My laptop will not boot up with its fully charged battery (pressing power button does nothing). Laptop/battery is about three years old.

If I then plug in the AC adapter, it still won't power on.

However, if I take the battery out, and keep the AC adapter plugged in, the laptop will power up.

And, if I then insert the battery, it continues to run without the aid of the AC adapter.

Do I just need a new battery?? Or is it perhaps something with the PC itself?

Thanks in advance! 

It's the processor in the battery not properly reporting the charge state to the computer, or possibly reporting some type of error state.

Either way, it almost certainly is a battery problem.

(yep, even batteries have their own CPU's these days) 

Once you get it booted up, you can probably check on the battery's internals with special software if you care to dig it up and install it.

Most laptops will allow that, but not all. 

Thanks Jay...it's either buy a new battery I suppose, or at least always boot with the AC adapater..... 

Batteries Plus can fix it.