[XPERIA X10 Q&A, Help & Troubleshooting] SysGhost:
First, a few things:

I'm well aware there are several other threads about this already, so before you go -"Arrhh!... not another one!". Read this: We need a more complete guide with more facts about what's going on, and details on how to recover from it.

Questions and new threads that involves bricking, red led of death, battery failures and so on, are very common. I felt it's time for this thread to spring to life.

If you have additional information, tips and/or corrections, please do post.

I'll complete/edit this post accordingly.

With that out of the system, let's go on.

The red LED blinking rapidly three times, from here on called FRLoD (Flasing Red LED of "Death")

The X10 have now reached an age, where battery failures, and strange behaviour as a result, are fairly common. Specially if one got the device early on. So if you're reading this hoping to get your device back into life, the whole thing might be a result of just that: Battery is failing due to wear and/or age.

Questions:

What's does the FRLoD mean?

Simple answer: The battery has "run out of juice".

It's so empty, the device cannot startup at all. Not even to show the offline charging screen. Any attempts to restart the device will only be followed by the FRLoD again.

Sometimes, one can let the device rest for a while, and restart can be achieved, but the luck soon runs out as the device simply shuts off during the startup procedure, and back into FRLoD mode.

If it can't show the offline charging screen, does it mean the device can't charge?

No. It will charge, it just can't show you that.

In fact, if you have the charger connected, it does receive a charge, but it will take some time before the battery has charged to a level where it's safe to show the offline charging screen again.

So what you might want to try, is to let the device charge blindly for a while.

It will take from a few minutes to a few hours, before the device kicks back to life and shows the charging screen. How long it takes depends how badly the battery where drained.

Yes it's quite possible to drain the battery beyond the "empty level".

I had my charger/USB connected all the time while in recovery/flash mode, shouldn't that keep the device alive even if the battery fails?

No.

When the device is in flash/recovery mode, it won't charge at all, even if connected to a power source. It will slowly deplete/discharge the battery, and it will run out of juice if one sits for too long in either mode. So when the battery drains/fails, the flash or recovery mode will fail with it.

This is why it's so important that the battery is fully charged before one attempts recovery/flashing the device.

Why it is designed like this, is beyond my knowledge. I belive it's the same with most, if not all, Android devices.

It seems the battery won't charge "blindly" at all. What can I do?

Charge the battery externally, with a proper Lithium Polymer Charger. One can even use an universal 3.6 volt Lithium Ion charger, as the charging method of either is basically the same.

Be careful: Never ever leave a lithium polymer battery charging externally unattendend. It can be dangerous. Keep an eye on the temperature. It should never exceed 50 degrees Celcius / 122 degrees Farenheit. If it does, disconnect immediately.

If charging externally is out of the question, you need to replace the battery with a fully/partially charged one.

There is also a rather "ugly" trick one can attempt:

Only perform this as a last resort. It's very dangerous, but might be worth it if everything else fails, including getting a new battery.

Pull out and shortly charge the lithium Polymer battery with an ordinary 9 volt battery.

Connect the positive terminal to the other positive terminal, and the negative terminal to the other negative terminal. Use a short pair of wires for this. Only charge it for a few seconds at a time, and never let the batteries temperature go beyond 50 degrees Celcius / 122 degrees Farenheit. (slightly warmer than your hand)

This will briefly kickstart the battery to life, so one can start charging it normally in the device.

I can't make the battery last long enough for this flash to succeed. Now what?

Simple: Get a new battery.

A fresh battery should be able to keep the flash/recovery mode active for several hours / a whole day.

What are the signs of a failing battery?

Here's a list:

Symptom: Device suddenly shuts of when used, even when battery had several percent left.

Reason: Battery can't take high load very well. the voltage dipped so quickly, the battery indicator didn't have time to update.

Symptom: The device only lasts for a few hours/minutes, before it needs charging again.

Reason: Old age. The battery has lost so much capacity, it's running on the last breath.

Symptom: Charging is incredibly fast.

Reason: Battery has, due to old age, lost too much capacity. Low capacity = Reaches full cell voltage quickly.

Symptom: Device shuts off, and it need resting before it can restart again.

Reason: Again. Old age makes the battery unable to keep the voltage at a good level. Resting lets the battery slowly build up the voltage level. Internal resistance is the "bad guy" here.

Symptom: Battery gets warm, even hot, while charging.

Reason: The battery is about to run out of charging cycles, and the charge is lost in form of heat.

Most, or all, of these symptoms are followed by the FRLoD, either temporary or permanently, depending on how bad the battery condition is.

I've charged/replaced the battery, and are having trouble with starting up. Earlier flash failed. Now what?

Don't worry. It's pretty darn hard to truly brick this device. Even if the screen remains black, there's still hope.

Once that the battery issue has been dealt with, you can move on:

Download the latest Flashtool and the proper global firmware (FTF-file) for your device from here:

http://xdaforums.com/showthread.php?t=920746&highlight=flashtool

1: Extract/Install the Flashtool to a folder of your liking.

2: Copy the ftf-file to the "firmware" folder in the Flashtool folder you just created above.

3: Start Flashtool. Click the Bolt icon (flash). A Window appear. There, choose "flashmode". Cllick OK.

4: A new window appears. In the list on the left, choose the Generic firmware you downloaded and copied into the "firmware" folder. Should have a similar name as the file itself. (Or manually open the ftf-file from the "..." icon on the top right.)

5: Hit "OK. Follow the instructions you get on the screen:

1 - Unplug the device.

2 - Power off the device. (Unplug and replug the battery to be sure it really off, and not stuck in black screen)

3 - Press the "back" putton, and hold it down while performing the next task.

4 - Plug the device into the computers USB.​

Forced flashing should now start.

Once done, the device should be back in it's original state, and you can now start over. Rooting, boot loader unlocking, etc...

Good luck.