Step One is to find out
what the blinking lights mean. They are a code, produced by the battery's on-board electronics. The blinking lights are Dell's way of
telling you something.
Over the years, many many batteries have been produced for many many Dell laptops.
How do you find out what the blinking lights are, and what they mean?
See the top link:
http://lmgtfy.com/?q=dell+l400+batteries