The battery life of the laptop is somewhere between 1.5 and 6 hours, depending on the use scenario. If you're getting near or below the low end, the battery might be nearing the end of its lifetime. You could get a 9-cell battery, which is heavier but offers better battery life.
Keeping batteries from older systems plugged in can cause them to degrade faster. It might be a hassle, but you can extend an old battery's lifetime by keeping the battery out of your system when you plan to have it plugged in for a long time.