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Been awhile but I definitely wanted to share this as I'm sure a lot of you will like it. I've successfully rebuilt another 4-cell battery for Thinkpad X60/61, also compatible with the X60s/X61s.
A lot of this comes from kfzhu1229's knowledge, he is on this forum too- and he's posted a few topics on this forum documenting his rebuilds. My method is pretty much based off of the direction he's provided on here and elsewhere.
I'll be summarizing the rebuild on here for the most part instead of providing a sort of walkthrough... but there is this guide I posted on our separate blog which has a LOT more photos and little step-by-step directions to help give you a much better idea what this rebuild entails. Please be warned, there is a lot of high quality photos on this blog, so if you don't have a high speed connection or ability to view large photos you may need to steer clear unless there's a way to load the page without them.
So- I've rebuilt a 4-cell before, but with these ones that have earlier Sanyo BQ8030 controllers... we've found that they have a hard time extracting a lot of capacity from the newer battery cell's chemistry. I also happened to have 4x Molicel P30Bs left, which are pretty high capacity cells(3000mAh) that are designed to handle extremely high current draw (30A continuous). In a laptop battery application that's clearly ludicrous, but I figured it would suit the X61 as the chassis is definitely able to outlast many of the batteries that exist for it, including OEM ones. TI BQ8030 proves to be a pretty hardy chip/BMS, so combined with cells that would be extremely understressed- I'd hope it could chase over a decade of usable lifespan, maybe the magical 1000 cycles? There's some of these 9-cell Panasonics in T60s that have over 600 cycles and still retain more than half of their original capacity which is quite impressive (either that or the Panasonic controller IC, the M37512, is quite lenient on what it determines to be a "cycle").
The battery is part number 42T4505, again which makes it a 4-cell Sanyo. These are one of the easiest laptop batteries to rebuild since the casing comes apart quite easily and... well, a 4S1P isn't very complicated. Just one single set of cells that all step up in voltage 4 times, in a planar configuration (2 rows, 2 columns) with no elevated row/column or perpendicular set of cells, or weird/confusing wiring. Thankfully this one was in great condition with no leaking, no corrosion, no messy insides and decent plastics. The Series 1 and Series 3 tap being so close to each other in the middle is a bit dicey, but I put my mind at peace by just using kapton tape and epoxy. Resetting the controller was done with BE2Works 5.73, and didn't really require any digging into EEPROM block locations like what kfzhu has demonstrated on his topics- since it has the nice luxury and convenience of a "quick reset" function. I would like to insert a caveat to those interested in maybe trying this and getting BE2Works- this "quick reset" does not work with every battery and should only be used if you know what you're doing. It can brick certain batteries if you're not fully understanding what's going on.
The data locations for these batteries are typically:
If you are interested in rebuilding batteries, please see this topic from kfzhu for more of a deep dive.
But after using the quick reset and writing the fixes to the ROM, the BQ8030 unlocked the MOSFETs and the battery began working completely normally again. I've since cycled it twice and it's calibrated to roughly 20% of design capacity, just a hair short of 30Wh. It does run out a lot longer when the meter hits 0%, it'll run for another 30 minutes before the battery actually cuts power to the laptop- but we've noticed this anyways with earlier Sanyo BQ8030 batteries in Thinkpads. Since the full capacity of the P30Bs is not even being used, combined with the very low current draw, the battery should last a very long time and also put up with the X60/X61's residual heat from the heatsink.
Anyways just wanted to share this. Love keeping these machines alive and I've been getting better at getting these batteries apart and back together intact too. I'd definitely encourage anyone who can, to check out the step-by-step blog I've linked above that shows you my whole process of rebuilding this battery with a lot of photos. And if you have any feedback, I'm all ears!