I tried the battery draining method, but it didn't work. In fact it is supposed not to work.
The password is a firmware password that is flashed to a memory on the motherboard. Once it is flashed there, you can drain the power as much as you want, the password lock will never be erased. Maybe this worked on older macs, but certainly not on the newer. Otherwise implementing this feature would be useless.
I was in the Apple store, with my macbook (and proof of purchase). He promised me it would be fixed in less than 4 hours. It turned out to be less than 2.🙂 🙂
The guy had excellent knowledge, and was very friendly. He also promised me the data would be intact, and it was. My macbook was in the same state, as before it was locked.🙂🙂
I told him, I had still a locked iMac at home, without a proof of purchase, as I bought it second hand. He told me, he would have unlocked it as well, even without the proof, as it was clear I was victim of hacking. Unfortunately I didn't have the iMac there, it is standing somewhere else.
He told me it would depend on the clerk in the shop at home, if he would accept to unlock it without the proof of purchase.