Apple iBook G4 14" Early 2004 Battery replacement

Due to its, age, the battery of my iBook from 2004 needed a replacement. As you can't find new ones online, I had to refurbish the one I already had in my device. Searching the web, there are some resources showing a battery rebuild for the G3 clamshell models, but none dedicated to the 14" G4 I had. The process is similar nevertheless.

Prerequisites

Before you begin, read this very good post first, as it details the general rebuild process quite well, and come back here to fill in the details regarding the G4 batteries.

As you have now learned, you can only rebuild the battery pack as long as the BMS still works correctly. To confirm this, get Coconut Battery on your device and check if the values shown there make sense. If not, you will have to hunt for a new replacement, as the BMS will not be able to work with new 18650 batteries anyways.

On batteries, I choose Samsung INR18650-35E (INR = Lithium Nickel Round, 35E = 3500 mAh) as they were readily available and offer the flat head design Apple used as well. With these batteries, the iBook should also last a little longer as an additional benefit.

I got some tools to open smartphones with which I used to open up the battery. Also, you'll need a simple spot welder and a soldering iron.

Opening the battery pack

First a word of caution: 18650 batteries can provide lots of amperes, so they are no joke if you physically damage or shortcut them as they will catch fire. Make sure your old batteries are flat before disassembling, and be careful when handling them. Having a CO2 fire extinguisher around while working on the battery pack is not a bad idea, but most likely not necessary.

With that out of the way, drain the battery pack completely, to have as little charge in there as possible. The battery pack consists of two plastic parts glued together. Some people on the internet said to heat the plastic, I preferred not to as I could not be sure I don' heat up the batteries inside too much. So i pried the two halves apart, starting at the external connections where the BMS sits. Try not to bend the plastics outside too much, just up, to make sure you can put them together again. Be careful with any sharp (metal-made) tools that could puncture the batteries.

You're in for a treat: The two halves are not only glued, but also feature some small noses to keep them together. I found no other way than to brute force my way in order to open them. I used different smartphone repair tools to tear apart the glue, then prying the two parts open. It took my around two hours to get in there, your milage may vary. I did not use heat or isopropyl alcohol or heat to make the job easier, next time I definitely would. Take a look at the picture to better understand how the parts are hold together to make your experience easier and less painful than mine.

After you're done, you will be greeted with eight 18650 cells, a BMS and some wiring.

Removing the BMS

It's always smart to take some pictures before you take something apart, so I did this first. Then, it's time to remove the batteries from the enclosure and remove the BMS. Desolder the cables. Here, use a big tip for your soldering iron as those can store more heat and should help to de-solder faster (again, you don't want to heat up the batteries too much). I've added some flux to the joints, the iron worked quite well for me at around 350°C. Before touching the joints and removing the cables, I added a bit of solder to the tip of the iron to improve heat transfer. This way it only took a second until the solder melted and I could pull the cable away.

Rebuild the strips

After you have removed the BMS form the batteries, I started to take a look at the connectors between the packs, took some nickel strip and replicated the parts. As my strips were not wide enough to accommodate the solder connections, I cut these part separately and attached them by welding to have the same shape as the original parts.
In total you should have prepared seven strips.

To make my life easier, at this point i sanded the spots where the soldering connection will take place. I added some flux to the very spots (check if you took the right side), and already added quite some solder I will later use to connect the cables. This way the soldering should be as simple as the de-soldering before. Don't be too shy with the amount of solder in this step.

Welding

Take your old batteries as templates and solder the strips onto the batteries. Double check the polarity here once more, as you don't want to mess this step up. I used tape, blu tack and some heavy objects to keep the batteries aligned and in place for the welding. Also try not to forget to put the two white spacers in place so your batteries fit perfectly when assembling everything again (and, rotate them correctly).
When done, you should have two battery packs, a 2x and 6x.

Reattach the BMS

Now, solder the wires back to you new connection points and do everything basically in the reverse order from the de-soldering process.

Put the pack back into the enclosure. I did not close the case already as I wanted to test if it works in general. The connecter moved, so I just finished everything, hoping it works and I don't have to open it up again. All went well.

Let's charge the battery up

Here, follow the blog post once more. What you want to do is fully discharge and fully charge the battery to reset the BMS values. Fingers crossed!

I had a weird problem. As the BMS was updating the numbers for the battery capacity,it only increased the existing value by 300–400mAh per full cycle. After multiple cycles I was back at 4400 mAh, which is fine to me. Too bad it only had 26 cycles on it to being with, as the preowner took good care.

Update
After a few full cycles and a PMU reset, the battery shows 99% capacity but won't go significantly higher.

Additional Sources
https://forums.macrumors.com/threads/12-powerbook-battery-info-and-upgrades.1932355/
https://blog.lazd.net/rebuilding-an-ibook-g3-clamshell-battery.html
https://forums.macrumors.com/threads/rebuilt-a-clamshell-ibook-g3-battery-battery-only-charges-for-12-minutes.2274217/