The message 'your battery is reaching the end of its usable life. It is suggested you purchase a new one' appeared on my Inspiron 1501 laptop. I know that the best thing to do would be to buy a new battery, but they are around £100 from Dell. If I always keep my laptop plugged in- I rarely use it outside or as a portable machine etc, do I need to buy a new battery? Or is the battery essential plugged in with the AC adaptor or not?