Latitude D420 battery life

I've had my Latitude D420 for about a year now and the one thing that really earns it its keep is the battery life. My concern is that the battery life seems to be falling significantly. When it was new I would get estimations of 5-6 hours when fully charged but I notice that estimates now are frequently 4-5 hours. This may not sound like a problem but once the battery goes under 3+ hours it will mean it won't last the time I spend away from the AC adaptor. What do you think: Am I just being paranoid or should I be worried?

 

I have a D420 and had to replace my first battery after 13 months due to a failure in one of the cells, so I would feel happy as long as that doesn't happen. It is normal for batteries to wear over time and not charge to the same extent as they used to. If you are heavily using your machine I would not expect the battery to last more than 2 years and hold a good charge. This is why I am glad I stocked up on batteries a year ago when they were still cheap...

 

I am aware that batteries wear out with time and charge cycles, I was just wondering if it was normal for them to go quite so quickly. Stocking up on batteries wouldn't seem like the solution to me, as they also wear over time.