Hello,my Dell d600 wont charge,it only runs on Ac power,previously it was working just fine and a friend came to borrow it and it stopped charging.i have replaced a new battery it only try to charge for some seconds and then goes to AC power again.now its has become a desktop instead of Laptop.I have sent to repair and they cant fix the problem,,i need serious help.
In a case of laptop, you should always go to the manufacturer's laboratory or make sure your laptop is sent to there.
It doesn't appear to be a problem of the battery itself since you tried a new one. The only thing that I can think about is that the faulty part is the sensor (or what ever the part is, that monitors the time of battery left). Unless you are a certified repair man or better a Dell's certified repair man I would recommend you not to mess with the hardware.
So, try finding Dell's laboratory or a laboratory certified by Dell and send your laptop there. Even if it costs you money it's better than ruining it (even a poor work of repair man can damage your laptop) and having to buy a new one.