OK, let's get the basics out of the way fast.
1. The stock battery is your 300 cycle unit so at one year with daily battery use the battery could do exactly what you noted and nothing is wrong. It's just worn out.
2. Even so, there is no known battery meter that can accurately predict how many minutes left given the old battery (one year is OLD!)
3. Try BatteryBar. The free version is nice and can show the wear on the battery.
4. Ask Dell how to calibrate the battery.
Hi I have a Dell xPS 15 LS502 with core i7 processor and windows 7. It is just over a year old. Recently I find that the computer dies instantly when the battery meter runs down to about 42% or so. When I plug it in, switch the computer on again and hover over the battery meter icon it says 0% remaining and it gradually charges up as normal. What seems to be the problem is that the computer seems to think there is more juice in the battery and registers the battery meter too high. Is there any way to fix or reset this? Also there is no warnings that the battery is faulty or needs to be replaced. Any ideas? It is driving me nuts!