And when a battery manufacturer offers a warranty of one year,
Here lies the crux of the problem I do not understand how an engineer would spec. Telecom sector does not use consumer grade products or materials. They do not buy a 1 year battery from Wallyworld, Agent Orange, Blue Glob or any other retailer. We use commercial grade products that come with 5, 7 and 10 year warranties. Since this battery will be left sitting for a year rules out FLA batteries period unless you can run the exerciser on the generator at least monthly, or use a small solar panel to keep them topped off.
That only leaves AGM. I assume this generator in question is like what most wireless carriers use at remote cell sites on the order of 40 to 55 KVA using diesel fuel. That means a pretty hefty AGM like a BCI size group 31 battery. For those we use two different manufactures Odyssey and Rolls.
The Odyssey PC2250 punches a wallop of 2250 CCA with a 126 AH @ 12 volt capacity and comes with a 4 year warranty. It is also a favorite of DYI EV builders.
The Rolls S12-160 AGM delivers a respectable 975 CCA with a 145 AH capacity and comes with 5 year warranty.
You will be hard pressed to find any other manufacture that carries a 4 and 5 year warranty on a SLI AGM battery. To get the 7 and 10 year you need to use FLA.