UPDATE: The system continues to perform amazingly but some of the components have evolved.

NOTE: It may not be clear from the text below (because I assumed it was obvious....) but the charging system philosophy is to have every possible charging source connected directly to the large and ever-hungry house Li bank then let that bank keep the smaller and more finicky starter battery topped up using a DC-DC charger. doing the reverse, as I have seen some do, VERY severely limits the ability to make use of charging sources to fill the house bank and, IMHO, completely negates the primary benefit of Li batteries which is their ability to take full charge rate until almost fully charged.

We recently bit the (financial) bullet and switched to Lithium batteries or, more precisely, Lithium Phosphate batteries, LiFePO4. This technology has been around for the skilled home builders and techies for a while but it has only recently matured sufficiently for the 'average' cruiser. I will go through the whole process below but first. "Why did we do it?" and "did it work?"

Why we did it:
Lead acid batteries have a couple of significant problems that, in combination, can make life miserable on a blue water cruising boat. The first is the need to get them to 100% charged pretty much every use cycle to prevent sulphation. The second is that their ability to take charge tapers off significantly as they become charged. This is not a problem if you are plugged into shore power and popping out for the weekend or the odd longer trip but when you are at anchor for months at a time, relying on solar power or the engine it is a real challenge. There are simply not enough charging hours in the day for the solar to get the batteries to 100% at a low rate of charge. It is particularly infuriating to have solar panels that can push 30A into the batteries only able to provide 10A or less due to charge tapering. Running a 59hp engine with a 120A alternator to provide 10A is even more annoying. The other issue is that you should only discharge them to 50% capacity and even that is not ideal. The end result of this is that a lot of time is spent managing a limited power budget and trying to keep the batteries 'happy'.

The key benefits of LiFePO4 batteries are that they can be discharged to less than 20% capacity without damage, they don't need to be charged to 100% and they take all the charge you can give them until they are almost fully charged. In theory we could hit the new batteries with 150A continuously until they are charged. This means the solar will be maxed out until the batteries are full and likewise the engine if required. No throwing needed power away due to charge tapering.

Did it work?
Amazing. Power management has gone from being a major preoccupation to almost forgotten. We can run the watermaker whenever we want. We can defrost the refrigerator whenever we want. It is pretty much a non-issue. Slight disclaimer, we did increase the solar capacity from 400w to 600w as well but without the battery change this would have been a largely worthless exercise.

Other benefits were shedding 170lb. We went from 425ah of AGM batteries (4 x 6v in series/parallel) to 375ah of LiFePO4 batteries (3 x 12v in series), more than enough given the increased ability to discharge. This weight loss was actually a mixed blessing as I'm still trying to get some weight migrated to the Stbd side to re-trim the boat.... The stable voltage (the batteries are theoretically capable of sustaining an astonishing 240A continuous discharge rate so there is negligible voltage drop) improves watermaker efficiency a little.

The planning:
This is not a trivial exercise. Every component in the power system has to be checked for compatibility with the particular requirements of LiFePO4 batteries. A few will need to be replaced. This is not my particular area of expertise and the consequences of getting it wrong are serious so I enlisted the help of Rod from Compass Marine . Over the years Rod has been helpful on a number of challenges, particularly when we were newbies. I would not hesitate in recommending him. We discussed various battery options and went for the Lithionics 12v 125A G31. They have a built in BMS (Battery Management System) with bluetooth communications that allow monitoring by app. They are not the cheapest batteries but they have a good reputation and the support was stellar. Importantly, the dimensions worked for our battery box. No regrets there.

Anyway, here was our system analysis.

Solar charger - Morningstar Tristar MPPT45 - reprogram. This was a doddle. Lithionics not only provided all the setup parameters but also a configuration file that I just had to load into the solar controller.

