You can pick up an early FL2 for a couple of grand or even less, that's banger money, run it for six months and throw it away - still a fraction of the price of buying one new (I pay £735 a month just for HP for the DS).

So yes it was a premium product, new owners aren't necessarily buying into that, they might just be buying a cheap run-around that can cope with muddy dogs on a muddy field.
Problem is the FL2 was designed and built as a premium product, the running costs are still going to be proportional to the original running costs, try running an old Full Fat Range Rover if you want a good example of this.
That is what will eventually ruin the FL2s reputation for reliability, run on a shoestring, run into the ground and scrapped - happens to all old cars. There are those of us who are passionate about our cars, and other like my wife who are convinced (wrongly) that they are JUST a means of transport.

Hence the reason for the original post - it's just a car, why should I pay for a fancy battery? just because it has a fancy stop/start that I don't even want.
The answer has been eloquently put by the posters who have replied - it's a sophisticated car that requires expensive bits, either pay for the expensive bits or buy a less sophisticated car, anything else will result in higher maintenance costs. Sorry. Tangiers Orange - gone, missing her
Replaced by Ewok what a mistake - now a happy Disco Sport owner

I agree with the above reasoning, thank you Andy. The FL2/LR2 is well made and even over engineered. For me, my year class of 2008 is worthy of keeping healthy for a long time unless repair costs go beyond reason. That's why I do proactive maintenance for the most part. Putting the wrong battery in any vehicle is penny wise...........................you know the rest.

AGMs are better batteries just don't let them discharge below 10.5 volts for any length of time. They also require "smart" chargers.

I started this thread because I thought my battery was failing. Now I'm not so sure:

I fully charged the battery overnight on Friday, and have checked the voltage regularly since then.

After 3 days with the car unlocked, the voltage had dropped from 12.7v to 12.6v. Perfectly acceptable, in my opinion.

Yesterday afternoon I locked the car for the first time, and checked the voltage again this morning. It was down to 12.3v.

This doesn't seem right to me. At this rate it will be below 12v by Thursday, and unstartable by Friday.

So I will hold off replacing the battery while I investigate possible issues.

Did you double lock it ?

No, just the one click. Would it make a difference?

Yes it doesn’t shut down all together unless double locked I think.

hmmm, but green house emissions are significantly reduced by this stop-start feature. And I think mother nature could use some of that.

But there is a need for an optimized electrical energy source to contend with the these demands. And there is a difference to consider that is more expensive to manufacture.

Its not a question of the money involved in fuel savings for me... its a point of each doing their part to hopefully reverse some of the detrimental things we’ve done. Guy Pensa
Cars with Personalities
Getting itchy. What's next?
2014 Land Rover HSE Lux (blend of practicality and aggressiveness)
2007 XKR (Jaguar Racing Green) (woo hoo!)
2004 XJR (Jaguar Racing Green) (deceased)
2002 S-Type R (British Racing Green, of course)
1951 Riley RMD (Black, time for that restoration)

in reading your follow up post (after I wrote my previous, lol). you may be right.
But as I travel often for work, sometimes my vehicles don’t move for months on end. And if the battery can’t hold a charge for that period, I replace it.

NTB, the jag is in the garage for 4 months every year. And only 1 year did she not start on demand. That was the call to replace the battery. Guy Pensa
Cars with Personalities
Getting itchy. What's next?
2014 Land Rover HSE Lux (blend of practicality and aggressiveness)
2007 XKR (Jaguar Racing Green) (woo hoo!)
2004 XJR (Jaguar Racing Green) (deceased)
2002 S-Type R (British Racing Green, of course)
1951 Riley RMD (Black, time for that restoration)

My 2013 FL2 has its original Varta Battery in though the stop start is a bit temperamental now when it does die I will replace with the same. https://www.tayna.co.uk/car-batteries/varta/f21/ £138 buy cheap buy twice. 2013 FL2 XS.
Defender 90 300 TDi.
Defender 90 300 TDi CSW.
1964 MGB Roadster.
1944 Willys MB "Jeep" with bullet holes. (gone)
17 hand Irish Drought Thoroughbred (mostly lame)
Nagging Old Boiler.