Hey, staying with my brother and he's got a Macbook 15". He's on his second battery now for the same issue. The issue is that the battery bulges out and causes major problems. For one, it's 1 cm outside the case in one corner. Secondly, the touchpad is extremely erratic because of the pressure the bulging battery is causing. The first battery was replaced for free by Mac but they won't replace it again. Now they are considering two possibilities.

1. the battery problem is supposedly a battery only problem, and replacement with a newer model of the battery should fix it. So they could try buying (yet another) new battery)

2. the problem is in the Macbook itself (overheating?) and it's a battery eater.

Does anyone have any knowledge of this major issue with Macbooks? (tonnes of others had the same issue, but I haven't found anything online about people putting a new battery in and having the same problem)

It's most likely the battery itself. similiar to the sony battery issue. I'd raise hell with apple, it's either the charging process or the battery in inadequate in powering the laptop and overheating.
http://www.appledefects.com/?cat=40
http://forums.macrumors.com/showthread.php?t=732142
google search

But I thought, "it just works".

thanks harry. I checked out those two pages too. must be using the same search engine

We got the problem figured out, though not entirely solved. The issue is the same issue and it should be covered under warranty. One of the dealers was giving us some issues though. So I think we'll just bypass that one and go to someone who wants to be more helpful. Unfortunately, they are living in Grenada and can't just walk in to the mac store to exchange it. It involves major time and shipping costs. But we'll figure something out.

At least it's just the battery and not the laptop. So it's just running on AC power for now.

tell your friend not to let them nutroll him.
bestbuy geeksquad is good for nutrolling people.

I'd walk in the apple store and holler like samuel L. jackson, "my shit is rotten, gimme another one right now"

You forgot M*therF*cker.

my wife says I use too much profanity, I was like what the fuck you mean I curse too fucking much, that's bullshit.

If it's a 15" Macbook pro there's a chance it's a bad one depending on how old it is.

About 2 years ago there was a run of pro's that were bad for acpi problem, and burned out batteries. Under applecare that counts as a logicboard issue but good luck convincing an apple tech of it.