UPDATE: I managed to blow up the solar regulator by the dumb practice of isolating the reg by switching both sides of the reg on a double poled breaker. Why? Because I had one and didn't think it through. Anyway I discovered that simultaneous off is fine but simultaneous on is definitely not. Hooking up the fairly substantial solar input before the regulator electronics are fully powered up is definitely NOT. Anyway the dead beast was replaced with a Victron MPPT 100/50. This has worked very well and the ability to monitor and configure it via a Bluetooth app is awesome.
MPPT Charger

Alternator regulator - Balmar MC612 - reprogram. Lithionics provided all the parameters but these things are a pain to reprogram using a magnet on a reed switch so I learned from the web that taking it out of the engine room, powering it up on the bench, reprogramming it and then reinstalling it was MUCH easier. We now have a custom loom set up in case we need to do this again in the future.

Alternator - AMP 125SE SER 125se2363 with serpentine belt - keep but derate. This alternator is theoretically rated for 120A continuous but lead acid batteries never really test it. LiFePO4 batteries WILL test it and, in a small and poorly ventilated sailboat engine room in the tropics it is not going to last. We derated the alternator to 50% via the Balmar regulator, or 60A max current. This reduced the risk of overheating and we replaced the defective regulator alternator temperature sensor as a backup. This sounds drastic but we rarely use the engine for charging and 60A continuous is still way better than the previous 10A or less.

UPDATE: The alternator eventually died. Possibly due to the additional load but it was a truck alternator with dreadful cooling and of uncertain vintage, probably more than 20yrs old. Anyway, that has recently been replaced by a Balmar XT170. EXPENSIVE but designed for the job with much improved cooling and less heat generation. This has been derated to about 100A to further improve it's longevity.

BalmarXT

Starter battery charger - Xantrex Echo Charger - replace. The echo charger needs both battery sets to have the same charging profile and we chose to leave the starter battery as an AGM. We replaced the echo charger with a Victron energy Orion Tr Smart DC/DC charger that can be (Bluetooth) configured to support a wide range of charging profiles. This has been problem free.

Battery Monitor - Xantrex - reprogram. Lithionics provided all the parameters and it matches the battery charge state reasonably well but with Bluetooth access to the batteries and the reduced need to monitor charge state this is only used to monitor current flow. Newer models probably have Li settings.

UPDATE: This sort of works but we never look at it anyway. It is just too easy to monitor the batteries in the app and it has ceased to be an obsession in any event.

Shore Charger - Xantrex Truecharge2 - toss. This (fairly new) shore charger is just not configurable for LiFePO4 batteries. We cannot envisage needing a shore charger for the foreseeable future so we will replace it when required. Apparently Xantrex are coming out with an Li compatible shore charger but others are already available.

UPDATE: We still don't use it but perhaps in Tasmania it will be useful so we have installed a Victron Blue Smart IP22 Charger. Works fine and is app configurable - love it.

Victron Shore Charger

Alternator Protection Device - previously not required - install. One of the less pleasant characteristics of the LiFePO4 BMS is it's ability to instantaneously go open circuit if it sees something it doesn't like. If the alternator is cranking out current when this happens the back-EMF will fry the alternator. We installed a Sterling ProProtect APD to address this problem.

The installation:
We procured new battery leads and made sure we had all the breakers and fuses needed for the different components then wrote up a step by step procedure for the changeover while we were waiting for the stuff to get delivered. This proved to be invaluable. We had basically rehearsed and reviewed the whole process and we managed to complete the change out in two days, including the solar upgrade. It has been trouble free ever since. Not a small project but definitely one of the most impactful upgrades we have made to Leela.

Final thoughts:
1. If you spend most of your time at a dock this is not a good investment. For anyone spending a lot of time on passage or at anchor it has huge benefits.
2. Unless this is your area of expertise get good engineering support. There are lots of potential pitfalls.
3. Plan very carefully, particularly if you are doing this in a remote area. It only takes missing a tiny thing to stop you in your tracks.
4. These are not Lithium Ion batteries. They have an entirely different chemistry and can be shipped internationally without problems.

There is a lot of good information about this and many other topics on the Compass Marine website.