I had to put a new battery in mine when I bought it three years ago. I picked up a cheap one from a place round the corner and did the same in my Spyder and after 3 years it's just starting to show signs of slowing down but it will do another year.
Realistically 3 years is all you can expect from a battery so I would just but another cheap one in.
My 4200 has a Lion battery in it, as does my 944. Lion is a budget brand, possibly even Euro Car Parts own brand even (suspicion, not known fact...)- it's usually the cheapest battery they sell, but at the same time often has better mAH and CCA than the much more expensive branded ones.
I had one in my Elise for years with no issues. My car lives on a conditioner though which I think prolongs the life nicely.
I admit though, for daily / cars that are never plugged in, I try to stick with a Bosch S5.
I guess my theory is if the battery is looked after / constantly conditioned, a cheapo will do the job and last a long time. If it's abused (run dead in winter with heated windscreens, heaters and headlights on a daily 5 mile journey) get a good one - so my daily sheds will have great batteries whilst my pride and joy will have budget ****
