[Tip] TF701 Will work when powered, but otherwise dead (battery won't charge)... - Asus Transformer TF701

Slightly long post...
Skip to [Symptoms] if you want to get to the meat!
[Background]
I had a TF701 a while back. After using the HDMI connection in a hotel (I blame this!), the battery was dead in the morning, and the tablet would not boot or charge from the dock. It *would*, however, boot when plugged directly into the charger.
Out of warranty, I eventually scrapped it (wish I hadn't!) and replaced it with a new one...
Move on a year - I bought a second hand TF701 off eBay - this had a broken screen but otherwise worked fine (bit of a bargain!). One replacement screen later, I had a fully functioning TF701 to play with (which I rooted and rommed)...
The other day, the display stopped working (I'd carried it on my bicycle pannier - well padded - but I suspected vibration had done something). Back home it started working again, but the display was a bit intermittent, so I guessed that the connector cable had become unseated, so I opened her up again to take a look.
Sure enough, the ribbon was slightly out, so I straightened it, secured it properly and closed up the tablet while it booted.
Just as I snapped the last clip into place, the tablet instantly powered off and wouldn't boot.
[Symptoms]
Tablet will not boot on battery (plugging into the dock makes no difference)
Pressing/holding the power button results in two flashes of the orange power LED then nothing
Plugging the charging cable into the tablet allows you to boot normally
Unplugging the charging cable results in instant power down
Once booted (on external power), the tablet functions normally, but claims that the battery is at 0%, claims it is charging, but the battery doesn't actually charge at all.
The symptoms above were exactly what my 'scrapped' tablet did, so I thought this was yet another dead TF701. The difference, this time, was that it was working fine right up to the point where the last case clip was snapped into place. In other words, I seriously doubted that this was an electrical failure.
[Resolution]
I'm not entirely clear on what resulted in fixing the problem, but here is what I did:
disconnected the display and digitizer ribbon connectors
reconnected the display ribbon only (i.e. not the digitizer) and tested (no change)
disconnected the battery connection
measured the voltages on each of the pins on the battery connector (roughly 4v on the live pins)
cried inside a little
carefully reconnected the battery connection (firmly!)
reconnected the display and digitizer connections
tried powering her on again (expecting nothing) - she fired into life!
I let her boot up while the display panel was not clipped in place - just resting on the tablet. The TF701 booted up normally, and the battery display showed that there was about 87% charge!
This time I shut it down, clipped the casing back together, and powered it all up again - she lives!
So far, the tablet seems to be working normally, docking/undocking is fine and the battery charges normally.
I suspect that the two flashes of the power LED is some sort of diagnostics signal (may be as simple as 'no battery detected').
Bottom line is that if anyone else out there finds their tablet with the same symptoms, open her up and try re-seating the display ribbon connectors and the battery connection. There is hope!
I'm now wishing I hadn't been so quick to dispose of the first one that died. Oh well.

