Like others I have had battery problems one sort or another over the years on different FL2s.
Last replacement was just over a year ago. For some reason it went flat in the summer, 4v and seemed okay until we had the cold snap. Was very loathe to hold charge. So I then got all three batteries, yes kept old ones, onto the workbench and charged them all up. My battery charger
stopped charging at around 13.5v. On disconnecting charger multimeter read around 13.2v
That figure did not last long. Within the hour was down to about 12.5v and overnight 11.5v
This is just sitting there nothing connected. So had a brainwave, very rare occurence, thought lets try my CTEK battery conditioner which is permamnently connected to my classic. Now this a very small unit
0.8A charge. Anyway gave it a go across all 3 batteries. After around a day all 3 batteries were showing fully charged with about13.2v on multimeter. So disconnected CTEK and after 24hrs all 3 batteries are still at 12.6 /12.7v. Still this voltage after a few days now.
So the conditioner has done something to them to bring them back from the dead, even the one that ended up at 4v. If nothing else has saved me buying yet another battery. I might even buy a bigger unit to replace my standard battery charger.

As long as the battery isn’t AGM the conditioner attempts to pulse the charge in an effort to stir up the sulphate that forms in the acid in a discharged battery and re-dissolve it. AGM batteries hold the acid in matting rather than as free liquid within the cells so it doesn’t work.
If you are using a CTek on a modern car with the battery still connected don’t use the conditioning setting as many battery monitors don’t like the 15v that conditioning uses and the system just stops!

Can I ask what just stops?

After having the ctek running on its conditioning setting while connected to the car the next time I drove the car the battery would not charge (no output to the battery) at all. Something in the cars many ECU’s disabled the charging system. A simple disconnect the battery to discharge and get a coffee then reconnect the battery and all was well again.

Interesting, was this on a FL2 or another vehicle?

Another vehicle and it was because the car’s bcm was not happy with the charger putting out over 15v which was more than the cars charging system should supply. I’ve installed a Bluetooth battery monitor in my cars since owning the FL2 and I spotted it straight away. I might have been in real trouble if I’d kept on driving!!
It doesn’t matter in your case other than there is no free liquid acid in an AGM battery for the ctek to try to dissolve the PbSO4 in to. The ctek will charge a battery nicely and then go into a float stage where it just keeps a battery at the optimum level for storage ready to use. A stop/start FL2 might have an AGM battery.

Try reading this about charge/discharge cycles of lead acid batteries.

As I remember (perhaps, it was 60+years ago) the length and speed of charging/discharging matters as it the plates react differently during each and every battery cycle.
So it is quite possible to alternate fast and slow battery cycles to bring a poor (coated plates) battery back to life (capacity-wise), but not a dead battery with damaged plates.

The normal symptom of a failing battery is high internal resistance from one of the 6 cells failing.

https://en.wikipedia.org/wiki/Lead%E2%80%9...ischarged. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 79k+ miles) (MY2015)
Metro in its 11th Year of (Extended) LR Warranty / Full LR Service History
(Expensive, but Trouble/Worry free - hopefully?)