On the back of the laptop see if you can locate a pinhole usually it has a small symbol beside it. This contains a switch that would disconnect the small bios battery press and hold for 60 seconds with a small paperclip with the battery and power disconnected. Proceed to press the power button a few times before connecting battery or power supply. Boot without battery until you can get into your operating system . After this computer boots and everything is working then you can install the battery while the system is on to see if the battery will start to charge.
Replace all the cooling fans . Mine did this a few times and right after I replaced the HDD. I now have went thro a few HDD becouse the HDD was not apple certified. To control the fan speeds . I tried fan control software but it sux. Just replace the fans and add apple cert. HDD and it should be fine. P.S. disconnect the cd drive and try a boot also.