It looks as though a couple of faulty connections may have done for the 3 year old AGM 220Ah domestic battery pack, so we are going to use our own yacht - a 2006 Bavaria 39 - as a test bed for what LifePo4 upgrade does and, more importantly, does NOT work.
We will use the lithium upgrade as an excuse to change our ancient 170w solar panel for a new one of 400w and we will replace the existing Epever 30A MPPT charge controller with a MUST 1Kw 12v MPPT charge controller/inverter. This upgrade allows us to have 220v throughout the boat and will much improve the performance of our on board solar water heating system. We have also taken advantage of having 220v available to add a small fridge able to make ice cubes for the G & T.
Before going ahead with the lithium upgrade, we posted some questions to three different FB groups. The consensus there was fairly clear - although with a couple of contrary views from people with directly relevant experience.
The clear advice was to keep an AGM starter battery and charge the lithium from there, using a dc/dc charger between the two. That means only connecting the alternator, and the hybrid solar charger to the starter battery. Because we have the new hybrid controller, we think that we can disconnect the existing 220v charge controller, but that would not be true for those with a simpler charge controller. (Most modern MPPT controllers are fine with lithium by the way). That kind of solution forces us to buy the dc/dc controller costing around €400.
The other big debate related to the alternator. There were replies to my posts that stated very firmly that if I did nothing, my alternator would at best overheat, and at worst burn out. There were also posts that said this was simply not the case, although they dis say that the risk was a function of cable length. My battery builder business partner says that he has spent the last 10 years supplying lithium battery systems to milk trucks, using their alternators to charge his lithium without a single problem. The cost of buying a device that controls the charge from the alternator to lithium is also around €250 even buying expensive Victron equipment.
Obviously the ideal would be first to change the solar panel and add the hybrid controller (zero risk) and then just add the lithium batteries to the AGM. It is certain that will slightly discharge the lithiums in favour of the AGM but I could also very cheaply create a completely separate charging system for the starter battery. I first need to check how the starter battery is charged at present.