I've never had much luck with wet batteries, and they seem to die with great regularity.
My Harley came with an AGM (absorbed glass mat) battery in 2003, and it lasted for nearly 10 years, despite frying away inside the oil tank, and being half the size of most Harley batteries. When it started getting sulky, I stuck it on the NF, and of course it's been just fine for that, as it has nothing much to do. Apart from the Dynastart models, the stock battery is several times larger than it needs to be. The Harley battery is much smaller than the stock one, so needs packed out.
I've never had any issue with a quality AGM battery, and bought a Motobatt recently for my Cali III, when the wet one did its usual, and died on me. Loads on eBay for under £50, but can't remember the size for the NF:
http://www.ebay.co.uk/sch/i.html?_odkw=&_osacat=10063&_from=R40&_trksid=p2045573.m570.l1313.TR5.TRC1.A0.H2.Xmotobatt.TRS0&_nkw=motobatt&_sacat=10063For the record, so-called sealed batteries are Valve Regulated Lead Acid, and can be either Absorbed Glass Mat (AGM) where the electrolyte is held in fibreglass matting between the plates, or Gel, where it's held by silica gel. Most automotive and motorcycle VRLA batteries are AGM, and they will last for many years, without any maintenance, providing they're not fried by duff regulators, or discharged by more than 60% or so, when they may fail to recharge. Not an issue unless you have an alarm (ho, ho) or are in the habit of forgetting to switch off your lights, and/or ignition.