My records say HX540-12FR. Is that this battery?

Yes. EnerSys's current range summary lists the model as 12HX540-FR, and HX540-12FR is an older ordering of the same designation that still circulates in purchasing systems, ours included. The FR marks the flame-retardant case, which is the standard build for this model. Whichever form your paperwork carries, the battery that ships is the 123 Ah, 540 watts-per-cell unit on this page.

When is the 12HX540 worth choosing over the 12HX505?

The two share one case, one terminal layout and one rack program, so the tray never decides between them. The 540 carries 123 Ah against 119, delivers 540 watts per cell against 506 at the 15-minute rate, and weighs three pounds more. Choose it when the runtime calculation lands close to the 505's limit and you want the margin; when the 505 clears your load comfortably, the smaller unit does the job and costs less.

Can I put new 12HX540s into my existing string?

Replace a string as a set rather than mixing new units into old ones. Units of different ages, and models of different capacities, settle at different internal resistances, so a mixed string shares current unevenly on discharge and float, which shortens the life of the new batteries and hides the condition of the old. EnerSys's warranty terms also assume a properly maintained, uniform string. A specialist can price the full set with volume freight in one quote.