There's a lot of crappy aftermarket batteries out there. You may have gotten a bad one or one with too low of a maH rating. Or there may be a problem with the charger or laptop. What prompted you to buy a new battery?
I've seen problems with some of the cheap replacement batteries so the last time I needed one for a Dell I bought a used Dell OEM (one that has the Dell logo) battery on ebay and it's been working fine. I can't say for sure that's the problem with yours as what I know about it only comes from your description but that's been my experience.
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