Related

Dead Screen or Nearly Dead Battery Issues

Okay, first thing to mention is that I have searched for information on my issue and while I have found (and tried) solutions that approach my goof, I need to post this:
Last night I finished reading from Kindle app, hit the power button to shut off the screen and laid it down. I know I should have plugged it to recharge but too late now. I am running CM nightly from 07172012, so I know I didn't download the nightly CM battery issue versions.
This morning the screen will not light up; I have tried charging it, hard reset with power and home screen buttons for 15-20 seconds (and longer), and nothing works. My concern is that the screen is dead. I connected the touchpad to my laptop and I get the connection sound from the laptop; it sees the TP but does not accept it as a portable storage device. Device Manager lists it as: QHSUSB_DLOAD with driver(s) not loaded.
When I try the volume rocker button I do not hear the small audible ping as volume goes up or down. Of course, with the resets, who knows where I am in the OS (or even if it is WebOS or Android).
I really didn't want to be a noob and post this, but I need some advice from the brethren here on XDA. Thanks in advance.
It seems to me that it's not even booting if your getting that strange device in device manager. Try the webos recovery tool from hp. It may help. Although it will wipe android off.
Sent from my SGH-I777 using xda app-developers app
my son-in-law is in the same boat. Nothing he does wakes it up. I believe there are tp's out there with this hardware issue. I let my tp battery drain completely. It powers down around 4% battery. I place it on the touchstone, it charges backup and boots into moboot, then webos. I've done this twice. no problems. If it's under warranty still, send it in.
If you have a touchstone charger put it on that and leave it for a couple of hours and it Will come back to life.
See this article by Reverend Kyle http://www.reverendkyle.com/index.php/articles/146-hp-touchpad-charging-issues-solution
I hope this helps.
I've had multiple instances of black screen of death failure.
The latest one I had, I was able to restore by using a standard USB to micro USB cable (and recently NOT the HP OEM cable) to connect to the computer I usually use to sync or use HP web doctor with.
Same symtpom as the OP, get the three dings when connecting the touchpad to the computer, no responses and the QHUSB driver load error. HP charger wouldn't chage it, touchstone would not charge it.
Anyhow, I left it connected for a few hours to charge connected to my PC. Then, I disconnected from the PC using the large USB connector and reconnected. Whamo, got to the bootloader.
I believe what happened was I trickle charged the touchpad enough to get the touchpad to boot.
You can try the various hard boot keypresses too, but wait for a few hours while trying to trickle charge the touchpad via the computer USB.
Update
Okay, so I followed the old rule of doing the simple (and/or free) solutions first and then move on.
1) I set the touchpad on the touchstone for several hours yesterday - no luck
2) I connected the touchpad to my pc through the USB and let it charge - no luck
3) I let it be plugged into the standard charging plug all night - no luck
4) I prayed to the great old god Cthulhu - no luck
My only option remaining is to use WebOS Doctor. One question on that: in all the literature I have read thus far (and perhaps I am mistaken), I need to be able to get the TP into USB mode to download the corrections/reboot back into WebOS. Is that right or can I expect that when the doctor sees the mangled mess of an OS through the USB connection, it will know what to do?
Again, as always that for the previous posts and for the support.
mnachazel said:
Okay, so I followed the old rule of doing the simple (and/or free) solutions first and then move on.
1) I set the touchpad on the touchstone for several hours yesterday - no luck
2) I connected the touchpad to my pc through the USB and let it charge - no luck
3) I let it be plugged into the standard charging plug all night - no luck
4) I prayed to the great old god Cthulhu - no luck
My only option remaining is to use WebOS Doctor. One question on that: in all the literature I have read thus far (and perhaps I am mistaken), I need to be able to get the TP into USB mode to download the corrections/reboot back into WebOS. Is that right or can I expect that when the doctor sees the mangled mess of an OS through the USB connection, it will know what to do?
Again, as always that for the previous posts and for the support.
Click to expand...
Click to collapse
Another piece of black magic that worked for me and is worth a try as it is free. Connect Touchpad to standard HP brick charger plugged into power outlet. Unplug cable from charger, count to 5, plug it back in, count to 5, take it back out. Repeat the cycle until you get bored or until you get a red charging symbol. If you get the latter then let it charge up normally and you are back in business. The theory behind this method is that it takes a few seconds on plugging in for the circuits to realise they "shouldn't be charging" and this allows a little bit of charging to take place on each insertion. Repeat enough times and you get enough to kick start the proper charging. In my case about 20 times did the trick.
As I didn't want to get into this situation again I then set up a Tasker process to power down th eTouchpad if the battey charge dropped below 8% and I've been OK since.
RIP Touchpad / EOT
Well, I had to wait a day to get my hands on a WinXP laptop; I upgraded my Dell to Win8 about 3 months ago and WebOS Doctor does not play well with Win8.
While that time passed, the touchpad finally gave up the ghost. There is no power left in the TP. When I run WebOS Doc the laptop cannot see the TP connecting via USB. I also checked by connecting the TB to the Dell, since it would at least see the device. Not any more.
My thanks again to those that responded. I did try the plug in, plug out with 5 second waits between, just in case. You never know. However, after 50+ attempts I had to call it a no-go.
does your home button light come on at all? Are you able to see moboot bootloader at all or are you able to start the tablet in webos recovery mode by holding the vol key while powering it on? are you able to ./adb connect? Also I've never had much luck with the resest method youre using, holding down the power and home key, I always hold down the power key and press the home key 15 times, when I get hard freezes and lockups, it's always worked.
