I planned to get a working BMS from (preferably) a non bloated battery to minimize puncture risks.

There's absolutely no insulation from the heatsink and the battery. Microsoft did put a huge chunk of the cooler on top of the battery (I strongly believe it has been done in propose). Nobody with a sane mind would do that but yet, Microsoft did it...

I think that I'll try to solder 18650 cells to the original BMS and see how it goes. If it does well, no need to reverse ingeneer it. If not... I guess I'll have to acquire some knowledge first to do it myself.

I've found a picture of a BMS someone took appart.

The left one is from a Chinese copy but the right one is the original LG one. Looks like both cells would connect to it. Comments say that the original BMS has a temperature sensor and charging control and that's why some Chinese copies won't get recognized by the Surface.

I think I'm getting somewhere with my idea.

You're right... I guess I'll salvage the BMS from another dead battery which hasn't expanded yet. I'll ask a few repair places around. Maybe they could even strip out a BMS for me.

But then comes the part where I'll solder new cells to it. Do you think it will get the extra capacity of 18650 cells ?

The tablet is already disassembled, thanks. I already dealt with similar devices to not break stuff while attempting a repair. I've just never dealt with such a bloated battery.

The idea was to fix that inherent design flaw the Surface Pro 4 has : A massive part of its heatsink is covering the battery and transfers heat to it, damaging it the more the Surface is used.

If I replace the battery, it will do it again in a few years if not sooner due to lower quality cells.

Buying good quality 18650 and making 3D printed parts to fit them outside the casing will cost less than a replacement part from Ifixit...

I not only want to fix it, but I want it to last longer.

The BMS seems to be located right between the cells. If I cut short each cell wires, there should be no short... But the cells doubled in thickness, I'm even afraid to remove them from the casing.

Yeah, that's what I was afraid of...

Any way to rip the BMS from the old battery without setting it on fire ?