Thanks for the feedback
Gosh, what have you done to 'knacker' them after just five plus years? Are they all knackered or just the one?
Bowvthruster, electric windlass, solar? These would makes a difference on how you wire lithium. If you have a bowthruster and windlass, wiring these from the engine battery would make sense. Solar would wire to the lithium. The alternator can't charge the lithium directly so you would need a DC/DC charger. In addition, you would want a smart shunt to monitor SOC of the lithium. Hope this helps
I wonder if my bolding is the root of your problem; lack of use.
If I had a small cruisers for use in the UK in would definitely have lithium. Lithium batteries are smaller lighter and will last many, many years. Far longer than lead. When space is at a premium on small boats, lithium allows you to have lots of Ah in a small space and you can use way more of the battery capacity than you can with lead. You would need the same good quality power management system for lithium. In many ways lithium is a better fit on small simple boats as you are unlikely to have large loads such as powerful inverter or electric cooking so you get a far simpler installation.
You can have a link from lithium to charge an engine battery but if its duff, that might not help. If you had lithium battery you built yourself, you can start from the lithium by bypassing the bms. This would only be for emergency use but lithium is capable of huge amps
The OP already has (probably) sufficient capacity for his needs so the smaller and lighter is a bit of a red herring - although valid if one wants greater capacity but are limited in space. The vast majority of UK cruisers with a usage pattern like the OP manage very well with LA, particularly with a bit of solar. Note he says he uses shorepower and does not seem to spend long periods at anchor.
Yep I have bowthruster and anchor windlass, and clipper shunt battery monitor, no solar yet. I have 40 ft van de stadt, and the batteries are placed near the areas they are needed. So anchor windlass and bow thrusters have the domestic batteries at the front of the boat and engine battery is at the back of the boat. So lots of battery wiring to consider if I went lithium. I am assuming I need to change my victron battery charger as well?
I didn't dive in and tell him he needed lithium. He said he was interested in lithium. Trying reading whatt I said. I actually answered Sandys post not the OP. I also said that if I had a small cruiser in the UK I would have lithium for all the benefits I pointed out. I actually asked the OP what other systems he had before answering his post
Converting your boat to lithium would not be straightforward. It would also be expensive. Due to the inrush current attributed to large DC motors such as bowthrusters and windlasses, you cannot simply install lithium batteries and power these devices from the lithium as the high inrush current load can/may damage the BMS. You would have to use a lithium battery that has an external BMS like an REC where the load doesn't pass through the BMS. These BMS cost circa £750, not including the battery.
The key thing about many decisions like this is to first determine what your current needs are and what constraints you have. Then look at how you might remove or minimise those constraints. So, for example reducing consumption (LED lights an obvious one) or increasing power generation - the obvious being solar. These 2 can have a significant impact on your net daily use and therefore on your buffer capacity. Your existing buffer is not out of line for your size of boat and pattern of usage so the first choice is moving from FLA to AGM which typically increases potential life by 50%+ for a 30% premium with minimal changes, maybe just adjusting the settings on your charger. Additionally AGMs have a higher charge acceptance rate and lower self discharge which is useful if you keep your boat in a marina and use shorepower or have solar to keep the bank fully charged when you are not using the boat (that is most of the time!).
Errrrr. Batteries like to be used.
Lithium doesn’t care either way, and for someone who doesn’t use the boat a lot would be the better option as you can turn everything off and leave for a couple of years and the battery will be fine. Do that with lead and results will vary but often the battery would be dead.