Mine is in the same boat for a couple of months, removed battery and charged. Boot loader is borked I get only get a qualcomm usb device but nothing thats of any use for webos doctor or qualcomm tools. We'll have to wait until someone figures out jtag or how to replace the bootloader with qualcomm tools. I'm amazed with the number of touchpads out there that something hasn't leaked from hp to fix it. Anyone got friend in hp or qualcomm :angel:
OP Update
Well, I thought this was a dead issue but here is the latest.
After wallowing it a pit of despair the likes of which can only be described as wretched, (I even coveted my wife's TP up close, almost giving into temptation and swapping decals to reverse ownership) I looked outside of XDA and did general Google searches and happened upon this site:
http://forums.webosnation.com/hp-touchpad/299256-dead-touchpad.html
After reading a bit I realized that the touchstone may be able to recharge the dead battery even though fully drained. I began the quest that Friday morning the 24th of August, after verifying that my Dell laptop did not see the TP when plugged in via the USB cable.
I just did a quick check and the laptop now "sees" the TP when connected. Still no luck on booting into Moboot or the USB icon screen - neither Power and Volume Up or Power and Home buttons achieve anything. But I am glad to see that the touchstone has placed enough charge into the battery that the laptop is aware of the TP when connected.
I believe that given a few more days on the touchstone I should be able to boot into moboot.
@ haxin - I have nothing. Not a single light from the Home button - my only evidence that there is SOMETHING going on is when I connect the TP to the laptop and get my connection "sound". Again, I have high hopes that the touchstone will get me where I am going, albeit at a snails pace.
@ paddy - Yeah, I hope that when (if) I get to a lit screen, I am able to move past where you are at. My big concern is that I am toast even with a semi charged or fully charged battery from the touchstone.
I will update later on. Again, thanks to all for the input and probing questions. You have made me think harder and work a bit longer on this project.
OP Update - 31 August
After 7 days on the touchstone, the touchpad is currently showing the low battery charge icon; I have now plugged it into the A/C adapter.
Each morning I have picked up the TP, held down the power button and held the home button for 15+ seconds. Today it worked, turning on the screen for the first time in a week and letting me see the low battery image.
In my quest to resolve this issue, I have read so many variations on what buttons to hold and how long to hold them that after awhile you end up considering the possibility that SOME of the button options are not correct. That the operator has booted their TP up, but they misconstrued the actual solution; how else can you explain the idea that HP built so many different button combinations and hold times to boot a TP from a nearly or fully dead battery state?
Anyway, I will continue to post updates until my beloved TP is back to running CM9.
And to all those who gave their TP up because the battery got too low; find a touchstone or other "trickle" charger solution, connect and let it sit for a week or so.
Thanks!
Glad the Touchstone worked for you , don't know why it has taken so long to charge but when your up an running properly Llama is a good app to install as
A:You can set alarms for when the battery gets to a certain level/levels
B:Have it shut down when the battery gets to a certain level.
I use 20% for an alarm and 10% to shut down which allows me to sleep easy and not worry if someone else is using it . .
I am up and running. I most definitely will load that app on my TP, plus my wife's and a coworker who bought a TP because of mine.
I don't know either why it took so long; I am just glad it worked and I have it back. I am on it right now.
Thanks again to all who provided feedback and guidance. This is why I love XDA.
Sent from my HP Touchpad using xda app-developers app
HP Touchpad sleeping and will not turn on
Just a note to those HP tablet owners who experience the Black screen of death or sleep of death.
I am the proud owner of 4 HP tablets and two of them so far have had the black screen of death. I have read forum after forum trying to get answers to this problem. There was only one posting that has made cense to me. Someone posted that the tablets symptoms are very much like when the Phones use to go to sleep. The solution was to disconnect the battery for a few minutes then plug it back in. Then simply hit the hold the power button down to turn on the tablet.
The problem is taking the TP apart. There is a video on youtube on how to disassemble the tp. It is a little annoying that you have to go to such lengths to reset the hardware but HP will charge you 250 dollars to fix the problem. After you watch the video on how to open the TP, remove the 3 little screws holding the battery. Now slowly pry the battery up, they use double sided tape to hold it in place along with the screws. Wiggle the battery side to side and try to disconnect the battery connector. This can be done without removing the boards in the TP. Wait 1 min then try to replace the connector in the socket. It is on the underside of a board so when you have it in place use something to make sure it is tight. Plug the charger in to the USB cable. Without closing up the unit hold the tablet power button in until you get the battery charging symbol on the display. Let the unit charge while it is still disassembled. Just in case you have a problem. Once everything is operating disconnect to charger and replace the battery screws, now snap everything together.
This was the only way I was able to fix two of my four TP's. I am sure I will have to do this again because my kids always let the tablets go dead and they don't pay attention when placing them on the Stone for charging. If the tablet is not properly placed on the stone the tablet will not charge.
Hope this helps.
This by far is the best tablet for the money anywhere.
msousa1 said:
Just a note to those HP tablet owners who experience the Black screen of death or sleep of death.
