Open source Thinkpad T420 battery design

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

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.

• 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

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.

• 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.

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.

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.

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

I’m going to make a controller board with a LTC2944 Battery Gas Gauge.