Open Source Thinkpad T420 Battery Design

ThinkpadBattery

Open source Thinkpad T420 battery design

http://beta.aceparent.me/#/battery

What is this?

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

Setup

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 (for programming the attiny85)
• 12v Lithium-Ion battery pack
• 3S Lithium-Ion BMS
• 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.

Limitations

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). I don't plan on fixing this, as the exact % is unimportant to me.

Future Plans

3D Printable Case

When my current battery becomes unusable, I'm going to create a 3d printable model to house nine 21700 cells that can be attached to the laptop.

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