The solution was to disconnect the battery for a few minutes then plug it back in. Then simply hit the hold the power button down to turn on the tablet.
The problem is taking the TP apart. There is a video on youtube on how to disassemble the tp. It is a little annoying that you have to go to such lengths to reset the hardware but HP will charge you 250 dollars to fix the problem. After you watch the video on how to open the TP, remove the 3 little screws holding the battery. Now slowly pry the battery up, they use double sided tape to hold it in place along with the screws. Wiggle the battery side to side and try to disconnect the battery connector.
Click to expand...
Click to collapse
I realise this is an old thread, but I recently had a reoccurrence of the black screen of death issue and thought given the number of touchpads still out in the wild, I would share my experience on the off-chance it may help somebody.
My first experience of the black screen issue was a few months after having my touchpad (many years ago). The screen just went black and no combination of holding the reset keys for any period of time would restart the unit. I had suffered lock-ups before and found the reset combination to be successful, but this time none of that worked and I had effectively what appeared to be a dead unit.
Having contacted HP, I returned the unit under warranty and a unit was returned working. I say “a unit” as I was never quite sure if it was the same unit or a refurbed unit, but to be honest given the speed the units went out of stock during the fire sale, I was just glad to have a working unit again.
Well this week after several years of use including the installation of Android, the black screen occurred again. As before, no combination of the keys / holding for a period of time / pressing them a number of times etc had any effect. The unit was three quarters charged and I was using it up till the point the screen went black. :crying:
Given that the unit has no warranty and it was “dead” anyway, I decided to take it apart. Despite watching YouTube videos, this appeared to be more troublesome than I thought, but eventually I was in. Interestingly, the backlight on the screen was still lit, suggesting the unit was still powered. I did consider disconnecting the battery, having removed the screws as per the previous poster, but mine seemed to be well and truly stuck down and I felt I may damage it prying it away from the base.
However, looking around inside the unit, I spotted what was clearly a surface mounted switch that faced upwards towards the back of the screen. Out of interest, I pressed on this switch with my finger nail and my unit powered off (shown by the backlight turning off). I then pressed the normal power switch and the unit powered up as normal. I have attached a photo of the switch which resides close to the popout tab with serial numbers on that can be found on the right hand side of the unit.
It does appear strange that HP put this “reset” button inside the unit, facing inwards rather than outwards where a small hole in the back of the case could be used to press it. So if you do get to the opening the case stage, consider this as an alternative to prying the battery loose. Also if somebody can work out a way to make the button accessible from outside the case it might be a useful modification.
JoeG90 said:
<.......................................>
However, looking around inside the unit, I spotted what was clearly a surface mounted switch that faced upwards towards the back of the screen. Out of interest, I pressed on this switch with my finger nail and my unit powered off (shown by the backlight turning off). I then pressed the normal power switch and the unit powered up as normal. I have attached a photo of the switch which resides close to the popout tab with serial numbers on that can be found on the right hand side of the unit.
It does appear strange that HP put this “reset” button inside the unit, facing inwards rather than outwards where a small hole in the back of the case could be used to press it. So if you do get to the opening the case stage, consider this as an alternative to prying the battery loose. Also if somebody can work out a way to make the button accessible from outside the case it might be a useful modification.
Click to expand...
Click to collapse
Oh I was so keen when I came across this post. A Reset Button! How great would that be?
Tried it, unfortunately it did not resurrect my dead as a doorstop TP. I was so hoping...
I just put a 'new' EBay battery in it a couple of days ago and tried trickle charging with a 1000mA charger for 24hrs. No joy there either.
All I get when I plug it into the PC is that it shows up as a "QDLoader 9008" (QHSUB_DLOAD driver) .
No flicker of life or light whats so ever...
_mr
middle_road said:
Oh I was so keen when I came across this post. A Reset Button! How great would that be?
Tried it, unfortunately it did not resurrect my dead as a doorstop TP. I was so hoping...
I just put a 'new' EBay battery in it a couple of days ago and tried trickle charging with a 1000mA charger for 24hrs. No joy there either.
All I get when I plug it into the PC is that it shows up as a "QDLoader 9008" (QHSUB_DLOAD driver) .
No flicker of life or light whats so ever...
_mr
Click to expand...
Click to collapse
Try to borrow a Touchstone conductive charger if you can. It's the only thing that has worked for me when it has totally drained down.
All the best.
sstar said:
Try to borrow a Touchstone conductive charger if you can. It's the only thing that has worked for me when it has totally drained down.
All the best.
Click to expand...
Click to collapse
That's what I was thinking - no joy there either... It is not even showing signs of getting the least bit warm.
I keep thinking and hoping that there is some way to resurrect this one but I may be wrong...
It's currently trickling on a 1000mA charger. Tomorrow I'm going to try the Touchstone behind it.
If that doesn't start something then I'm going to yank the new battery and along with the old battery charge them like an RC device.
(if I can ascertain the proper way to do that --- )
_mr
After several days on the trickle, still no life.
Took it apart at lunch and with the battery still connected to mobo it is showing 3.1V across the red/black. So I figure the battery
is good. Looks like the mobo is gone.
Not sure what else to try.
_mr

