I know this is an old thread, but I have a solution. I was having this same problem with an HP laptop. With the battery inserted, it acted like no AC power was connected and eventually drained the battery. With the battery dead, the unit would no longer power on since it was ignoring the AC. With the battery out, it ran fine on AC. I swapped batteries and that did not fix the problem. Then, I swapped the AC adapter and, with the battery in place, the laptop powered on and the battery charged as expected. So, the problem in my case, was definitely a faulty AC adapter.