???) be programmed for a lithium battery. You probably need to adjust the alternator field output to limit your alternator at 70% of max output to avoid it overheating.
At a minimum, you will probably need/want a new alternator, an external regulator, an alternator temp sensor, new alternator to battery wiring, etc....
If all this seems a bit overwhelming, you could consult a local marine ABYC certified engineer or book a remote consult with Pacific Yacht Systems (Vancouver BC).
https://www.pysystems.ca/electrical-design/
The recent ABYC standards have made this whole proposition a little more complex and expensive, as the standard requires communication between the batteries and the alternator and a method to warn the user before the battery BMS disconnects. Very very few lithium batteries and alternator regulators can do this. Although ABYC is not a requirement, most marine engineers would follow the standards if they are certified.
Once you figured out the DC side of the house, you'll need to consider your AC charging needs. The majority of marine AC chargers don't support different chemistries on each output. This means you'll either need only one AC charger and rely on the DC-DC charger or you'll need two seperate AC charging sources.
Here's a typical install with an inverter, solar panels, alternator and separate AC charger for the starter battery (note that most Victron inverters have a secondary DC charging circuit which can be used to achieve this).
For my H380, I ended up with a system very similar to the attached diagram. I already had a 100A high output alternator and external regulator. My alternator charges the start battery. (this is for redundancy, as my solar system charges the house battery). When the ignition is ON (wired in a relay) AND the alternator outputs power, the two DC-DC converters charge the house lithium bank from the alternator (start batt). The voltage rules in the DC-DC chargers makes it impossible to accidentally discharge the start battery. I created a DC distribution bus bar and a DC charging bus bar, all are fused. I have 2 separate AC charge sources (Xantrex inverter and small AC charger for starter batt). Before batteries, probably cost about $1500 in "stuff" and about 50 hrs of work to rewire everything. We have a 410 Ah lithium bank. I have yet to see it go below 80% at anchor in the summer. Looking forward to see how it does this winter (in BC) with the forced air heater running and very little solar output (200W of panels). Underway, my battery bank is fully charged in under two hours.
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