Red LED flashes when charging [Solution]

My z1 had a bad weekend and ended up with 1% battery which finally died. It was weird, it started with the phone not detecting when it was unplugged from the charging device, and I figured it was just a one off thing so I reset the phone, reflashed the phone and finally wiped and reflashed the phone (all off the charger since I was on the road at the time). Well nothing worked and it would not even power off. Eventually the phone died with the red led flashing away. Once I got to a charge I then I got the famous red light flashes while charging or no led when plugged in. For me both these meant the same thing. My phone would not charge and would not accept a charge.
I tried: leaving it plugged in, switching to the side port for charging, trying to get it to repair mode... NOTHING WORKED. It all boils down to the fact that the charger on this phone cannot charge a drained battery. Anyways needless to say I was desperate and searched for a solution. I found 3 and made up a fourth
SOLUTIONS
Non invasive
1) If your phone is under warranty bring it to a repair center.
2) Send the phone (to be repaired at your expense) to Sony repair center
OR
The following require a that you take the back cover off, ergo void warranty
3) Replace internal battery
OR
4) Get the battery out, carefully untape the LION battery package from the plastic frame (do not use anything sharp because if you pierce the battery shield you have a fire on your hands). Once the plastic frame is removed from the battery you can see two large connectors going into the pack, attach alligator clips from those to a "MICRO USB 1A Battery Charging Module" (available on ebay). Make sure polarity is correct and plug the charging module in.
I was able to do item 4. My battery before being charged was reading 2.6v unloaded, I left it for an hour or so and it was 3.7v, I put my phone back together and voila it powered up. The battery read 19% charged when it got to the lock screen. The battery is still charging (at 35%) but so far so good
Nz
What also worked for me is to hook Xperia Z1 to a magnetic charger.

