A few years ago, my MBP's trackpad started acting up. My research suggested that the symptoms could be caused by a swelling battery pressing against the underside of the trackpad. But I didn't see any swelling when I removed the underside panel, and battery diagnostics came out fine. When I had the machine get a flat-rate service at my local Apple Store (for an unrelated problem), there was no indication that the diagnosing tech noted any battery issues. (In retrospect, at that point it should have been noticeable by a tech. They ended up replacing the motherboard, so any battery swelling on the keyboard-facing side ought to have been visible during the repair.)
Several months ago, I noticed that the lid wasn't quite closing right. Not long after that, iStat Menus gave me a warning that my battery needed service. The max charge it was holding at that time was 85%, and the USB ports would no longer provide enough power to run a bus-powered external drive when the laptop wasn't plugged in.
I researched Apple's "battery service" and wasn't happy about the fact (as conveyed by the Genius I spoke with) that it would take two to five business days. They would not pre-order the correct battery for me and do the swap while I waited. I don't know if that's Apple's policy, that particular store's policy, or that Genius's own laziness, but it irked me.
So I did more research and came across iFixit's guide to doing the repair oneself. It was surprisingly straightforward, simpler than swapping the hard drive or adding RAM (both of which I have done before on this machine). The only thing I needed that I didn't have (besides the battery) was the Tri-point screwdriver, and I could buy that from iFixit with the battery.
Even so, I kept putting it off. I've been planning to replace the hard drive with an SSD in the hopes that it will speed things up (I'm maxed out on RAM, so there's a lot of VM disk activity), and so I wanted to do both at the same time. I finally had the money to buy both the battery and the SSD last week, so I was finally good to go.
As it turned out, I decided after all my parts arrived that I would wait to change the hard drive. I still needed to do some more prep and research (most importantly, making sure everything I have is Sierra-ready—I'm still on Yosemite, and I figure that there's no better time to jump forward two years in OS versions than when installing a new drive), and I realized that it might be a good idea to change the battery separately first so that I wouldn't have any trouble with powering the external enclosure I'll be putting my old boot drive in.
My first clue that this was long overdue was when I was removing the case screws: the panel started to pop up as screws were removed. By the time all the case screws were out, the underside panel was no longer flush with the rest of the case on any side. My first two pictures show what it looked like after I removed the panel. The bulge on that side of the battery pushes out roughly 5mm. Clearly, this battery's condition was terminal.
I ran into a small problem when unplugging the battery from the board. The plastic tab the instructions say to pull on wasn't present. There wasn't any protruding edge of the connector that I could pry on with a spudger, so I ended up having to pull it out by the wires. I rationalized that this was okay because that battery is never going to be plugged in to anything ever again.
Once I got all the screws out and the battery removed, I found that the battery was bulging so badly that it had actually burst the plastic case. You can see this in my third picture (which is rotated 90 degrees so that I wouldn't have to crop out any of the carnage). The edge where it split open is the interior edge on the keyboard-facing side. No wonder my trackpad was acting up—the center of the battery was more than twice as thick as it was supposed to be. I think that if this wasn't a unibody aluminum case, the laptop itself would have been visibly distorted.
BTW, that black-and-white pen-like object visible between the green spudger and the pack of screwdrivers is an OXO electronics cleaning brush. Retractable static-free soft-bristle brush on one end, capped soft silicone wedge tip on the other. Awesome at removing crud. I gave the entire interior an air puff and a sweep with that brush before installing the new battery. You can find the brush on Amazon. (I have no connection to OXO except being a loyal customer, and I'm getting no benefit from plugging this. I'm not even posting an Amazon Associate URL for it.)
The only other issue I had was with plugging the new battery into the motherboard. The plug is tricky to line up with the socket, because there's just no good angle from which to see the socket, even with a magnifier, and there's no real tactile clues either. I was maneuvering the plug by the plastic tab to try to line it up, but that didn't work for long because the tab came off. The adhesive holding it to the plug and wires isn't very strong. Now I know why the tab was missing from the old battery's plug—it probably came off when Apple's tech unplugged the battery when they replaced my motherboard. I ended up using my spudger to line up and push in the plug.
After that, finishing up was easy peasy. Sealed 'er up, plugged 'er in, booted 'er up. The battery was reading about 50% charge at login, and it didn't take long to reach full charge. My trackpad feels normal again. The lid closes flush again. USB ports deliver full power again. I should have done this two years ago.
If you suspect that your battery is swelling, don't remove the case screws sequentially around the perimeter. If you do, the underside panel is likely to go sproing once you round your first corner, and you'll probably lose the screw you just released. The safest order to remove the screws in is back-and-forth across the panel. (I recommend the center screws on the two short sides first, then one of the four non-corner screws on one of the long sides, then the diagonally opposite screw on the other long side, then the same screws on the other diagonal, then the corners in the same diagonally-opposing pattern. When tightening the screws later, follow the same pattern in reverse.) Even if you don't think the battery is swelling, following an alternating pattern reduces the stress on the case and makes it less likely that you'll strip a screw or screwhole.
While you have the case open, take a moment to clean the cruft out. Dust bugs will clog the fans and make the computer run hotter, and they can also cause shorts on the circuit board if you're unlucky. A soft-bristle brush like the OXO I mentioned and some careful puffs from an air can will do wonders. And before you spray air into the fan blades or hinge vents, open the laptop's lid and put a couple of paper towels between the keyboard and screen, right up against the hinge, to catch the gunk that comes out of the upper vents; you don't want to be cleaning that off the screen later.
I cannot stress enough how easy this repair is. I marked the difficulty as "moderate" only because you shouldn't try it if you're not comfortable with opening the case. Apple insists that this battery is not user-replaceable, but if you're out of warranty (and pretty much every model this particular guide covers is likely to be out of warranty by now), there is no reason for someone who's comfortable adding RAM or changing hard drives to pay Apple $150 to take two to five days to replace it. The only downside is being stuck with an $8 specialty screwdriver that you'll probably never use for anything else. Small price to pay for the benefits.