I have posted intermittently about this project of mine here in various threads, but let me give a summary.

I bought a few weeks ago a Thinkpad 755CX laptop: pentium 75, 40MB RAM, built-in FDD but no CD (I use a docking station for it in which I installed a SCSI CD unit), and Mwave DSP/modem/sound card. The laptop works otherwise fine, but because the BIOS wasn't upgraded to the latest revision the Mwave card only works in DOS and Win3.1, not in W95/98. I tried to flash the latest BIOS, but IBM decided to make the flashing program check for a fully charged battery before it will allow you to flash. I don't have a working battery for this laptop (have 5 or 6 dead ones) so flashing the BIOS was impossible. I actually have a running thread on forum.thinkpads.com about this, tried many things suggested there none of which worked.

It comes down in the end to getting a working battery. There's no way around that.

I only need a battery that will take enough charge to be read at 100% for a few minutes while I do the flashing. I don't need one that is completely functional and allows the laptop to run off it for extended periods of time. The Thinkpad will also be connected to the AC charger during the flash, so none of the battery charge should be actually used during the process.

I have given up the thought of reviving the Li-Ion batteries I have. Once those are dead, they're dead. But there's a hope with a NiMH pack.
I looked for a long time for one of those, finally got one a few days ago. It appears to have been stored improperly for many years (it has an on/off switch which is supposed to be put in the "off" position for storage - but this pack was "on" when I received it). It wasn't dead but pretty low, reading 6V in open-circuit and down to about 4V afrer a very brief test discharge. The battery pack is nominally 8.4V, 3.5Ah.

I put the pack in both the laptop, and in an external Thinkpad quick battery charger, but none would charge the battery. I tried the freezing trick (froze the battery overnight in a ziplock bag then thawed it the next day to room temperature) but that didn't do anything.

I then resorted to try and charge it with a constant current source. I let it charge at a constant 100mA for 8h; the battery initially needed 13.6V to achieve this charging current, which after 8h decreased to 12.2V and the battery now read 8.5V in open circuit. I put it in the Thinkpad at that point, and the laptop's battery indicator was reading it as 26% charged. However the laptop would not boot off the battery alone, wouldn't even try to power up with the AC disconnected.

So I connected the battery back to the constant current source, let it have 150mA for 12h (voltage was around 13V to get that current) and now the battery reads 9.2V in open-circuit. I put it in the laptop again, but this time it won't boot at all with the battery in, regardless whether the AC is connected or not. Without the AC connected it won't power up at all (as before). With the AC connected it powers up briefly, powers down, powers up again, powers down and so on.

I'm not sure where to go from here. Does anyone here have experience reviving old NiMH laptop batteries, and if so, any tricks I could attempt? I have read about zapping the battery with a high voltage, but I'm reluctant to do that. These batteries are very rare and hard to find, not to mention expensive when you do find them.

Otherwise the only idea I have is to attempt a number of discharge/charge cycles on the battery - discharge it through a 100ohm/1W resistor down to 6V, charge it back, discharge again etc.

I would appreciate any help. I don't have any hope of getting sound working on this laptop in W9x unless I flash the BIOS, and I need a charged battery to do that.

(update) with a 100-ohm resistor connected I can only get about 50mA from the battery, because under this load the voltage decreases to 5.6V. That means the battery has a high internal resistance i.e. it's in bad shape but not hopeless. I estimate the internal resistance to 60-80ohms.