I bought our JD X520 in July 2006 so based on your timing, I suppose I'll be in the market for a new battery soon myself. Quite frankly I'm surprised it has lasted this long after sitting idle in a cold shed from November - March each year.
I may just go ahead and proactively find a replacement at Auto Zone or Advance and see if I can save a few bucks....
One of dad's favourite sayings was "he who buys cheap pays thrice"
Lawn mower batteries are very different to car batteries as they go into a body that has no suspension.
As such they are exposed to sharp shock loadings which cause the battery grid to fracture and fall to the bottom of the case.
Each cell has 3 to 5 grids in parallel so the battery will still show 13V but looses capacity.
Naturally the more grids that fall off the less capacity the battery has.
BEcause a mower battery discharges quickly and recharges slowly the chemical composition of the mower battery is different to that of a car battery
The same applies with standing time.
Starting batteries like in your car will charge up to 14.5 V and hold that for about a week but after a month may be too flat to start your car.
A mower battery can not go over 14 V but will hold 13 V for months a charasteric of storeage batteries.
At the battery factory every battery is carefully tested before they are branded.
The cheaper the battery, the lower the quality of the battery.
Note this is the factory wholesale price which may or may not be reflected in the retail price.
Thus the battery sold by the mower shops with a mowe brand on them will usually always be better than a one with a battery company's name on it from a discount store.
By far the best battery will be a AGM ( Adsorbent glass matt ) PVR (pressure valve regulated ) battery, they are also the most expensive.
And the top of these are the spiral cell AGM PVR batteries fitted std to some Toro commercial mowers.