macbook unibody (late 2009) does not recognize new battery

So - this question has been asked a bunch in the past, however I have not found a great solution yet that works. I have an old Macbook (late 2009) unibody that my son uses. The battery was finally dying - not keeping a charge and their was an alert saying to change it. We bought a replacement on amazon, swapped the battery out, and now the computer will not recognize the new battery. We just get the icon of a battery with an X through it. Seems that this is not an uncommon problem. From what I read, it seems unlikely I bought a bum battery ... and some folks think it could be a logic board issue (but really? the computer was working fine (ish) with the old battery) and others think there is some other issue related to resetting PRAM and SMC (each of which I tried).

Anyone have a solution out there? Someone must have figured this one out, back in the day.