[Edited Message Follows]

Hi all,
12 months ago I ordered 12x new Rolls S12-128AGM batteries to replace Stella's (A54-154) waning 5 year old battery bank.  
My thinking at the time was that I would seek another 5 years from AGM's before seriously considering lithium.
The pandemic happened and I was unable to return to the boat (in the UK) in 2020, so I cancelled my battery order.

Fast forward 12 months and I am thinking again about my battery replacement. During the pandemic I bought a caravan (RV) which is fitted with a 12Vx 200Ah lithium battery, and it has given me some good first-hand experience of using a lithium system in an off-grid scenario.  In practical terms, I am impressed by its ability to accept high charge rates even when SOC is high, and also to operate effectively with stable voltages when in low SOC states, even when delivering high currents.     My boat use-case is ideally 6 months constant cruising and 6 months in storage, although the current cycle will be 18 months in storage due to the pandemic. I am least impressed by the logistics of maintaining a lithium battery during long periods of storage.

That said, I am now considering making the jump to lithium when I (hopefully) return to the UK in 2020.
I don't want to get involved in a lithium "project" and I'm not particularly interested in turning Stella into a "smart boat".
I'm also not interested in retiring my generator, and I am happy to continue using this for AirCon and Laundry appliances, so I don't need a big invertor, and I don't want to replace the cabling between batteries and Engine Room charging devices.

My preferred path would be to install "drop-in" replacement lithium batteries, and then upgrade any other components necessary to ensure safe and reliable operation.
I believe the main thing I will need to re-configure/augment is my 175A charger to ensure it doesn't overheat with too much constant load, and doesn't destroy itself in a HV battery disconnect.
Any other critical things I should consider here?   

The lithium batteries I would prefer are the 24V models since they should always be properly balanced by the BMS, whereas the 12V models need to be separately managed for proper balancing in a 24V system (just like Lead Acid batteries do).  I understand about not having 12V redundancy for engine starting, and I'm comfortable with my contingencies there. 

I considered the BattleBorn 24V/50Ah batteries, and apart from the strange terminal configuration making them not "drop-in", these would seem to be a good choice. However I cannot find a supplier in the UK. Has anyone purchased these in the UK?  Has anyone purchased them for shipping to the UK from the US?

My other serious contender is the Relion RB24V50.  This has a slightly lower max discharge current (50A) than the BB battery (60A) which is important particularly for bow thruster operation but the terminal arrangement means that it is a true drop-in replacement AND it is available for purchase in the UK.  I wonder if anyone has any experience of this battery, or other Relion batteries? 

Cost vs power
The set of 12x AGM batteries is GBP2800 and provides about 250Ah of usable capacity at 24V without taking the batteries below about 65% SOC (which is my preference). So lets say about GBP11 per Ah of usable capacity.

A set of 8x 24Vx50Ah Relion batteries would cost about GBP8000 and give a usable capacity of about 280Ah without taking the batteries below 30% SOC. That is therefore about GBP28 per Ah of usable capacity.

The rule of thumb seems to be that the lithium option is about 2.5 times more expensive than AGM for equivalent capacity. 
Does that stack up with other experiences or did I miss something?

Weights
The 12x AGM batteries total mass is 393.6kg whereas the 8x lithium batteries weigh in at 105.6kg
That's a weight saving of 288kg.
I know that Stella, like other A54's, has a slight list to one side, but I just can't remember if it's port or starboard, can someone please remind me which it is?

Hmmm, I didn't intend this post to be quite so involved, but just typing it has helped to clarify my thinking :-)

Cheers
Dean
SY Stella A54-154
Currently (still) ashore in Ardrossan, Scotland.