battery not being recognized

for some reason, my macbook is not recognizing my battery, theres just a black X. i thought it was because i had an old battery, so i purchased a new battery, but the problem's still there. i can turn on my macbook only when connected to the power cable just not from the battery, but when i disconnect the power cable, it still shows the black X, and the macbook continues to run. any ideas why or how to fix it?