Galaxy Tab S2 SM-T710 overheating when OFF and plugged in!

Hi everyone,
I'm really hoping someone can help me.
I have a Samsung galaxy tab s2 8inch. It is the SM-T710.
It was working fine playing music on a Bluetooth speaker system then the battery died.
Tried to charge it and it was acting funny, said it was charging then got to one or two percent and i attempted to power on while the device is on charge and it powered on then turned off within a few seconds.
Let the tablet charge for a while now and its not really going up and the screen has gone off and its not turning on and i now notice its getting really hot at the bottom part of the screen slightly above the home button.
Removed the charger and left it for one hour, then came back and tried with a different new genuine Samsung charger and cable, and still getting extremely hot within a few 5 - 10 seconds just above the home button.
Unplugged it and left it till the next day and attempted to charge it again same thing.
I though it could be either: A faulty battery or A faulty Micro USB charging port flex (possibly short circuiting).
Now Ive opened up the whole device and taken apart every item on the mother board following a tear down video on YouTube.
I then connected a brand new genuine battery to the board and removed the old one, and the device was still overheating however this time i could pin point the exact source of heat on the motherboard which i will circle on an image but cant attach to this post as i need ten posts.
The images are on my public google drive but i cannot share the links here any ideas?
I then thought the micro USB charging port flex is causing a short circuit and so i completely disconnected that and the overheating problem still occurred.
Then i disconnected each and every component one by one and still the same thing on the motherboard keeps overheating excessively within seconds of connecting the battery to the motherboard.
Now Ive completely removed the motherboard from the frame and Ive connected the battery to it and same overheating issue.
Now Ive reconnected just the micro USB charging port back to the motherboard and attempted to plug a charging cable into the port and exactly the same thing still overheating excessively.
I really do not want to replace my motherboard simply because of the data on the device etc.
Can someone please tell me what the red circled item is in my image so i know what is overheating!
Could it be the CPU?
Is there ANY other way to replace JUST that part, even in china? Can anyone advise or help me?
Many thanks to the people who patiently read this and helped.
Kaiser
London, UK
+1
I guess the motherboard got corrupted
First off, do not boot up an almost empty device. Second, it is entirely normal that the bottom area (charging port, home button etc) gets hot while charging. It will fade after the battery got some essential energy. Third, do not deplete the battery completely, if possible. In the long run it may damage the longevity of the battery.
Was there ever a solution to this issue? I'm having the same problem.

Nook HD Charge Light only stays on ten seconds then switches off

My nook HD ha been sitting for a few months, and worked fine. Then the charger broke. I stuffed it in a box. Then recently realized i had enough money to pick up a third party charging cable that had decent amazon reviews.
Ten seconds charging: Five seconds with green, five seconds amber, then click.
I hold the power button and nothing.
Unscrew the tablet and battery's seated. Plug power in without a battery attached just for funsies. Red light after a fw seconds of rapid amber/green flashing.
Is there any way to revive this dead unit short of buying another battery? I halfway don't want to given the nook's charging cable design seems almost intended to fail with snapped wires. I just want to see if I can revive this thing or try pulling it apart to see if I can repurpose the screen or not.
Goggles2114 said:
My nook HD ha been sitting for a few months, and worked fine. Then the charger broke. I stuffed it in a box. Then recently realized i had enough money to pick up a third party charging cable that had decent amazon reviews.
Ten seconds charging: Five seconds with green, five seconds amber, then click.
I hold the power button and nothing.
Unscrew the tablet and battery's seated. Plug power in without a battery attached just for funsies. Red light after a fw seconds of rapid amber/green flashing.
Is there any way to revive this dead unit short of buying another battery? I halfway don't want to given the nook's charging cable design seems almost intended to fail with snapped wires. I just want to see if I can revive this thing or try pulling it apart to see if I can repurpose the screen or not.
Click to expand...
Click to collapse
I had to double check this wasn't a post I done a few months ago and forgot about...
I lost my charger, foolishly tried to force a regular 30 pin in before realising it had a proprietary port and bought one online that acted exactly like yours. Assumed it was due to me forcing the 30 pin in but maybe not!
Anyway, the solution for me has been to apply a bit more pressure when putting it in. So I'd insert it into the port until it felt comfortably in place and then just give it a little bit more of a push in than seems reasonable. In general after doing that I've had no issues for the duration where it's connected (I seem to recall it disconnecting once or twice afterwards a bit easily but not a common thing), it's not the most scientific of solutions but it has worked every time for me.

