Replacement Battery Dropping Suddenly to 7%

I’ve just replaced my battery using the kit from iFixit. Everything seemed to go well according to coconutBattery , and I calibrated it per the instructions (full charge -> use it until it goes to sleep and wait -> full charge).

For some reason, when I start using it, the battery discharges normally for some time, then drops suddenly to 7%. It will stay usable on 7% for quite a while, then it dies.

30 minutes later:

This happened on the first cycle, and after I charged it fully (which was slow, took ~ 5hrs - not sure if that is part of the same problem), it has happened again.

It seems other folks have had similar problems with these replacement batteries - is there any way to fix this? Are there any reliable sources for these batteries? It is quite an involved process to try new batteries until finding one that works if so many being shipped are unreliable.