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189 Stars 11 Forks MIT License 50 Commits 4 Opened issues


Open source Thinkpad T420 battery design

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Open source Thinkpad battery design

What is this?

This is a fully open source laptop battery design. The design uses an attiny85, which can reply to the T420 motherboard's SMBUS requests.


Configurable Constants

  • BATTERY_CAPACITY | Your battery capacity in mAH
  • V_HIGH | The highest output voltage of your battery, normally 12.6v
  • V_LOW | The cutoff for when the battery will stop working, normally around 10.8v

Creating your own

Parts Needed - Arduino Uno/Mega (for programming the attiny85, Mega needs different pin assignment) - 12v Lithium-Ion battery pack - 3S Lithium-Ion BMS (it might be possible to use the existing bms if specification is available but communication needs to be implemented) - Components from schematic - PCB from schematic

Once you have all these parts, assemble the pcb and attach a BMS to your battery pack. Solder the 12v battery output/input to the large pads on the pcb, then program the attiny85 with the included ino file. The output pins at the bottom of the PCB are a 1-1 mapping of the motherboard pin input.


  • The laptop won't supply power to the board unless it detects there's a battery attached, meaning if you plug the board in to the battery without having an external battery to power it, it won't work.


Use the ATTinyCore for compiling. Without it, Wire.h won't work.


Voltage Reading / Remaining battery

Currently remaining battery %'s aren't exact because I'm assuming there is a linear coorelation between voltage and capacity (Which is wrong). This is probably fixable using a battery gas gauge, I'll have to investiage more.

MCU Power Consumption

By design, the MCU will always be on as long as a battery is connected to it. This can cause the battery to slowly run out of power if it isn't connected to a laptop. A potential fix could be to implement a low power mode for the attiny, or have a seperate disconnect switch for the battery power on the battery pack itself.

3D Printable Case for T420 only

The case for the cells has been completed. The case model and stl files are included in /model

Future Plans

iam4722202468 - Design and manufacture a PCB with current,voltage and battery capacity measuring capabilities, an easily programmable microcontroller, and a 3s balanced charger

pj1976 - Find a bms that would fit, or try to talk with bq29312pw that was in the original pack


iam4722202468 - Initial work, T420 9 cell model

pj1976 - Added support for R50

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