Fix for broken power button (Mi 9 Pro, but will probably work for Mi 9)

Maybe not the right sub-forum for this, but for some reason I show as not logged in on the Guides sub-forum.....
Long story short, I removed the shattered back of my phone for replacement, and in the process it appears I did something that broke the power and volume buttons. I don't know what broke, it all looks good, but I used a blade to lift the cover and in retrospect, I think heat and a suction cup would have been better.
I tried rebooting prior to installing the new rear cover, and lucky I did because I needed access to finally fix it. The buttons were completely dead, nothing worked, but if I plugged in power, it started charging, so I knew the battery and screen were still good. From adb, the device was there but unauthorized, so I was unable to boot that way (I've since rebooted and turned on USB debugging, but evidently it uses the vendor keys still when in recharge mode, and it still showed unauthorized).
None of the local shops were any help, I suspected that if I could get the boot sequence to start all would be good, and this turned out to be the case, but they were all clueless about anything other than changing the battery or the screen.
Anyway, onto the solution. The first time I got it to boot, I simply disconnected the battery, then plugged in the power using the Xiaomi charger, it wouldn't boot at all from USB power, it would only charge. As soon as I plugged it in, the power sequence started and the phone booted and ran fine. I pushed the battery connector back into place and all was good, except the problem of what happens if I power it off.
I went to developer options, checked USB debugging, and powered off, confident that even if I still couldn't see it in charging mode, I'd at least be able to manually power it on after disconnecting the battery. Well, of course, the second time it didn't work. ADB was still unauthorized, and the battery disconnection trick didn't work, it started to boot but then appeared to lose power and the screen faded. This gave me a clue that it might be possible to use the wireless charging to get it to boot, but the problem with that was that it wouldn't wirelessly charge with the battery disconnected. In the end, what worked was to loosely hold the battery connector with my finger over the contacts, while holding the phone against the wireless charger (I have the Xiaomi 65W wireless charger), in this state the battery reads 15% even though it's full, but the phone boots. I then pushed down to fully connect the battery, at which point it read 100% and was booted. It's super-fiddly to hold it all in the right position, but perseverance pays off. If you lose the connection to the battery completely at any point, the phone simply dies.
To get to the battery connection on the Mi 9 Pro, lift the wireless pad, at the top right corner of the battery you'll see a short flat black cable that runs towards the centre of the battery and then up to connect to the motherboard. Lift up on that connector with something thin to pop it off. When rebooting, hold your index finger on the connector with enough pressure to make a contact without fully reconnecting it, while holding the wireless pad against the charger. Why the hell this works I have no idea, but it does.
To operate the volume/power, just download widgets. It would be nice to have a physical volume button, but doing it from the screen works.
This was a life-saver for me because I friggin' love this phone. I have the 12/512 model, and it's so good I've finally stopped fiddling with ROMS because it just works perfectly. Unfortunately, it took me a week to figure this out, and I ordered a Mi 11 Pro in the meantime!
If anyone has any good ideas about an easier way for me to reboot in case it ever powers off (a simple reboot is fine), I'm all ears. I'm currently looking at having to remove the rear cover again and go through the whole rigmarole if it ever powers off.

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