A DC Power Meter based on the popular INA226 36-V, Bi-Directional, Ultra-High Accuracy, I2C Current/Power Monitor with Alert. 

While INA226 breakout boards are available from eBay/Banggood/AliExpress (e.g. CJMCU-226) they do not allow for easy connection of the bus voltage. Additionally, once this design has been proven, this layout can also be forked into larger designs.

Specifications

• Input Voltage: 0-36V.
• Current Measurement: +/- 20A. (Phoenix Contact connectors rated at 12A)
• Bus: I2C
• ADC Resolution: 16bits
• Bus Voltage Resolution: 1.25mV
• Current Resolution: 625uA

Design Notes

The device is powered via a separate 2.7 to 5.5V power supply connected to J3. Typical power consumption is just 330uA @ 3.3V. 

The signed 16-bit bus voltage register has a least significant bit/resolution of 1.25mV allowing for a full scale voltage of +/-40.96V.  Note, however that the device is rated to 36V (40V absolute maximum) on the input pins.

The analog input for the shunt resistor accepts a full scale voltage of 81.92mV. With a 4mOhm shunt resistor, a maximum current of +/-20.48A can be measured with a 625uA resolution.

Shunts

Below is a table of suggested shunts.