Laptop doesn't recognize the new battery, and then neither the old one

I followed all the instructions to change my battery, and I did it correctly. Then I tried to switch on my computer but it didn't do anything. So I tried to replace the old one and it happened the same thing! So I tried to reset the SMC as the apple website said. When the computer started it seemed to be everything ok but then the toolbar said that there was no battery detected. The same thing happened either with the new and the old battery following the same procedure of resetting the SMC. What do I have to do to let the battery to be recognized?