It could be because it a very old battery. My Dell Latitude E6400 is nearing eight years old and it was rated with around 4 hours of battery life when new. Now, I get an alert saying I need a new battery and a full charge lasts about an hour on a good day.
Try getting a replacement battery. This should be perfect:
http://www.laptopbatteryhq.com/dell-inspiron-9300-laptop-battery.html?gclid=CjwKEAjwv7HABRCSxfrjkJPnrWgSJAA45qA27VLREFqBNpHZkGJZNoyxxDyp2xe50wmSWY4OL3mTERoCOyzw_wcB
Hope this helps -
Jono