I would like to add my two cents on the screws: I found that the upper left screw was slightly larger than the others, so it was not 4.3mm, but something like 6mm. Of course, it could just be my model (X551MAV).
Good advice! Too late for me, alas. However, as long as I had done the most difficult part (really, those tiny, delicate cables are the worst), I took the opportunity to do a full set of upgrades: replaced the HDD with a SSD, added a DVD/CD burner, upgraded the RAM from 4GB to 8GB, and now the thing is humming like new!
Mine was 11.25V and I had no problem, but I have seen 14.4V used as a replacement as well without issue.
The only thing I can think of is that the new battery was faulty. It is probably way too late to return it, but you could try buying another and see if the same problem recurs with the new, new one.