Youtube video: https://www.youtube.com/watch?v=IJ120djyGn4

This project gave a second life to my old useless Acer tablet. It had a dead battery, but what's worse - this tablet doesn't work from an external micro-USB power supply (it can only use the micro-USB connector for charging the battery).

But there is a pretty easy way to give a second life to such tablets - one just has to replace the internal battery with an external power supply.

Here are the steps I took to do the conversion for my Acer (can also work for many other cheap tablets):

• First I cut the 3 wires between the battery and the connector, inside the tablet. They are red (for +power), blue (for -power), and yellow (for the thermistor inside the battery - to monitor its temperature). I measured the thermistor to be a 10k type. The voltage of my battery is 3.7V.
• Then I very carefully dislodged and removed the battery from the case (it is glued there) - be very careful not to puncture or break it; use goggles and gloves just in case.
• I soldered a 10k resistor between the yellow and black wires going into the tablet (not the ones in the battery!).
• I connected a bench power supply providing 4.0V / 2A to the red and blue wires going inside the tablet (if you don't have a bench power supply, you'll need to use one of those cheap DC-DC step down voltage converters from amazon/ebay, to go say from 5V to 4.0V).
• Now my tablet can be powered up, despite the battery being dead!
• I rooted my tablet (google for instructions for your particular model)
• I installed two free Android apps: "txtpad" and "init.d scripts support".
• I used "txtpad" to create a script (a text file) in the default location, containing one line command (do not put quotation marks): "dumpsys battery set level 99"
• I ran the second app (init.d), which can execute any script at boot time, and pointed the app to the script I created in the previous step. Make sure it has root previlegies when it runs the script (an option in the app).
• I tested my setup by rebooting the tablet - ~30s after rebooting, you'll briefly see a message on the screen that the script was executed. At this point, check your battery status in Android settings - to should show 99% level, which will stay fixed (will not decline with time). If I didn't do the above tricks, the power indicator would show the battery level going down (even though it's powered from and external fixed power source!), and the tablet would eventually shut down - making it unsuitable for applications where it needs to be on 24/7 (like security monitor).
• Now the only remaining thing was to replace my bench power supply with an AC adapter. I happened to have a small 3.3V/2A AC-DC adapter. I measured it's voltage when there is no load - it was 3.5V. Under load (with the tablet connected) it was around 3.4V. This is still enough to run the tablet. You can use any other power supply, just make sure the output voltage never goes outside of the range 3.3...4.0V, and that it has at least 2A rating.
• The final touch was designing and printing a small adapter for a power socket
  (similar to this one) I happened to have.
• I drilled 5 small holes in the back of my tablet, fixed the power socket there using my adapter, and soldered the wires from the socket to the battery connector which connects to the tablet. Don't forget the 10k resistor (between the black and yellow wires).
• Voila - you got a nice 24/7 monitor with WiFi! I installed TinyCam app to continuously display my front door security camera.

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