New coil packs, plugs and knock sensors. Checked gas

goes on. Am told car doesn't keep engine ck codes when turned off?

Getting a battery not charging warning periodically.Drained the battery once and then went away again.

i cant seem to pull gas tank out far enough to replace fuel pump? does the rear seat need to come out,so i can push it? seems to be hanging up in the middle?,with the key on,should i be able to hear the pump? not sure if its filter or pump bad?

How dificult to remove and replace