HA Dashboard on Android Tablet - Battery Bloated - Use capacitor and a diode to trick a tablet into booting off USB with no battery

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Thought this might be useful for others.

I have been using a Teclast p80x (8 inch tablet) for a permanent HA dashboard. in the home. Been using it for about 2.5 years.
Now I have a swollen battery because it was plugged in 100% of the time. (I have since read threads around putting in a power plug to cycle charging)

I have been able to convert the tablet to run without a battery, using a capacitor and a diode, and still using the standard power USB C port on the tablet. i.e. no extra external cables.

Battery in the tablet had 5 wires connected to the circuit board, 2 power (red), 2 ground (black) , and 1 data? (white) - (not sure this is relevant - but added for completeness)

  1. I removed the battery, but left the battery control module (had to remove a bunch of tape, and cut spot welds to battery tabs.)
  2. Plugged in tablet with USB A to USB C cable
  3. Couldn’t use a USB C to USB C - something special about these and the power management on the tablet main board. I could only ever find up to 0.1 volts anywhere on the board
  4. Use a voltage meter to find 5 volts on the tablet main board
  5. Connect wire from tablet main board 5 volts via diode (i had a spare 1N4007 available) to the +ve battery terminal on the battery control module (diode points toward battery control module - didn’t want capacitor pushing any voltage back to tablet main board and diode usually rips out about 0.8 volts, so battery control module will only get about 4.2volts. i.e. same as orig battery)
  6. Put ceramic (104) 0.1uF capacitor across +ve and -ve battery terminals on the battery control module

This worked - Using the normal tablet USB C charging port (with USB A to USB C cable)

  1. Tablet booted.
  2. Could log in, change settings etc.
  3. Battery percentage shows 100% - Doesn’t show its charging. (Doesn’t bother me)
  4. However, as soon as I tried to run Home Assistant Companion app, that I expect drew too much peak current, the tablet would act if the (imaginary) battery had been disconnected and the power cable had been pulled out. I.e. back to pre boot up stage. i.e. it was off - not in standby.

So I upped the capacitor size - only one I had available was a 400v 10uF Aluminium Electrolytic.
Now it seems to be working no issues at all. Been running for about 16 hours as of this post.

Problem I have is the 400v 10uF cap is way too big to fit inside the tablet case, hence not able to mount back on the wall with my current mount.

So need to ponder how do I determine the correct size capacitor?

  1. Ceramic 104 (0.2 uF) was too small.
  2. Electrolytic 10uF 400V is too big.

As 10uF is working, I am going to try and use a Tantalum 10uF 25V cap.
It’s small enough form factor to fit inside the back case of the tablet (5mm by 3mm)

Will post back after I purchase one and try it.

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Decided to give a Tantalum a go.

I was able to pick up a a few 10uF Tantalums today.
I have substituted the 10uF 400v Electrolytic for a single 10uF 35V ($1.15 AUD) Tantalum.
Tested it - Seems to be working ace.
Soldered it all together and added heat shrink, and put the rear case back on.
It’s working and back on the wall!
No case mods required.
No extra power supply required.
No battery.

It was going to cost me about $20 AUD for a battery (delivered - and not until March…) - and might only get another 2 years out of it.
Sorted, piecing togther a few youtube videos and a few forums, for a little time and about $3 AUD worth of parts.

Love the internet!!!

If any of this has helped, pop a reply in down below.
I will post some pics next.

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While testing with the 10uF 400v capacitor. Can’t see diode in this pick, it’s hidden behind.

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Now with Tantalum 10uF 35v cap.
And heat shrink to ensure no shorts.

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And back on the wall.
With battery ready to be disposed. :grinning:

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Fantastic work bud. The way I approached this issue after one tablet blowing on me.

I have the charger for the tablet plugged into a zigbee plug, the tablet is loaded Home Assistant so it’s sensors are exposed including battery level.

Then I have wall panel loaded as the main launcher to lock it down.

Then I have automations set up within Home assistant to monitor the battery level, when it drops below 5% the automation turns on the plug, then when the battery level hits 99% another automation turns off the plug.

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Yep, planning on doing that with my other wall panels. Looks like the Ikea Tretakt at $12 AUD might be the way to go. Don’t need the power monitoring.