Yep, I've done it. (Be sure to read this whole thing, I didn't put all the info in a good order.) Bought one of those cheap after-market batteries and it wasn't long before the firmware screwed up and my MacBook started displaying all sorts of bizarre symptoms, related to the battery. So here I had an almost new battery that the firmware was messed up in and an old Apple branded battery with worn out cells. I did what any redblooded engineer would do and took both batteries apart. This is not easy to do. Apple glues the cells into the bottom of the battery holder and they are tough to get out. I used a paint scraper to get them out. Still damaged one of the cells, but no matter cause they were trash anyway. Take your time and you'll get them out, just be carefull not to damage the board in your original Apple battery. Ok, you got all the cells out of their cases. Apple uses spot-welding to attach the tabs of the cells to the board (the correct way). You can't remove the tabs without messing up the board so I took a Dremel tool with a cut-off wheel and scored the tabs where they entered the cell and just left the welded tabs attached to the board. Luck would have it the tabs on the Chinese battery were soldered in, so it was easy to free them. By the way, the cells in both batteries were identical Sanyo cells.
To solder in the new cells to the Apple board you are going to need a really Large soldering iron. The idea here is to transfer heat as fast as possible and get off of it before heating up the cell to much. I used an old iron from the 50's that is huge. I think you could sweat copper pipe fittings with it. First sweat some solder onto the old tabs still on the board and onto the tabs of the new cells. Here is the tricky part-getting the cells lined up, soldered, and flipped back together to have the whole package fit correctly back into the battery holder. TAKE YOUR TIME with this. I should have mentioned this earlier, but when you are taking the cells apart you will notice you have 6 cells, 3 in series and 3 in parrellel. Unfold the cells like a butterfly and you will have 3 cells on each side, facing each other with the connecting board running through the middle. When laying out your 6 new cells be sure to get + and - going to + and - on the the corresponding cell (each pair of cells are in parellel). One set of 3 will be upside down from the other 3. Getting things lined up here is the tough part. Lay the tabs where they belong along the connecting board. With a sharp pointed object press down on the stack of tabs and hit the whole thing with your large soldering iron. Be QUICK, lipo cells can go into thermal runaway pretty easily and burst into flames! It took me a couple tries before I had all 6 cells line up perfectly. Apple used a couple of thermal sensors (and associated wires) from the controller to the connector board. Leave these intact, they use some sort of special glue on these. Watch their wires when soldering. Run a piece of electrical tape across your soldered tabs and fold the cells back together. Now fit the whole assembly back into the battery case (may take some persuasion), and your done. Time to check your work. At this point the firmware in your Apple battery may be confused (having gone from cells that were shot to having new ones). You may have to reset your SMC. Go to your Apple menu and select 'About this Mac', then select 'More Info', then select 'System Report' (I'm running Lion, to get to the system information utility you may need to do something different), the menu that comes up will have 'power' about 15 items down. select it.
Your full charge capacity should be between 4,000 and 5,000 and the voltage should be above 11 volts. If your full charge capacity is significantly below 4,000 mah then one or more of your cells are not making a good parellel connection. (Each cell has roughly a 2500 mah capactiy, so two in parrelel should come up to 5,000 mah, usually will be between 4,000 and 5,000 in the real world). If your voltage is significantly lower than 11v, likewise you have bad series connection. Like I said, your firmware may be confused and your numbers probably won't look like what you would expect. It will take a while for your firmware to 'catch up'. Mine started out at around 4100 mah and over a period of about 6 weeks it kept coming up as I used the new battery. It is now sitting at about 4800 mah. After installing your new cells be sure to do a battery calibration, or two. There are directions all over the place on how to do this. Be sure to follow the directions and allow the battery to sit for 5 hours at each end of the charge/discharge cycle. It is required by the TI gas gauge to put the controller into a calibration cycle. Hope this helped.