I just changed RAM from 4GB to 16GB in a MacBook Pro 13" Unibody Late 2011. Followed all the steps here, nothing broke. I was very careful on the instruction "the edge of a spudger to pry the battery connector upwards from its socket on the logic board", and also to connect it again.
What happens is that my computer works fine except for the battery. On the upper side (next to clock) says that there are no batteries available. It only works with AC adapter connected.
Please, Help me!!!