I did something drastic today. I ripped apart the old TF-101 keyboard (already broken prior) to get at the battery pack inside, opened my son's Transformer, and swapped out the dead battery inside with the one from the keyboard, only to find out that... the other pack is just as dead as the one already inside the Transformer.
I get the same red battery icon when I try to turn on the Transformer I just reassembled using the other battery. Now, I could in theory remove the battery from the second Transformer I have in order to do this, but opening up one of these things and extracting the battery is a non-trivial task to say the least, and I don't relish doing it again. A new battery is rather expensive, at around $40 + delivery, and to be frank I'd rather just spend that $40 as part of the cost of a new tablet. However, it turns out that there are some files inside that thing that I want to be able to recover (mostly photos and videos), so I'd ask what options I have for attempting to power up that thing one last time to extract the data from it. One way would be to use the working Transformer's battery, but that is obviously quite out of the question. I could put the thing on a lab bench somewhere (just assume that I have the facilities to be able to do this) and feed it with 7.4 Vdc along the same 8-pin header that the battery uses, to fool the tablet into thinking it had a working battery, allowing it to boot. What do the wires do, and what voltage levels should they have in order to make the tablet think it has a good battery?