<p>The battery life diminishes with use. It may last 10 hours the first year, 6 hours the next, etc. The actual rate of decrease depends on the battery and laptop used, but no battery keeps its longevity forever.</p>
<p>Usually when people buy an 8-cell, they do it either for longer battery life than possible with the stock battery, or they stick it in a freezer and store it until their current battery’s life reaches a point where it’s no longer usable.</p>
<p>I actually did that with my current laptop. I bought a spare battery for it, sucked all the air (and henceforth, the water) out of it as best as I can, and tossed it into the freezer.</p>