Why won't my battery charge?

I bought a new charger and I bought a new battery, neither fixed the problem.

I brought it to apple and the guy said it may be the battery cable but I didn't think to ask what else it could be. What else could it be?

I had planned on changing the cable myself but after seeing how elaborate the tear down is I'd like to be more sure that is the problem and maybe not the io board or something else. Is there a way to pinpoint the source of the problem before I start taking it apart, I don't want to have to do it twice.

I changed the hard drive two weeks ago so I think I am handy enough to change whatever I need to fix this.

Thanks for the help.